what does TPS/IAC exactly do?
 

what do they do? sense how much gas is being given, and give gas according to that or something? or something else?

i think MAF tells how much air is comnig, so it gives it that much fuel, so what does the TPS/IAC and the front o2s do?

TPS is the throttle position sensor. It tells how much throttle is being used. IAC is Idle Air Control, it controls how much air to let in during idle. The front O2 sensors are used during closed loop to let the PCM know how well the air/fuel ratio is and adjust it if necessary.

TPS also controls the transmission shift points on an A4 (4L60E)equipped car.

Frank

so if it goes bad, the TPS might think the throtlle isnt open all the way? and what do you mean closed loop?

When my TPS went bad, the car revved really high befor it would shift to 2nd. When it finally went into 2nd, it lurched into gear. Litterally put me back into the seat. Scared the crap out of me when it first happened.:eek:

Think of the TPS readings as the "demand" for horsepower sent to the PCM (computer) by the driver. At larger throttle openings, the PCM is going to think in the "maximum power/maximum acceleraton" mode and dump in more fuel and maybe timing. As far as the automatic trans goes, high throttle openings result in higher shift speeds, and firmer shifts. On non-electronic trans (700R4 and 4L60) a TV or throttle valve cable was used to tell the trans throttle position. On 350 and 400 trans. engine manifold vacuum, which is a good indicator of throttle postion and power demand, was used.

If the TPS goes toes up, the computer is confused (becomes a "confuser"?), and can only guess at the demand for power.

"Closed loop" generally means that O2 sensors feed combustion information back to the computer which adjust spark and fuel to get best emission and maybe fuel economy. It's an in-process, constantly changing situation achieved during normal driving after the engine warms up.

"Open loop" means that preprogrammed fuel and timing maps are used by the computer. This is typical at high throttle openings (WOT), or maximum "demand" as well as startup and warm up.

My $.02

Here is some good info from Injuneer that might help you out.

http://members.aol.com/injuneerzz/scanmast.htm