EFI explained

EFI or electronic fuel injection has been with us for quite a while now and has proven to be far superior to the old carburettor. The easiest way to describe how it works is that inputs recieved from various sensors around the engine (and sometimes transmission) send a signal to the ECU or electronic control unit, the control unit processes these signals and sends an output to the fuel injectors, air bypass valve and on distributor-less engines, the coils. Lets take a brief look at the input, output and control unit functions of a common multi-point type injection system used on most main-stream cars today. To simplify this post ( and not bore the hell out of everyone ) we will leave the ‘hardware’ side of things to another day.

Most input or information sensors are resistors that modify the voltage applied to them by the computer, this voltage is called the Reference Voltage.


The computer sends this reference voltage to the sensor and as the resistance in the sensor changes so does the return voltage to the computer. The ECU or computer then uses these changes in voltage to make the necessary adjustments to keep the engine running at it’s best. Typical input sensors found on todays vehicle will include;

  • Airflow meter – placed in the air intake tract and used to measure the amount of airflow and adjust the fuel delivery appropriately.
  • Manifold Pressure Sensor – often referred to as MAP sensors and used in place of air flow meters to determine the load on the engine and the fuel required to keep it running.
  • Air and Coolant temp. sensors – used to provide a rich air/fuel mixture for starting and warm-up and to measure air density which affects air/fuel ratio.
  • Oxygen sensor – regarded as the most important information sensor. Located in the exhaust pipe or manifold. Used to measure the amount of oxygen in the exhaust gasses, an indication of whether the engine is running lean or rich.
  • Throttle Position Sensor – tells the computer whether the engine is at idle or wide open throttle or any position in between. Once again used to calculate the amount of fuel required.

The ECU or computer is the brain that processes these voltage signals and adjusts the outputs in accordance with it’s program or ‘map’.

Typical outputs are;

  • Fuel Injectors – the principal component in the output system. The computer reads from the input sensors and calculates when the injector should open and for how long to maintain the ideal air/fuel ratio.
  • Air bypass valve – used to correct idle speed and air/fuel mixture at idle. Allows air to bypass the closed throttle body butterfly.
  • Coils – on a distributor-less ignition the computer is used to ‘fire’ coils that are located either on top of the spark plug (via an insulator) or close by with a small plug lead. The instant the coils are fired can be dependant on engine speed and load.

That sums up our brief look into the EFI system. Obviously this is only a basic overview of things, but my idea was to give you an understanding of how and why it works. And besides, I could write a whole book once we get into the nitty-gritty of it all!

4 thoughts on “EFI explained

  1. Hi Craig. That is a great introduction to EFI. Some people may ask why would you use a MAP sensor in favor of a Mass Airflow Meter. Although a Mass Airflow Meter does measure the actual amount of air going into the engine, and hence gives an accurate measurement for the ECU to determine how much fuel needs to be delivered, they can be restrictive. This can lead to a reduction of engine performance at the top engine of the engines operating RPM range.

    MAP Sensors on the other hand measure the amount of vacuum or pressure in the intake manifold and the ECU uses that to determine the amount of fuel to deliver. The great thing about MAP sensors is that they do not cause any restriction in the intake system.

    Regards, Steve

Leave a Comment