SpeedTECH The definitve guide to EFI transplants


EFI for Dummies 

EFI (Electronic Fuel Injection) has become increasingly popular on new vehicles over the past two decades to the point where the vast majority of new cars are now so fitted. I don’t know about you, but I look at all new cars as a potential source for engines etc in 5-10 years time, so I figure getting to know EFI better is a good thing.

I know a lot of very smart people who design and build very impressive cars yet they appear very wary of EFI. The aim of this article is to explain how it all works in an attempt to remove some of the mystery that leads to people shying away from EFI as an option for their car.

Page Contents 

History Fuel System Injector Duty Cycle Load Signals RPM Signals
Crank Position Injection Duration Ignition System Control Correction Factors Engine Temperature
Exhaust Sensor (Lambda) Knock Sensor Boost Sensors Throttle Position Cold Start
Idle Up Speed Sensor Starter Signal Neutral Start High Altitude
Stop Lamp Oil Pressure Communications Diagnostics Conclusion


Fuel Injection systems for petrol engines have been around for almost as long as the Otto cycle engine itself. The reality was that the manufacturing processes did not exist until the 1930s to make it viable. Fuel injection was developed primarily for the aircraft industry as an escape from the problems of carburettor icing. In the 1950s it began to spread to up market cars, notably the Gull Wing Mercedes. It is no coincidence that Bosch became an early market leader, one of the perks of being the people to design the first practical systems.

The early mechanical systems were not too far removed from a diesel fuel injection system, relying on high pressures to direct inject the fuel. Electronic injection first appeared in the late 1960s and became increasingly established over the following 2 decades, to the point where EFI is now the predominant system.

The dominance of EFI is closely linked to the tightening of global emission control regulations as EFI is actually the easiest means of meeting increasingly stringent emissions criteria.

The good news is that virtually all EFI systems operate in the same manner - the vast majority are either Bosch systems or derivatives thereof. Accordingly this article deals specifically with BOSCH systems but it is directly relevant to damn nearly everything.

In simple terms an EFI system does two things - it controls the amount of fuel going into the engine and, if the system was designed after about 1982, it controls the ignition advance also.

Fuel System 

The fuel side of things is achieved by (you guessed it) an injector. The injector is really just a solenoid operated valve. It is supplied with fuel from an electric pump that is either mounted in the fuel tank or right next to it. The fuel pump discharges at approximately 33-40 PSI into a fuel rail that feeds the individual injectors. The fuel rail has a pressure regulator attached that normally incorporates a pulsation damper as well. Early systems had a separate pulsation damper near the fuel pump. The fuel pressure regulator has a diaphragm that is linked to the inlet manifold via a small bore vacuum tube. The idea is that the pressure regulator maintains the fuel rail pressure a set amount (normally approx 38 PSI) above manifold pressure. In this manner the spray pattern of the injector is constant whether you are on or off throttle, or somewhere in between. Excess fuel is fed back to the tank.

A cunning little bit of circuitry called a circuit open relay shuts the fuel pump down if the engine stalls. This prevents a ruptured fuel line becoming a flame thrower if you crash.

Of course there is a fuel filter as well because the clearances in the injectors are very small. As an aside the requirement for a swirl pot is a load of bollocks - manufacturers just incorporate baffling inside the fuel tank. If you are retro-fitting an EFI system to a car I strongly recommend using an in-tank pump - they are generally quieter and less exposed to external damage. Easiest way is to get a complete tank with fuel pump and cut the bottom and top out of the tank. The you will get the baffle etc as well - just weld them into your tank. Hey presto - factory set up. Modifying the pump pick up to cater for a different height tank is easy. Some pumps (especially on bigger engines) are two speed, controlled by the computer switching in or out a resistor.

The fuel injectors function by the EFI computer, properly termed ‘Electronic Control Unit’ (ECU), switching them on for a specific duration. As the injectors are constant pressure the longer the injector is turned on the greater the amount of fuel injected. Injectors are normally switched to earth by the ECU. Injectors come in two types - low resistance (1.5 - 3 Ohm) and high resistance (13.8 Ohm). Low resistance injectors have an external resistor pack.

An injector cannot be ‘on’ more than about 80% of the time otherwise the operating coils overheat. The ratio of on time to total time is termed ‘duty cycle’. Smart EFI systems will have one injector per cylinder, pointing directly at the intake valve. This is called port injection or multipoint. Dumber systems will only have one injector for the lot. Really smart systems will operate the injectors sequentially immediately prior to the valve opening. Simpler systems will trigger the injectors in groups with no real correlation to valve timing.

Because different engines require different amounts of fuel injectors come in various sizes. The injectors themselves fall into a couple of basic external design types so it is relatively easy to put larger flow injectors into your car if you require more performance. Problem lies in getting the ECU to drive them properly so the car runs right across the entire rev range and not just when flat out.

Derivation of Injector Duty Cycle 

So, now we have a fuel injector set up we need some way of calculating the injector duration. Luckily the ECU does this for us. Now, here is a bit of information you probably did not know - the computer does not actually calculate the injector duration - instead it looks it up in a table and then adds some correction factors. This method is simpler and faster for the way the ECU thinks. There are two main factors the ECU needs to know to find the base injector duration: engine load and RPM.

Load Signals 

Load is ‘simply’ a function of air flow. There are two main methods of determining air flow. Actually, air flow is not the correct term - mass flow is what we are actually trying to measure. The first system is pressure based. Pressure in German is ‘Drucht’ (I probably spelled that totally wrong, but I am working from memory), hence ‘D-type’ EFI. This system uses a MAP (Manifold Air Pressure) sensor. Simply put, the pressure variations in the intake manifold deflect an electronic thingo that sends a linearly related signal back to the ECU. OK - it deflects a silicon wafer. I am trying to keep this simple.

Because the throttle body area (ie throttle butterfly size) is known the volume of air entering the engine can be determined. The only complicating factor is air temperature which obviously effects density, hence a manifold air temperature sensor, termed THA, is added in for good measure. This is just like a water temperature sender - resistance varies with temperature.

MAP sensor systems are good because you can stuff an air filter directly onto the throttle body which helps greatly in fitting an engine into a cramped engine bay. They are also bad because they cannot provide as accurate a measurement of airflow as the next method.

A smarter way is direct air flow measuring (L- type, after ‘Luft’ for air). This method incorporates (you guessed it) an Air Flow Meter - AFM for short. The most common type (certainly on Toyotas) is the flap or vane type, where airflow moves a spring loaded flap. People that tell you these are no good because they increase inlet drag are talking a load of rubbish - the flow loss caused by an AFM is something like less than 1/10th of a percent. Flap position corresponds to resistance. Once again temperature correction is required and most AFMs incorporate a microswitch that shuts down the fuel pump when the flap is shut. Be warned that some have a positive resistance coefficient while others are negative, ie resistance decreases as airflow increases.

A second type of AFM is the ‘hot wire’ type. This is a cunning piece of kit that uses a very fine (something like 0.3 mm) wire that is maintained at precisely 100 degrees Celsius above the ambient air temperature. The ECU senses the current required to maintain that temperature and from there figures out the airflow. These beasties don’t require temperature correction and are probably the way ahead. Every time you turn the engine off the wire glows red hot for about a second to burn off any dirt that has settled on it. If you are paranoid about intake restrictions then this is the sensor for you. I believe the Lexus V8 uses these.

Finally there are the ‘optical Karman vortex type’. These are really smart. A pillar (called a vortex generator) is placed in the middle of the air flow. This vortices created behind it impinge on a mirror contained on a very thin metal foil leaf spring. The foil vibrates at a frequency relative to the air flow, and a light emitting diode bounces a beam off the mirror into a phototransistor that translates the frequency of the vibrations into a voltage. Yup - magic literally achieved with mirrors!

Most of these sensors actually compensate for engine wear due to the slight changes in air flow as piston ring blow by increases, hence EFI cars realistically do not require tuning. I drove an 18 year old EFI Celica for 12 months without doing anything other than a couple of oil changes and an initial spark plug change. Tune ups are generally speaking not required and so a complete waste of money. The 20 Valve Toyota 4AGE in my Leitch is still on the original 1991 factory platinum sparkplugs, and has only ever had oil, filters and cam belts replaced - and my engine would now have over 120,000 km on it...

RPM Signals 

RPM signals are relatively simply generated via a toothed wheel and an inductive pick up - termed a ‘Hall Effect’ sensor. Most place this pick up in the distributor. Either way, in simple terms the faster the engine rotates the higher the generated voltage is - up to about 100 volts AC. This signal is termed NE and generally the toothed wheel has 24 teeth although some have just 4.

The RPM signal can also be used to trigger such things as RPM limiting which is usually achieved by fuel injector cut (as opposed to ignition cut), RPM triggering (such as cam advance ie Toyota 20 valve 4AGE) or throttle butterfly opening (ie Toyota Variable Intake System - TVIS - early 4AGEs).

Crank Position Signal 

For cars with electronic ignition control the ECU needs some means of determining the location of the piston in relation to top dead centre in order to fire the coil at the correct time. This is achieved via a crank position sensor. There are two main ways of achieving this. One is to use another toothed wheel in the distributor, which typically has 4 teeth. As the teeth pass a pick up a voltage spike is introduced. The spike is read by the ECU as a datum that tells the ECU exactly where the crank is every 90 degrees of rotation. In this manner the ECU can accurately trigger the coil. Some systems use the starter motor ring gear, but have one tooth missing instead - this interruption in signal is registered by the ECU.

Distributor-less systems still require the crank angle indication, which can be achieved by either of the above methods. These systems are often termed ‘waste spark’ as they fire on each revolution of the crankshaft, hence cylinders are paired ie the number of ignition coils is half the total number of cylinders. In this manner a cylinder fires on both the power and exhaust stroke. This makes it easier for the ECU and the loss in energy is actually quite small due to various electrical goings on that I won’t go into, because it is all a bit too complex! (In other words I don't quite understand it myself)

Basic Fuel Injection Duration 

The ECU now goes to a look up table - which is just a grid full of numbers - where it looks up RPM on one axis and Load (air flow) on the other. This table then returns a value, which is the injector duration. Typically modern tables are in 50 RPM steps and the ECU interpolates between the nearest RPM value above and below to get the best fit injector duration.

Ignition System Control 

Smart systems control ignition timing as well, by the ECU referring to a different look up table to get basic ignition timing dependent on RPM and load. The table value dictates the discharge point of the current built up in the coil and hence the ignition advance.

Correction Factors 

Sounds too easy, doesn’t it?!

Well, now you know why ‘cheap’ aftermarket ‘fuel only’ ECUs require only a MAP sensor and RPM signal - and also why they go well flat out but are hopeless anywhere else - because they are unable to modify the basic injector duration. A number of correction factors are utilised - and some are more important than others.

Engine Temperature Correction 

Engine temperature correction (THE or THW) is the most critical correction factor. If the engine is cold both injector pulse width and ignition timing are modified to make the engine run smoother. Additionally exhaust emissions are hugely effected by a cold engine. Engine temperature is sensed by a temperature sensitive resistor placed in the engine cooling system on the engine side of the thermostat.

Some systems also have a water temperature switch (TSW) that activates when the engine begins to overheat. When this happens the ECU switches off things like air conditioning and backs off the timing to reduce heat build up.

Exhaust Sensor (Lambda) 

The percentage of unburned oxygen in the exhaust has a major effect on the chemical composition of exhaust gas emissions. A conflict arises whereby an engine wants to run an air:fuel mixture ratio of about 0.8 to 1.2 stoichiometric (from my fading memory) depending on whether it is accelerating or the throttle is shut, for optimum performance. Unfortunately unless the air:fuel ratio is maintained stoichiometric the exhaust gas emissions increase dramatically. A decrease in oxygen increases carbon monoxide and nitrous oxide emissions, while an increase causes an increase in hydrocarbon emissions. It is for this reason that the air:fuel ratio must be kept rigidly at stoichiometric.

The exhaust gas sensor basically reacts with the oxygen in the exhaust gases and generates a voltage that is sensed by the ECU. The voltage changes dramatically with any movement from stoichiometric and so the ECU alters both ignition timing and fuel to maintain the correct relationship. With the Lambda sensor in circuit the ECU will operate in a closed loop, however, if the sensor is defective or if the throttle is wide open the ECU operates open loop and enriches the mixture slightly to avoid lean out and possible engine damage. It is for this reason that certainly most if not all Toyotas function quite well without the exhaust sensor connected. Running leaded fuel coats the surfaces of the sensor and stops the voltage being generated. For this reason an unconnected sensor is better than a faulty one in many systems.

There are two basic types of lambda sensor (if we don’t consider the metallurgical composition or positive Vs negative coefficients) - heated or not. Because the sensors don’t function until they are above approx 400 degrees Celsius an electric heater is fitted to newer designs to improve response time with a cold engine and hence reduce start up emissions.

Cars designed for leaded fuel markets do not have a Lambda sensor - instead they have a variable resistor that is used to adjust emissions at idle.

Knock Sensor 

As manufacturers strive to attain tighter and tighter emissions criteria the requirement to operate closer to stoichiometric air:fuel ratios means that engines run very close to the knock (pre-ignition) limit. A knock sensor (KNK) is fitted to many modern engines to detect the onset of knock. Generally one sensor will be fitted to four cylinder engines with two on six and eight cylinder engines. When these sensors are triggered the ECU will retard ignition until the knocking stops. With modern systems knock control is individual cylinder related ie the ECU will modify the ignition advance as required for an individual cylinder rather than apply the same correction to all. Obviously knock sensors are critical for turbo or supercharged cars.

Boost Sensors 

Boost sensors are fitted to turbo or supercharged engines and are really just a MAP sensor with a wider range, to advise the ECU of the amount of boost and hence the required degree of correction to the fuel and ignition signals. Early boosted cars also have a separate fuel cut switch which activates above a pre-set limit to prevent the engine grenading if boost goes too high. Later ones just monitor the boost being produced via the boost sensor and cut fuel injection if the limit is exceeded.

Throttle Position Sensor 

The throttle position sensor (TPS) generally incorporates a number of microswitches that switch a voltage supplied from the ECU to earth. Contacts are made at idle and at wide open throttle, while some also have a variable resistor track. In simple terms the microswitches inform the ECU when the throttle is closed which may cause the ECU to stop injection if the car is coasting to minimise both emissions and fuel consumption, and when the throttle is wide open so that the ECU defaults into open loop operation. The variable resistor track tells the ECU how far open the throttle is.

Cold Start 

Cold starting for early EFI systems relied on a ‘cold start injector’, which is basically an additional injector that is fitted into the intake manifold plenum. It is controlled by a thermostatic time switch which is also located in the water jacket near the thermostat housing. If the water jacket is cold the thermo time switch will cause the cold start injector to operate for approximately 3-5 seconds to provide start up enrichment. The injector will then shut off. Some people trigger the cold start injector to provide additional fuel (for example if they are running additional boost on a turbocharged car or nitrous oxide), however, this is a very crude means of achieving the aim.

Newer injection systems dispense with this system and instead pulse the main injectors twice as often whilst the engine cranks.

Next step is increased idle speed once start is achieved. This is often accomplished by an ECU triggered solenoid valve (itself triggered by the ECU engine temperature sensor) that opens up a throttle butterfly bypass passage thus allowing additional air into the engine. Some newer engines have a 'rotary slide valve' (RSV) that is pulsed to open or close varying degrees to regulate the bypass air. Because the ECU senses the additional airflow it increases fuel (and hence idle speed) accordingly. As an alternative some cars actually have a servo motor to drive the throttle open slightly.

Idle speed on early cars was adjustable, however, later cars are done electronically hence there is no idle speed adjustment. Also some cars (especially those with flap type AFMs) incorporate a bypass channel controlled by a screw that will adjust mixture at idle. Again, later systems have dispensed with this adjustment and do it via the ECU in closed loop operation.

An alternative system is a wax element controlled throttle body bypass system that allows more air to bypass the throttle butterfly when the engine is cold, therefore increasing engine idle speed as described above. This system is easily identified by the supply and return lines from the cooling system to the throttle body. Toyota 4AGEs utilise this system. Earlier systems utilise an electrically heated, bimetallic strip operated air valve. This system is used on older engines like 2TGEUs, 3TGTEUs, 18RGEUs etc.

Idle Up 

Various devices such as air conditioning, power steering and automatic transmissions place a high load on an engine at idle, hence most EFI cars have a signal input that lets the ECU know when these items are turned on or when the transmission is put into drive at idle. The ECU increases the fuel and operates another air bypass (via a Vacuum Switching Valve - VSV) to maintain idle speed (Idle Speed Control - ISC) without risk of stalling.

These devices trigger either electrical load signals (ELS) or Air Conditioner (A/C) signals into the ECU. The AC signal often delays the triggering of the AC compressor electromagnetic clutch for half a second until the engine has had the chance to increase idle so as not to stall when the load comes on.

Vehicle Speed Sensor 

Many vehicles (especially Japanese) have a vehicle speed sensor (SPD) fitted. This is an inductive pick up that is part of the speedometer or transmission. It sends a pulse to the ECU and this is often used to limit the top speed of the vehicle, again by injector cut.

Starter Signal 

When the starter motor is engaged a signal (STA) is sent to the ECU (in reality the feed to the starter solenoid from the ignition switch is branched off to the ECU). This signal lets the ECU know the engine is cranking and hence ignition and fuel timing are modified to assist in starting.

Neutral Start Switch 

Cars with an automatic transmission have a contactor that feeds into the ECU to prevent the engine starting unless the transmission is in park or neutral. This is generally a voltage feed from the transmission. Numerous people using automatic EFI engines with manual transmissions cannot get the engine to start due to lack of this voltage. Simple fix - whenever the ignition is switched to start apply a voltage feed to this connector.

High Altitude Compensation 

High Altitude Compensation (HAC) is also achieved by a sensor similar to the MAP sensor. This is used to modify the injection duration and timing for high altitude areas (strangely enough) where the air thins out. These sensors are predominantly fitted to earlier injection systems that run air flow meters.

Stop Lamp Switch 

The stop lamp switch (STP or BRK) informs the ECU when the car is braking and in this manner the ECU can cut back the fuel injection further to reduce emissions and save fuel. Corresponding this signal with the vehicle speed sensor enables the ECU to differentiate between the vehicle braking and being stopped with the brakes on.

Oil Pressure Switch 

Some engines feed an oil pressure signal to the ECU which is used as part of the logic for controlling the idle speed control (ISC) system. I have yet to find a car with this system fitted.

Communications Signals 

There are a number of communications signals between the various ECUs present in more complex cars. Features such as cruise control, traction control, ABS braking, electronically controlled transmissions, water/air intercooler systems and speed sensitive power steering often have signals shared with other ECUs. The majority of these systems will have negligible effect on a transplant where the features are not fitted. Occasionally you may need to supply a signal (as in the case of the neutral start switch) otherwise no signal will be interpreted by the ECU as the system being turned of.

Apparently the Mazda 20B triple rotor intercooled turbo beastie is a major pain in the butt in this regard as it has twin sequential turbos and also only comes with an auto trans. By all accounts it is nigh on impossible to get the engine to run on the standard set up unless you use all the original components due to boost limiting depending on gear selection and all sorts of stuff. Being stoopid I would like to have a crack at ‘fooling’ one to run the stock engine, ECU and turbos, but with a manual transmission. Anyone want to lend me one to play with?

Diagnostic Terminals 

The vast majority of modern EFI systems contain onboard diagnostics which are normally triggered by earthing a ‘check connector’. This results in the ‘check engine’ warning light flashing a number of times in a set sequence. The number and combination of pulses equates to a trouble shooting code. Large diagnostic plugs often contain leads to enable the fuel pump to be run in the absence of the EFI system as well as providing a direct tap into various sensor outputs to avoid having to unearth the ECU etc to take a direct reading.

The reality is that most modern systems have all the diagnostics required onboard and anything else can actually be diagnosed with a bit of patience and a multi-meter. The rationale that you have to plug the car into a mainframe computer is one of the biggest misconceptions in the industry. Also, don’t over-estimate the amount of ‘tuning/maintenance’ an EFI system requires (unless you are unfortunate enough to get one of the many dodgy AC Delco systems).

Faults normally fall into two main categories - terminal and non terminal. Minor faults may not trigger the warning light, so you may not even know you have a problem, however, the ECU will log the error code in anyway - otherwise you will never be able to find an intermittent fault. Slightly more major faults will trigger the warning light, but most likely only when the fault condition is occurring. The car will most likely still perform OK but the fault will have been logged.

Major faults will bring the light up and either kick the ECU into limp home mode, where drive-ability will be impaired, or else the car will just plain stop.

I have had major fault occur in a car without a warning light connected (my Leitch) and I can assure you, you can tell the car is playing up even without the benefit of a warning light! The car still ran but was a pig until it warmed up - fault being a defective water temperature sensor and air temperature sensor. Car had clicked into limp home mode.


This brief article (believe it or not, it is!) just skims the surface of modern EFI systems, but should still provide a reasonable explanation of what is going on in a modern car. I firmly believe EFI is not really the rocket science that a lot of people think it is. I also hope it now becomes apparent why many of the simpler aftermarket EFI computers are really quite crude and lack the crisp response of factory systems, particularly when all the correction factors are taken into account.

I have a friend with a fairly modified 4AGE in a Corolla that runs EFI and an aftermarket ECU with no ignition advance capability. When it gets on song it really goes well, but it lacks crispness at lower revs and idles at 1800 RPM. My 20 valve, which puts out similar power, could not be a more different engine to drive - benefits of a smart factory system, to say nothing of the better economy it delivers.

Hopefully this article also makes it apparent why modifying injected engines is a complex business, especially once you start to get increase beyond the scope of the ECU - which by the very nature of the way it operates does not have a lot of scope to alter timing/injector duration beyond what it is programmed with. As always I return to my basic argument - the cheapest way to reliable horsepower is to install the entire factory set up from a vehicle that has the performance you require as a standard feature.

Finally, a basic EFI system is just that - basic. The number of corrections and additional systems vary from car to car - an early RWD 4AGE is about as basic as you can get  and damned effective too. If you seek more information check out your technical bookshop - they carry some very good manuals, but I strongly urge you to thoroughly examine them first, before you buy, to ensure you are getting information relevant to you.

OK - now you know how the engine works - on to the next step: 

 How to Wire a Complete Car 


Copyright © 2000 SpeedTECH Last modified: January 23, 2000