Sensor question

[RagtopRay]

What exactly does the IAT (or air charge) and the M.A.P. sensors do? This is on a 4.3

Thanks

[SteveH95Formula]

IAT is the intake air temp. sensor and is mounted by your air filter. Naturaly it is a sensor that tells the computer what the intake air temp. is. MAP stands for manifold absolute pressure sensor. I'm sure Fred can give a better explanation than I can.

[Injuneer]

If the PCM controls the engine using "speed-density", it needs to know the inlet air temperature (IAT) and the manifold absolute pressure (MAP) so it can calculate the density of the air entering the cylinders. If you took chemistry or physics, you may remember the "perfect gas law", that tells you that the density of a gas varies in direct proportion to its absolute pressure, and inversely to its absolute temperature.

I don't know if the 4.3 is a speed-density or a mass air flow (MAF) setup. If its got an MAF sensor, it doesn't need to calculate the air density when its running off the MAF. But it needs that information as a backup, so the engine can continue to run in speed-density mode, if the MAF sensor fails.

Its also possible that the PCM may adjust the ignition timing based on inlet air temp. And MAP is used as one of the axes on the ignition timing lookup tables. In a speed-density setup, MAP is also one of the axes of the volumetric effciency (VE) tables. MAP is used as a measure of engine "load"..... wider open the throttle is, the higher the intake manifold pressure.

[RagtopRay]

Thanks for the replies. It's in a '95 Astro that failed state inspection for high hydrocarbons. I wasn't sure if these could cause the engine to run rich. The van only has 193K so it should run perfect! The county in Texas I live in just recently started emmision testing. I had the injectors cleaned and did notice an improvement, but I want to check all my options before I try again.