STARTING MODE

When you first key on the ignition, the ECM loads the information off the PROM chip and performs a quick, self-diagnostic test. Some later ECMs also pulsed the injectors during this period to aid in what is known as “quick start” (obviously helping the engine startup more quickly). All of this happens in a very short amount of time, usually much less than 1 second after the key is turned on. Also during this period the fuel pump is activated for 2 seconds to “prime” the system. Next, the ECM waits until it receives reference pulses from the distributor (or crank sensor). Once it receives reference pulses (indicating the engine is turning over) it will use air/fuel ratio values stored in the cranking fuel pulse tables (in the PROM programming) which are based on the coolant temperature to determine how much pulse width (amount of injector ON time) it needs to command the fuel injectors open. The lower the coolant temperature, the richer the Air/Fuel ratio. The higher the coolant temperature, the leaner the Air/Fuel ratio. Ignition spark advance and IAC valve position output functions use similar tables.