Automotive diagnostic strategies:
Barometric pressure, manifold pressure, intake air temperature, volumetric efficiency and exhaust gas recirculation these items are all involved along with modifiers like battery voltage, coolant temperature and throttle position. Of course you have to look at trim, short term fuel trim and the adaptive strategy being long term fuel trim. The most important input to the PCM needs for most any calculation is engine speed. This input comes from a sensor that is located in the distributor. The PCM must see what the engineer designed it to see of the vehicle won't run.
-Barometric pressure, when the key is first turned to the on position, the PCM reads the BARO sensor to determine the altitude the vehicle will be operating at. Higher altitudes have less dense air and less molecules of oxygen in a given volume of air.
-Manifold absolute pressure or (MAP) is the measurement of potential air. This measurement tells the PCM how much air is ready to enter the combustion chamber when the intake valve opens.
-Intake air temperature or (IAT) hot air is less dense than cold air. Volumetric efficiency, this value is stored in the PROM. It is a value measured by the engineer when the vehicle was developed that represents the amount of air that has the ability to enter the cylinder on the intake stroke at a given rpm.-
-Exhaust gas recirculation or (EGR) the PCM needs to know how much exhaust gas is being recirculated into the cylinders to effectively know the real weight of the air in the chamber. This action performed by the EGR system actually cools the cylinders to reduce NOX emissions and engine ping. Modifiers tweak the calculated values to compensate for operating conditions that always change. I recommend a good emissions shop for any repair EGR related.
-Battery voltage, the PCM looks at battery voltage as an input to modify parameters, which is important to all systems and will maintain fuel efficiency and catalyst. Therefore a good battery in is crucial to all operating systems!
-Coolant temperature, when cold the engine coolant temperature or (ECT) will cause the PCM to add time to the base pulse width or (BPW) to compensate for lack & fuel vaporization. The PCM will never subtract time for BPW, it will only add. The ECT is the main control sensor when the engine is cold or in closed loop. When the engine warms up, it remains in closed loop and the o2 or oxygen sensor takes charge.
-Throttle position sensor or (TPS) affect the BPW in two ways. The PCM looks at throttle position to determine which strategy to use (idle, cruise or wide open throttle). The PCM looks for rate of change from the TPS to effect BPW and cause either and accelerator pump effect or a fuel cut effect.
