Pontiac Repair: Intermittent high idling, throttle position sensor, pontiac bonneville

Question
Hi Todd,
I own a '92 Pontiac Bonneville SE 3.8 NA. The car is serviced regularly. There is an intermittent problem of high idling. The car usually starts off ok and then will start to idle high after 10 minutes of driving. At other times it drives normally with no irregular idling. No codes were set. I replaced the IAC valve with a new one (original gm product) but the car still idles high. I checked the voltage reading at terminals B and C of the IAC valve connector with the engine On and then again with the key on engine off. I noticed that sometimes the reading would remain at a fixed 12-13 V and at other times would vary anything from 0.5-12V. What sort of reading should I expect? Will 12 V at both terminals with the engine on indicate a grounded circuit?
And if so how would I go about checking for the bad circuit. And if I'm not on the right track please advise me.
Thanks for your help
Tarek

Answer
Hi Tarek:

well first off an IAC motor does control the idle. but it is a bi directional motor it has 2 sets of coils in it. one to extend the pintel and one to retract it. the voltage difference is the computer changing the polarity of the coils to extend or retract that pintel. using a volt meter is just going to verify that the wiring is good and the power/ground path is okay from the computer to the valve. there is actaully a tool to check this circitry it has LED's in it. I don't think your issue is withe the wiring or the valve. I think you have an issue with a bad sensor somewhere or a vacuum leak. Without a scan tool this is going to be hard to find the problem. if you had a IAC issue it would set a code 35 for IAC issue. The most likely culprit for a sensor would be a Throttle position sensor or someone messed withe the throttle screw. A vacuum leak will cause a high idle as well. Again with out a scan tool you are shooting from the hip. if you have one I would #1 look at the IAC counts at idle they should be around 15-30 engine hot with no A/C on. The Throttle position sensor should be low around .35-.40 Volts at idle and 0% throttle angle. If the voltage is high or there is throttle angle % that's the issue. if the IAC count are low below 15 there is a vacuum leak or someone messed with the idle screw and if the screw was messed with the voltage would be higher than the volatge I gave you. these are jjust baseline #'s every engine is different. hope that helps. Good luck :}