High idle

When I start my car in the morning, it idles extremely high.I can idle down the road at 40+ mph.If I shut the car off and restart after several minutes of driving,the idle settles down.l took the car to my mechanic and he said he tried a different throttle body and a few other things and settled on the need for a new PCM(power control module?) and a TPS(throttle position sensor?).Anyway $600 later,I started the car this morning and no change in the car. Im still idleing down the road at 40mph.Before I take the car back, anybody have any thoughts on the problem? Thanks.

brutus-got the same problem with a 1999 buick century. Mine does it only sporatically though.You helped me in deciding not to replace the pcm and throttle position sensor, though. Still looking for an answer. Let you know if i find it. Thanks

engines typically run on a higher idle speed when cold than when warmed up. Many are set up so that the gas peddle will release the high idle if you depress the peddle and release it, but not until after the engine temp allows it. It used to be a mechanical linkage and a dash pot that mediated this drop in idle speed, but nowadays it is the computer that controls it. But the computer must depend on the data it receives from various sensors, and so I would guess it is a faulty sensor that is the culprit, but which one I dont know. You might be able to get a code that will tell you. wait here and see if one of the top guys can narrow it down for you. In the meantime, please post the make, year, milage, and whether or not it is carburated or fuel injected, it will help them to home it on it.

Could you tell us the year and mileage.

I think you need a new shop. This one is just throwing parts at the problem. While the throttle body comment suggests to me that this is pre-OBDII, components could still be checked. Diagnosis should preceed parts changing.

Your idle is controlled by the PCM taking in signals from the temp sensor, the mass airflow sensor, the manifold absolute pressure sensor, the crank (or cam) speed sensor, and the crank (or cam) position sensor, and the throttle position sensor, putting them into a program, and driving the idle air controller motor to control the idle. When the engine is cold the oxygen sensor loop is bypassed (signal not used) to allow the engine to run rich until it warms up. You may ultiamtely need a PCM, but first the shop needs to check the various sensor signals and the IAC circuit.

My wild guess is that the temp sensor is not telling the PCM that the engine is cold, the oxygen sensor loop is thus not bypassed and the engine is running too lean, the oxygen sensor is detecting too much oxygen in the exhaust stream due to the lean operation, and it’s telling the PCM to add more fuel. Then, when the engine actually warms up, it starts adjusting based on the “warm” parameters.

Mine is a wild guess. Others might call me crazy. I can live with that. But I’d start by checkiing the temp sensor output.

I fully agree with the above comments. The shop has no clue.

I echo the comments about the temperature sensor and (particularly) the IAC (idle air control) motor. The latter component can malfunction enough to give you a high idle but not enough to set off the check engine light.