An introduction to code reading and vehicle systems analysis

Electronic fuel injection and ignition systems on modern vehicles have a computer (called an ECM) to provide the proper control of the fuel-air mixture. The ECM works by measuring many different characteristics of the car, then, using this information, it calculates the correct quantity of fuel, the time of the fuel injection and the time for the ignition spark.

Because it works so quickly the calculation is done for each cylinder and firing stroke separately no matter what speed the engine is doing!

The main purpose for this degree of control is to achieve the best engine power output with the lowest possible exhaust gas emissions of the two poisonous gasses, carbon monoxide and unburned fuel (hydrocarbons).

Electronic fuel injection and ignition are much more reliable than the equivalent mechanical systems of carburettor and contact set ignition. These older methods required regular and frequent maintenance to keep the car "in tune". However, life is not so simple! The increased reliability is offset by a dramatic increase in complexity and in the number of components needed to keep the ECM in touch with the car ... these components are the SENSORS used by the ECM to measure the operating conditions on the car. For example, the ECM needs to measure the accelerator pedal position to "know" how fast the driver wants to go. The engine speed and engine angle are used to work out when and where the sparks are needed.

The ECM is a computer, like any other you could buy from computer store, but instead of a keyboard it has SENSORS as inputs and instead of a TV screen, it has ACTUATORS. These are the output of the computer; for example the ECM controls the fuel injectors by opening and closing them for a few thousandths of a second; during this time fuel is sprayed into the inlet manifold (near the inlet valves) in the exact proportions required for efficient combustion. The ECM also controls the engine's idle speed with the excitingly named "idle speed control valve"!