QuestionQUESTION: have a 90 chevy pickup with a 350 that randomly dies when stopped at lights after it warms up. doesn't do it all the time, just on occasion. does this sound like a module problem or something else?
ANSWER: HEy,
It could be a problem with the IAC valve. Introduce a small vacuum leak at the throttle body and it will increase idle a little bit and see if that rectifies your stalling problem. Clean throttle plates and remove and replace and clean IAC valve.
w
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QUESTION: Where would I find the IAC valve? If it's what I'm thinking of it's actually inside the throttle body? Do I have to disassemble the throttle body to get to it?
ANSWER: Hey,
Its in the throttle body. It can be removed without any disassembly. it has a black 4 pin plug on it.
Idle air control valve is a four wire stepper motor. PCM makes precise continuous changes in IAC position to maintain desired idle. PCM extends the IAC pintle to reduce idle and retracts the IAC pintle to raise idle.
Plug: (harness side backprobing)
-[--]-
A B
D C
A=IAC coil winding a +
B=IAC coil winding a -
C=IAC coil winding B +
D=IAC coil winding B -
windings a and b are actuated by the pcm depending on desired pintle movement and position. There is no feedback from the IAC to the PCM therefore the PCM uses a 'stepper motor count' to calculate known and desired position. For example. The PCM extends valve and retracts quickly for startup vacuum bypass. Should the PCM command retraction an additional 3 steps it would now recognize IAC position as 10 + 3 = 13 and base its next position on 13 counts.
It sounds very complicated but what should be known is there is a reset procedure that may need to be done if PCM looses positioning. Chock wheels, apply emergency brake, start and run vehicle in drive until ac cycles a few times and engine is at operating temperature (usually 5 minutes) This procedure needed to be done frequently on older GM vehicles especially the 2.8 and 3.1 early V6 MPFI engines. It is rare to have to do it to a 4.3 or 5.0 but once the valve is out and cleaned it is a recommended service procedure.
Wayne
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QUESTION: Ok, so we have replaced the IAC valve, did a complete tune up (plugs, cap, rotor, wires), put a new pcv valve in (old one was completely plugged) and a mechanic removed the egr and throttle body and cleaned them both. The mechanic put it on an analyzer and the only thing he could find was possibly a flakey plug and wire. With those replaced and everything else done, it still stalls when you stop at a light or sign. It's as if it won't hold a low idle at all. It runs great at speed though, you'd never notice a problem when not stopped. Any other thoughts? Fuel filter/pump? Any other sensors that might cause this issue that we should check?
Thanks,
Jay
AnswerThe IAC valve should STILL be trying to keep it running and if that circuit is not working properly it will stall. The computer has a 'target idle' that it wants to see all of the time and will increase idle with the IAC valve by retracting the valve when it dips below a targeted threshold. The valve circuit needs to be tested more thoroughly. Have you tried to introduce a vacuum leak like I originally mentioned? If the computer can't retract the valve then it still isn't working.
