EFI Basics
Back Home Up Next


EFI Basics

This page is a primer on EFI swaps. It it primarily based on some research via Google and lots of web sites, plus my experiences with both Ford and GM TPI and GM TBI EFI swaps. It is designed to contain information that is generic to most EFI swap efforts, in particular, to prevent me repeating information on each of my EFI swap pages. Much of this was originally written for my 1964 Ranchero EFI conversion, so many of the links are for Falcon-specific parts, but the details provided are applicable to pretty much any EFI system.


Information Sources

A DIY EFI swap is, like most other DIY projects, about saving money by doing things yourself and/or learning about how to do those things. You are spending your time to learn about something and do it yourself instead of paying someone else who already knows how to do it. This means that you will be first and foremost learning how to do the work involved, and that means reading up on details, making decisions, and then doing the work. Since the first step is the reading and learning, the first place I am starting here is by listing all of the places that I found helpful when I got started. If you do not like to read and learn, then I suggest you get a pre-made EFI swap kit or have someone do the EFI swap for you. EFI isn't terribly complicated once you understand it, but it's the "once you understand it" part that some folks may object to putting their time into.

For the Ford side, the fine folks over at http://www.fordfuelinjection.com/ helped me understand a lot of details. I highly recommend reading through their entire website for anyone considering a Ford EFI swap.

For the GM side, both TBI and TPI systems, I learned a lot from http://www.customefis.com/. He sells a "do-it-yourself" EFI manual for $30, and if you do anything with GM EFI - even for retrofitting to GM EFI to non-GM vehicles - it's well worth the cost. When you buy a manual, he also offers access to a separate section of the website with all kinds of great information. He also has great links to other pages such as the DIY EFI website.

Google is a great tool to go find out about things. Tons of folks have details about their EFI swaps online, and many places have extensive websites devoted to particular swaps. Search and read, then think and learn. It's free and often quite enlightening.

Amazon is a great place if you need to buy books. Good prices, lots of stuff available. I've bought several books as I've learned about EFI swaps, and I highly recommend reading up on the subject before you do a swap. Magic bullets are rare, luck is not always with you, and the best weapon you have against the random problems and issues that can arise is your mind. Fill it with as much ammunition as possible before doing battle with you project. :-)

Lots of parts, manuals, and even books are available on eBay. Once you decide what kind of swap you want to do, this is a a great source for parts, though you have to watch out for prices on "hot" items as the prices can quickly get unreasonable if people get into a meaningless bidding war.

The last and possibly most important source of information is...your garage. Get some parts in your greasy hands and see how they work. Try things out, see what fits. Get your EFI system running and experiment with things to learn more. Hands-on experience is very useful when contemplating questions like "will this part fit in my vehicle?" - nothing answers that better that getting the part and seeing if it fits! You have to be careful about expenses, but used parts are often available for low cost. This is very helpful when getting started on doing an EFI swap - having actual parts in hand makes it very easy to tell if you are heading in the right direction or not, or if you may be facing problems down the road. If the part you want to use doesn't even fit onto your engine, then you have a problem to resolve long before you get to worrying about if it will work with your computer, wiring harness, etc.



Like any specialized area, EFI systems have a lingo all their own. There are many terms and acronyms that are used that will no doubt sound foreign and mysterious to the newcomer. Learning these various terms and knowing what they mean is critical to being able to decipher the various pieces of documentation. If you don't know a MEMCAL from a HEGO, then it will all just sound like gibberish. You'd never know that the MEMCAL is a GM-only thing that plugs into the computer and the the HEGO is the heated oxygen sensor that mounts in the exhaust system. Most sites (including mine) will make an attempt to document the terms they use as they use them, though some will not. If you do not know what something is, stop and go look it up, only then keep reading.

One potential "gotcha" to be aware of is that different brands of EFI systems often use slightly different terms for the same basic thing, so it's best at first to learn the terms that apply to the system you are using so that the various websites and books you read will make the most sense. As an example Ford calls it's vacuum sensor a Manifold Pressure Sensor (MP) while GM calls it a Manifold Absolute Pressure Sensor (MAP). They both do the same thing - translate the amount of vacuum present in the intake manifold to an electrical signal the computer an understand, but if you were reading along and saw "MP" instead of "MAP" you could easily get confused. I've worked on both Ford and GM EFI systems, and as I've created these web pages I've made an effort on this page to call out both sets of terminology where there are clear differences, and on the GM and Ford specific pages I've attempted to use the proper terms for each of those systems. If you find obvious discrepancies, please let me know so I can fix them.


Safety First

Now that you are ready to learn, maybe even have read up a bit on all things EFI, and even know a few new terms - I need to say a few things about safety. Yes, it matters. Yes, you should read this. No, the lawyers did not make me do this. Staying safe is just good common sense.

EFI fuel systems are under very high pressure - 20PSI on up to 50PSI or more depending on the system - and this is very different that carbureted systems that run on a mere 3-7PSI. Make sure your fuel system is safe right from the first design you dream up for it, and make sure you work on it properly. Nothing hurts more than watching your pride and joy burn to the ground because you skipped $50 worth of wiring along the way or because you did something dumb while working on it. Nothing would hurt your loved ones more than knowing you burned to death in your pride and joy because you skipped $50 worth of wiring along the way or because you did something dumb while working on it. Be smart, be safe, and make sure both you and your ride are around for a long time to come.

First up is the question of making the fuel system safe in the event of an accident. Why? Because as long as the fuel pump has power (aka, the ignition switch is in the "on" position), it will happily keep pumping fuel - even if a fuel line ruptures. The effect of gallons of gas being sprayed in a fire-hose-like fashion all over a hot engine compartment is a recipe for a very unhappy Hollywood-style fire and explosion. In the event of an accident, you want the fuel pump to stop immediately. And if the engine stalls, you also want the fuel pump to stop.

The factory solved the "in the event of an accident" problem with a simple device called an Inertia Switch that is wired in-line with the fuel pump relay coil. If the inertia switch receives a harsh enough physical jolt - as happens in an accident - then a small spring-loaded set of contacts opens and cuts power to the fuel pump relay. There is a small button on the inertia switch to allow it to be "reset" in case of a false triggering (such as an overly enthusiastic door, trunk, or hatch slamming) or real triggering (such as a fender-bender or other accident), so it must be mounted in a reasonably accessible location. Wiring an inertia switch is incredibly easy and is something I consider to be a mandatory safety addition to any EFI installation. It's actually a good idea on any fuel system that is using an electric fuel pump - even the pumps designed to feed a carburetor.

The "in the case of the engine stopping" is usually solved by a connection to the oil pressure sender from the factory computer. It uses this to tell if the engine is, in fact, running, and that helps the computer to decide if the fuel pump should even get power. Some EFI systems use this type of wiring and some do not, so unless you have strong opinions one way or the other and the expertise to act on those opinions, I would recommend that you follow the wiring diagrams from your "donor" vehicle. If the computer you are using does not monitor the oil pressure and you want to, there are several ways to wire one or more relays through an oil pressure switch to get the same basic effect. Holley and other companies sell these oil pressure switches to do this on carbureted vehicles that use an electric fuel pump, and it is very easy to do this.

Lastly, the high pressures involved means that you need to take special care when working on them to ensure the fuel pressure in relieved before loosening any fittings on the fuel system. Any decent manual for working on an EFI system will contain the proper instructions for doing this. In the case of an EFI swap, typically you should follow the instructions in the factory service manual that go with the "donor" vehicle the EFI system originally came from. As an example, I am transplanting the fuel injection system from a 1989 Mustang GT onto my 1964 Ranchero. I have a 1989 Mustang GT service manual to refer to and follow when I am working on that particular EFI system. Yes, the manuals cost a few bucks, but the added diagnostic information and the extra safety details the manuals provide make it well worth it to purchase and use them.


Fuel Pump(s)

Most OEM EFI installations typically install the gas pump inside the gas tank. While it may seem an odd design to put the pump inside the fuel tank, it actually works incredibly well. It quiets the pump and protects it from the elements - it's basically inside a sealed box. It has the advantage of cooling the pump by surrounding it in a cool recirculating liquid - also known as the gas that's in the gas tank - which helps the pump live a long and trouble-free life. While noise and long pump life are important for any EFI system, especially a modern OEM system, putting the fuel pump in the fuel tank poses challenges when converting from an earlier non-EFI system. There is a good discussion of this at http://fordfuelinjection.com/Inject_your_horse3.pdf - you should go read it. They're talking about a Ford system, but the fuel system information is generic to pretty much any EFI swap. The tank needs special internal baffles and/or a "sump" to keep fuel near the pump pickup even at low fuel levels, and these are hard to retrofit into an existing gas tank.

For the fuel pump, the "ideal" thing to do is to be like the factory and use the in-gas tank style pump if possible, but on an older car this would require custom gas tank work or a lucky find of a reproduction gas tank built specifically for an in-tank pump. If you can not or decide not to use an in-gas tank style pump, the preferred solution is to use a low pressure pump near the gas tank to feed a custom "surge tank" and then have an external high pressure pump to feed the injector rails from the surge tank.  http://www.sdsefi.com/techsurge.htm has some good information on surge tanks and the paid section of http://www.customefis.com/ also offers some good details on building your own surge tank. If that's not enough for you, then Googling for "efi surge tank" should net you plenty of reading material and ideas. I also have a section on surge tanks below to give you the basic overview of them along with some ideas on how to make and mount one.

Keep in mind that a high pressure EFI fuel pump will draw 15A of electricity or so at full load, and that means you need good wiring with larger sized wires, a relay, and a big enough alternator to power things. If you're running a separate low pressure pump, that will draw around 5A, which puts your fuel pumps at 20A, minimum, so plan on at least a 30A capable circuit to feed them.


Gas Tank

Another consideration is that the EFI system will recirculate all of the fuel in the tank on a pretty regular basis. This is very different than the older style carbureted systems or a non-return style EFI or system where only fuel that is used by the engine gets pumped out of the tank. Why does this matter on a retrofit installation? Because all of the crud, rust, and junk in the bottom of your gas tank will get churned up and sent through the fuel system within just a few miles of driving your newly EFI converted vehicle. This make a good fuel filter mounted before the high pressure pump essential, and if running a low pressure pump, you should mount one before that pump as well - no sense letting the crud eat into the pump or get into the surge tank. It's also a good idea to drop the gas tank, empty it, and clean it out thoroughly as part of the swap. Even things like a bit of water in the bottom of the tank - as many older cars seem to have - can cause problems. In short, when you drop the tank, make sure you clean it very thoroughly before you re-install it!


Return Line

Most earlier EFI systems (pre-mid-1999 for Ford and around the same time for GM) use a return style system where the fuel pressure regulator diverts excess fuel back to the fuel tank through a second "return line" and the fuel is constantly recirculated. There are later "returnless" EFI systems that act more like a traditional carbureted fuel system (aka, no return line), but they are not commonly used in EFI swap applications. That means that In virtually all EFI swap cases, you must add a return line from the engine to the gas tank - the exception being if you are using a "returnless" system. If you are using a surge tank, the return line from the engine goes to the surge tank and the return line from the surge tank goes back to the gas tank. To do this, you have to hook the return line into the main gas tank somehow. One option is to cut and splice into the original filler neck assembly with a kit like the Moroso 65385. If your filler neck is the right size, this can work for you. You can also have a new line brazed into the original tank fuel sending and pickup tube assembly. Note that because you need to drop the tank anyway for this conversion so you can clean it out thoroughly, and that includes removing the sender and pickup tube assembly, it's not such a big deal to take the feeder tube assembly out to have a return line installed into it. It's also a good time to change the filter sock on the pickup tube and perhaps replace or refurbish the fuel level sender so your gas gauge reads correctly. Sagging floats and erratic fuel level senders are not uncommon in older vehicles, and dropping the tank is a hassle, so it's just common sense to do all of this work while you have the gas tank out of the car to clean it out. You may even find that your tank needs a complete cleaning and resealing inside, or as a worst case, a complete replacement.

One nifty idea I've recently seen is to use a '99 Corvette fuel filter, regulator, and return fitting assembly - GM P/N 10299146. It mounts along the framerails somewhere upstream and the high pressure pump and filters the fuel before the injectors as well as providing a regulator that maintains 58psi. A return line fitting is right there to send excess fuel back to the fuel tank. With this, you only need one line to go up to the fuel rails. The advantages are that the fuel in the tank is not exposed to underhood heat and thus stays cooler. The disadvantages are that the fuel in the fuel rails is not immediately flushed out with fresh, cool fuel on a hot start, which could lead to temporary poor running until it was replaced. GM used this on the Corvette, so it must work OK, so it's an interesting idea. A lot depends on where the surge tank, and thus the high pressure pump, gets located at. If they are along the framerails, then this makes a lot of sense. If they are already in the engine compartment, then this makes less sense but can still be used.


Fuel Line Fittings and Hoses

EFI systems use high-pressure fuel systems - all of the hoses and fittings on the pressure side must be rated to handle the pressure. That means you need the right fittings for the fuel lines. The Ford EFI systems I was working when I originally wrote this use what are called "Springlock" fittings on it's fuel lines. These require a special tool to disassemble them, and you must use them to connect to the fuel rail, and to any other late-model EFI fuel system parts you end up using. Find a car or truck in the junkyard with the nylon/plastic fuel lines, and simply cut off the end of the hose with the Springlock fittings attached. Then remove the hose and you have a fitting with a Springlock on one end and a barbed connection on the other end suitable to hook up to a piece of high pressure fuel line. Various GM systems may need custom fittings as well. Don't miss these small but important fittings when doing your junkyard scrounging.

For the fuel line, the goal is to run as little rubber fuel line as possible on the entire vehicle and use hard line for the long runs. It's cheap, easily formed, and very durable. You want most joints in the system to be able to accept fittings to connect to a hard line, or else be able to connect two pieces of hard line with as short a piece of rubber fuel hose as possible. Depending on where you mount the high pressure pump, you may also want to run the outlet of the pump in hard line all the way up to the point where the fuel line has to connect to the engine. That connection has to be flexible to allow for the engine to move, so plan on using rubber hose there or maybe even braided stainless steel line if you want the "racy" look. The original gas tank outlet accepts a rubber fuel hose, so I'll need to either change that when I add the return line or use a small length of rubber hose there. That part is only low pressure and does not move around much, so rubber hose will last a very long time without any problems.

The high pressure section of the fuel system - from the high pressure pump outlet to the regulator input (which is typically mounted to the the fuel rails) - must be done with high pressure fuel line rated for the intended pressure in the system. A part of this line will need to be rubber or some other flexible line (to go from the frame to the engine), and it is mandatory to use the a fuel hose here that is rated for the high pressure it will contain. High pressure fuel hose is different than low-pressure fuel hose, and much more expensive, so you want to minimize the amount you use for cost reasons. Some hoses can cost upwards of $5/foot, so running more than a few feet of means the cost adds up very quickly.


Surge Tank Overview

The surge tank is simple in concept, but can be a bit harder to actually install, particularly if you want a very "neat" installation. It has to be safe from accidents, heat, and road debris. It also has to fit into or under the existing vehicle - unless you want to make sheetmetal changes underneath the vehicle or in the engine compartment. You want to minimize the amount of rubber fuel line you use on the high pressure side of the system. It also has some appearance concerns if the surge tank is in a visible location - and some folks consider the underside of their car "visible" enough to warrant putting lots of effort into a "neat" installation. You get the idea. Custom surge tank solutions are expensive, I don't like the size of the BC Broncos solution, and the only "DIY" solution I know of is a bit ugly, though it should work just fine. Read on for design details and to find out what will work best for your application.

The fittings on the surge tank should generally be placed so that when the surge tank is installed, there are two fittings at each end, with one each at the top and the bottom of the tank. The two important details are that the EFI pump must come from the bottom front of the surge tank so it is "gravity fed" and the return line to the main tank must leave from the top rear of the surge tank so the surge tank is automatically full of fuel and purged of air. The remaining two lines are somewhat of an "as needed" thing, and I've arranged them so that fresh fuel from the main tank is fed into the surge tank directly above the EFI pump feed and the return lines are at the same end of the tank. This ensures a more constant turnover of fresh fuel from the main tank - aka, the entire system recirculates and the fuel in the surge tank is constantly being changed. I've diagramed the basic idea below with different colors for each line and directional arrows to make it clear what they are all for and what direction the fuel is flowing.


Surge Tank "DIY" Solutions

The "DIY" solution is to take a section of galvanized steel pipe 3" to 4" in diameter about 6" long, thread both ends, and put caps on each end. Then drill two holes - one on each side - of each of the end caps and tap them for the proper size NPT fittings. Install fittings to adapt from the NPT to your favorite outlet type, use gasoline-rated tape to seal it all up, and you're done.

A buddy of mine came up with what is a pretty neat suggestion - use an old small household size fire extinguisher or other small air/gas tank as the basis for the surge tank. If you can mount it vertically and upside down, the single fitting on the original "top" of the unit becomes a perfect gravity feed hookup for the EFI pump, you can drill and tap the original "bottom" to mount the return line to the gas tank. The other two lines can be drilled and tapped into the tank in any reasonably accessible place along the sides as required to fit your mounting position - any pressure rated tank should have enough thickness to tap threads into without welding on any additional bungs. Many of these tanks are rated for a decent amount of pressure, are built to handle some serious abuse and stay together, and if you want to hunt around you might even find one made of aluminum (an old scuba air tank, maybe?), so this seems like a really good idea. For horizontal-mounted use, if you were handy with a welder, you could take two, cut off the top sections, and weld the two bottom sections together to form a tank that is uniform from end to end. The surge tank is not under any pressure - any excess fuel returns to the tank via the return line - so you don't have to create something capable of holding high pressure fuel.


My Preferred Fuel System Design

Based on all of my research and the skills, tools, and money available to me, I believe a two-pump system with a surge tank is the best choice on my vehicles. Here are the notes I'm keeping as I piece my systems together.

The low pressure pump will be a small carb-style electric fuel pump mounted near the tank - the pump I use will be largely dictated by mounting space and price considerations. Others have recommended the Carter P4070 or P4594 pumps which are a reasonable $60, but I'm not real keen on the 1/4" inlet and outlet sizes - they just seem way too small for future higher horsepower increases. The Carter P4600HP and P4601HP have 3/8" inlet and outlet sizes, but are a bit pricier at around $90 and $100 respectively. The overall size of these pumps is also somewhat prohibitive for under-car mounting - at approx 5 1/2" tall plus a bit of clearance above and below , that's a bit much to put on an ~4" tall (or shorter in some areas!) frame rail. They seem to be well-made pumps and as of the time when this research was done, I couldn't find any non-EFI inline style pumps at a reasonable price, so you'll have to see what you can do when you get underneath your vehicle. Maybe mounting them in the rear axle area will work out OK. The P4600HP is the pump I like the most in this batch - it's got the 3/8" inlet and outlets, flows 100gph, and is only about $90. That should feed the highest horsepower monster I would be reasonably stuffing into any of my vehicles - it just has to keep the surge tank full, after all.

Some other choices I've recently stumbled across for a low-pressure pump include the Airtex E8016S and the Carter P60504. These are both low-pressure inline style fuel pumps that should suffice to keep a surge tank full. The flow ratings

The high pressure pump is the source of much debate, though lots of folks seem to recommend just a few choices. One is the Carter P74028 external inline pump. It's a replacement part that was originally used on various 1985-1991 F-Series pickups with the 5.0L and 5.8L engines. As far as I can tell, this pump or nearly identical to the Walbro GSL-392 pump, which also seems to be intended as a replacement for the same F-Series trucks. Both of these pumps mount in a very compact area and by mounting them and the surge tank near the engine, it minimizes the mount of high pressure fuel line you need to install - you only need high pressure line between the high pressure pump and the fuel rail. The return lines are all low pressure as are the lines from the tank to the low pressure pump inlet, from the low pressure pump outlet to the surge tank inlet, and from the surge tank outlet to the high pressure pump inlet.

The fuel filter is not decided yet, and the Carter low pressure pump has NPT fittings for the inlet and outlet, so I can use hard line there with adaptors such as a Weatherhead 202x6x6. The high pressure pump does have fittings on it, but it's not clear what size they are from my searching on the web, so I'll just have to buy one and find out, then decide on the right adaptors to use.


Fuel Filters

It is mandatory to add a fuel filter between the tank and the low pressure pump. Various styles are available, and you just need to find one that fits your specific needs. Since it will be mounted under the vehicle near the fuel tank, a metal cased filter is a smart idea to offer some protection from flying road debris, rocks, etc.. Select a filter with an inlet and outlet size at least as big as your chosen low pressure fuel pump. Your existing line from your gas tank may be a different size, and if it needs upgrading, be sure to do this when the tank is down for other work. A filter with flare or NPT style fittings is preferred but not mandatory.

You should also use another filter should be used between the high pressure pump and the engine. The injectors are even more sensitive to "crud" in the fuel than the high pressure pump is, and you want them to work right for a long, long time. Use a high pressure filter that matches the output fuel line size on your high pressure fuel pump. Ideally, your fuel rails will also have the same size as your fuel lines, but some factory fuel rails may be a tad on the small size for higher horsepower applications. They are much easier to upgrade than the rest of the fuel system, so if engine upgrades are in your future, size as much as possible of the fuel system to match your future needs and then upgrade the "weak links" later on as needed.

A NAPA 3481 filter is a high pressure all metal fuel filter with hard line fittings on each end. Unfortunately, those fittings appear to be metric, and you'll need to use NAPA WH 1446 fittings on each end to mate it to a 3/8" flare line. This is the original equipment filter used on some of the GM TBI truck engines such as a 1989 Suburban.

A NAPA 3270 filter is a low-pressure all metal fuel filter with 3/8" barb fittings on each end. It's suitable for a "pre-filter" between the tank and the first (or only) fuel pump.


Throttle Body, Injectors, and Fuel Rails

For a "throttle body" style EFI system, the throttle body and injectors are housed in a single unit that bolts on much like a traditional carburetor and there are no separate fuel rails. Mounting the throttle body assembly is pretty simple - either use a manifold specifically designed to mount your throttle body, or use an adaptor plate to mount your throttle body onto your existing manifold - these can be readily made from a piece of aluminum, or for many engines, bought commercially for very little cost. This ease of mounting to the engine is what makes throttle body injection systems very popular for retrofit applications.

For a "port injection " style EFI system, the throttle body and injectors are separate units. The throttle body just controls airflow into the engine and there is one injector for each cylinder, typically mounted in the intake runner very close to the cylinder head. There are separate fuel rails (two on any "V" of "flat" style engines, one on inline-style engines) that provide fuel to the injectors and hold them in place. If you are lucky enough to be working on an engine with OEM or aftermarket support for this style of injection (as was the case for my Ranchero EFI swap), you can simply start with a pre-made manifold. If you are working on an engine that does not have any readily available support for port fuel injection, you will need to have an existing manifold modified to accept the fuel injectors and fuel rails, as well as fabricate the fuel rails and a way to mount your desired throttle body and connect it to an air cleaner.

As far as injectors go, Mustang 5.0L injectors and GM TPI injectors are physically and electrically interchangeable which is great news. There are always plenty of used injectors floating about on various sites, and if you are careful about cleaning them, they usually work quite well. Oddly enough, the Ford injectors seem to be somewhat preferred over the GM injectors - and they're cheaper and more readily available from what I can see. One thing to note is that the lb/hr rating on the injectors you happen to grab with your donor system may or may not be appropriate for your needs. Assuming they are clean and they work, they should be fine for mock-up and even some initial testing of the system at idle and lower-power levels - aka, cruising your neighborhood at slow speeds to do a basic "drive test". You need to be really careful here - if the injectors are too small, they will not be able to "keep up" at higher RPM and/or engine loads and the engine can go to an extremely lean condition resulting in costly engine damage. The morale? Be sure you get an appropriately sized set of injectors to use for your final tuning. I found a neat online calculator you can use, others information sources and possible other online calculators are around if you Google around some. If you're getting Ford injectors, some common ones are usually identifiable by their color - tan or grey is 14lb/hr, orange is 19lb/hr, and blue is 24lb/hr. Note that these are Ford's flow ratings at 39PSI, and GM rates injectors 43.5PSI, so you need to adjust for this when doing injector size comparisons so you get the injectors that are sized correctly for your needs.

Whatever injectors you get, unless they're brand new or already reconditioned, they will need to be cleaned. First, get familiar with an eBay stored called Mr. Injector - they sell "injector service kits" for the Ford/Bosch top feed pintle tipped injectors for about $20. There is a good page up on FordFuelInjection.com detailing how to use these kits, an easy-to build test harness, and some readily available injector cleaner to clean used fuel injectors. Basically, you remove the o-rings, remove the internal filter on the injector, and let the injectors soak submerged in full strength cleaner overnight. When they come out of the cleaner, hook them up to the test harness to activate the injector, and then blow compressed air backwards through then injector to blow out the remaining crud. Make sure the other end of the injector is pointed down into a bucket or other container suitable to receive the cleaner + crud mess that will come out! Remove the injector from the test harness, install the new injector filter, and install new O-rings onto the injector.


Wiring, Sensors, Actuators, and Other Such Things

All EFI systems use a computer, a bunch of different sensors, and a bunch of actuators. What's an actuator? It's a device that does the exact opposite of a sensor - instead of telling the computer about something, the computer tells it to do something. The fuel injectors are a classic example of this - the computer tells them to open and close appropriately to provide the right amount of fuel to the engine. There are other relays, solenoids, and motors in the system as well - particularly for emissions controls - that the computer actuates to make things happen. The exact sensors and actuators used vary a bit from one EFI system to another, but the basic details are the same. All of those devices need to be mounted on the engine, transmission, exhaust, or someplace on the vehicle (as required by each device) and there needs to be wiring to connect it all to the computer. The result is a non-trivial number of wires going to the computer, which makes careful planning of the computer location important - the largest bundle of wires starts at the computer and heads out to each different area of the vehicle, with the harness getting smaller as each group of wires leaves the main harness to head out to whatever it needs to go to. For most cases, the computer goes in the passenger compartment and requires a fairly large hole through the firewall for the wiring. For other cases, the computer is weatherproof and can be mounted in the engine compartment, but this requires careful planning of the location of the computer so it stays cool and doesn't come into contact with anything - fan blades, flying debris from the tires, motor moving around on it's mounts, etc. In most "hot rod" or "customized" retrofit installations, the appearance of the wiring is as important as it's function - running that huge wiring bundle as neatly and as out-of-sight as possible makes for a much nicer looking installation.

Below is a list of common pieces and parts you will find on EFI systems along with a basic description of each one. Not all these will be present on all systems - in fact, some of them are mutually exclusive and which one you have depends on the type of EFI system you have. This list is intended to give you a basic primer on what is included in these systems so you have some idea of what's going on.

Name Purpose
Throttle Position Sensor (TPS) Tells the computer how far open the throttle is. Usually mounted to the throttle body where the throttle "butterfly" valve is located. Used to adjust the amount of fuel and timing for the desired throttle position.
Coolant Temperature Sensor (CTS, ECT) Tells the computer how warm (or cold) the engine coolant is. Usually mounted in the intake manifold or cylinder head. Used to adjust the amount of fuel and timing for cold starts, vs. warm engine operation.
Intake Air Temp Sensor (IAT, ACT) or Manifold Air Temp Sensor (MAT) Tells the computer how warm the incoming air is. Mounted somewhere in the air intake system - usually the air cleaner or the ducting, sometimes in the intake manifold itself. Used to adjust the amount of fuel and sometimes timing for hotter or colder incoming air.
Manifold Absolute Pressure Sensor (MAP, MP) Tells the computer the amount of engine vacuum that is present. Usually mounted somewhere in the engine compartment, but can also be mounted in the passenger compartment. It has a vacuum hose that connects to intake manifold. Used to adjust the amount of fuel based on the current engine load. Used in "Speed Density" style EFI systems where the amount of fuel is looked up using stored tables in the computer.
Mass Airflow Sensor (MAF) Tells the computer how much air is coming into the engine. Mounted so that all incoming air goes through it, usually somewhere ahead of the throttle body. Used to adjust the amount of fuel to match the amount of air coming into the engine, which changes based on engine load. Only used in mass airflow style EFI systems where the amount of fuel is calculated directly based on the amount of air coming into the engine.
Barometric Pressure Sensor (BP) Similar to the Manifold Pressure Sensor, except that it is open to outside air instead of being hooked up to manifold vacuum. Tells the computer what the density of the outside air is. Used to adjust the amount of fuel based on altitude or other changes in air pressure. This is how EFI systems automatically adjust to run properly at high altitude vs. sea level in ways that a carburetor can not.
Knock Sensor (KS) Tells the computer if the engine is currently "knocking", "pinging" or experiencing detonation. Used to retard timing and prevent engine damage under heavy load with poor quality gasoline. Usually mounted to the lower part of the engine block and is basically a highly refined microphone that sends a specific signal to the computer when there is a problem. May not be present on all systems.
Oxygen Sensor (O2, HEGO) Tells the computer the amount of oxygen in the exhaust, which indicates if there is too little or too much fuel currently being put into the engine. Used to "trim" or adjust the amount of fuel. Mounted in the exhaust system as close to the engine as possible. This is what enables "closed-loop" mode where the computer can learn about your engine and over time fine-time the amount of fuel to compensate for engine wear. Only works when sufficiently hot to produce a signal - some sensors include a small heater to allow them to warm up and begin working faster.
Vehicle Speed Sensor (VSS) Tells the computer how fast the vehicle is traveling. Used to adjust the amount of fuel for deceleration conditions (where much less fuel is required) and is used to tell when the vehicle is actually in motion - some programming in the computer only takes effect when you are actually driving the car. Usually mounted on or near the transmission. Later models may only have this sensor and no speedometer cable - the speedometer may be electronic and driven directly by the computer.
Inertia Switch Turns off the fuel pump in the event of a crash. It is a simple mechanical device that is wired in-line with the fuel pump relay and operates as a fail safe device.
Oil Pressure Switch Tells the computer is the engine is actually running and producing oil pressure. Usually used to help control the fuel pump relay - no oil pressure and the fuel pump is turned off.
Park/Neutral Switch Tells the computer is the transmission is in Park or Neutral, or conversely if it is in Drive or Reverse. Used to adjust fuel and idle speed as you transition in and out of gear to maintain a smooth idle. Also used in some cases to tell the difference between driving the car and simply revving the engine in Park or Neutral.
High Gear Switch Many earlier computers - mostly GM - did some rudimentary control of the torque converter clutch that allowed the transmission to "lock-up" and operate without slippage when driving at highway speeds. May not be present on all systems, notably Ford engines that used the AOD transmission. This tells the computer if the transmission is in it's highest gear, usually overdrive.
Cranking Signal Tells the computer when the starter motor is engaged. Used to adjust the amount of fuel and the timing when the engine is cranking. Typically just a wire that is connected to the starter solenoid so the computer knows when it has power.
AC Compressor Signal Tells the computer when the air conditioning compressor is on. Used to raise the idle speed slightly when the AC is running. Typically just a wire that is connected to the air conditioning compressor clutch so the computer knows when it has power.
Electronic Ignition Tells the computer how fast the engine is spinning. Some models may use a sensor that reads the position of the crankshaft directly, others may simply report an RPM signal much like a tachometer hookup. Can be very simple with a single coil and traditional looking distributor or very elaborate with a multiple coil "distributorless" style system. Most systems also allow the computer to control the specific timing (advance/retard) of the ignition system, thus making this both a sensor and an actuator. Your system may have discreet components to do each part or it may be a single module that handles everything.
Idle Air Bypass Motor (IAB) Allows the computer to control the amount of air coming into the engine at idle and is used in conjunction with the amount of fuel to control the idle speed. Usually mounted on the throttle body, but can be mounted anywhere and be hooked up to the engine via a vacuum hose.
Fuel Injectors Allows the computer to control the amount of fuel going into the engine. Mounted in the throttle body with one or two injectors for "TBI" style systems or in the intake manifold near the head(s) with one injector per cylinder for "TPI" style systems. The injectors are simply very fast on-off valves that spray a very fine mist of fuel when they are on.
Fuel Pump Provides highly pressurized fuel to the engine to give the fuel injectors something to inject into the engine. Can be mounted near the engine, along the frame, or even directly in the gas tank. The computer controls the fuel pump via a relay.
Check Engine Light, Service Engine Soon Light (SES) A indicator light in the instrument cluster that tells the driver if something is not right with the system. In many systems, it can usually be instructed (via a test harness hookup) to flash what are known as "trouble codes" for a repair technician to use as an aide to diagnosing and repairing problems. The computer tracks a great deal of information about the engine, and is programmed to recognize when something is not right. The SES light is how it informs the driver that something is wrong, and is the most basic way to tell a technician exactly what it thinks the problem is. The same "trouble codes" can be read by more expensive diagnostic equipment and displayed on a screen, but often a simple jumper wire is all that is needed to get the SES light to give a basic set of information. This can be quite handy for doing EFI swaps on a budget - the first step in diagnosing problems is just a jumper wire and some time spent looking up the trouble code. Of course, you still have to confirm what the computer says and then fix it, but it's a good start.
Lock-up Torque Converter Clutch Control (TCC) Many earlier computers  - mostly GM - did some rudimentary control of the torque converter clutch that allowed the transmission to "lock-up" and operate without slippage when driving at highway speeds. May not be present on all systems, notably Ford engines that used the AOD transmission. The computer activates this if it decides the TCC needs to be on.
Emissions Devices
EGR Solenoid Allows the computer to control if the EGR valve is open or closed.
EGR Sensor Allows the computer to tell if the EGR valve is working properly.
AIR Solenoid Allows the computer to control the air injection system. Also known as the Thermactor system on Ford vehicles.
Charcoal Canister Solenoid Allows the computer to control when the fuel vapors in the charcoal canister are allowed to be drawn into the engine and burned.


Oxygen Sensor

For the oxygen sensor, also know as an O2 sensor, you can get 1, 2, 3, or 4 wire sensors. They all do the same basic thing, but have some different features to them. One and two wire sensors are unheated, with the two wire sensors providing a separate ground for the sensor. The three and four wire sensors are heated, with the four wire sensors providing a separate ground for the sensor. The heating element allows the sensor to get up to operating temperature faster and to stay there even at low exhaust flow readings. This helps the engine run better. The separate ground for the sensor is also helpful, particularly on older vehicles where you are doing an EFI swap and that may have a questionable ground path for the sensor location. An oxygen sensor generates a small but very important voltage of 0V to about 1.1V depending on the amount of oxygen present in the exhaust stream. With such a small voltage range, any fluctuation in the grounding of the sensor can really confuse the computer, and I prefer to run the O2 sensor ground separately just to be sure. Similarly, a heated O2 sensor will perform better in swaps and help ensure it's putting out accurate data as much of the time as possible.

A 4 wire O2 sensor was used on the 1995 Corvettes and is available as AC Delco part #AFS75. The pigtail for the OEM connector for this sensor is AC Delco PT368. Various other units are available.


Vehicle Speed Sensor

This monitors the vehicle's speed and allows the computer to adjust things as needed. Sometimes you can get away without using one, but it can cause odd behavior of the EFI system in certain cases, like stalling out when you let off the gas while cruising and other weird things like that. It's an easy thing to add, so you should do it.

On Ford's, the VSS generally connects to the transmission right where the speedo cable normally goes in, and then the speedo cable connects to the back of the VSS. The speedo gear goes on the end of the VSS that sticks into the transmission. The speedo cable connection to the VSS is different than the speedo connection to the transmission, so you generally need a proper length speedo cable from a EFI equipped vehicle. These are not hard to find, and the speedo cable connection at the speedometer was the same push-on style connection from the late 60's all the way into the '90s, so getting a used cable for most swaps is pretty easy - just grab it from a donor vehicle. For earlier speedometers with the screw on style connection, you can get a custom speedo cable from various places.One other interesting idea for Ford's is to try and use an "early style" cruise control speed sensor that mounts inline with the speedo cable. The cruise control modules are the same all the way until around 1987 or so, but the VSS changed to the newer style around 1985 in some cases. So, the same cruise control module can work with both early and later style sensors, which would indicate they are electrically compatible in the signal they generate. This could solve a problem for a tough swap in some cases, if it works. I have not verified they are identical, but this seems plausible as an idea to try out. The EFI system doesn't care what the speed is, just that it sees a reasonable signal, so this seems like it ought to work.

On GM's, the VSS generally mounts into the transmission in some way, but some of the TBI systems use a VSS that is combined with the speedometer, and this not really suitable for retrofitting into something else. JTR makes a good collection of different GM VSS-related parts and adaptors, but it's not super-cheap. It is all right there and ready to order, though, so it's work a look.

I believe all of these stock Ford speed sensors (early or late style) are 8 pulse per revolution, also known as 8000 pulses per mile (PPM). As far as I can tell, they are all basically small generators that put out an AC signal on the two output wires. The higher the revolutions, the higher the voltage you see. It's a purely analog setup. This is different than the TBI GM stuff where the 2000 PPM VSS is a a "digital" unit that turns on and off twice per revolution. The later GM TPI stuff uses an analog 4000 PPM VSS that is similar in design to the Ford units, just with a different number of pulses per mile. You simply have to match your VSS to your EFI computer so the computer gets what it expects. The folks over at JTR have a great document explaining all this.

Comments? Kudos? Got some parts you'd like to buy/sell/barter/swap? Nasty comments about my web page so far? You can email Mike or Debbie.

Pretty much everything on this website is copyrighted, if you want to use something, ask first.

Page last updated 01/02/2009 01:51:39 PM