1999 GT will not idle

[hagglerock]

I have a 1999 Grand Prix GT 3800 with 150K miles. The car started and ran fine this morning, moved it on the drive way, then went to restart a few minutes later and no luck. I can get it to start if I give it some gas and can keep it running all the way down to about 900 rpm, but when I let off the gas completely it stalls immediatly.

Check engine light is not on. I have a code reader but have not hooked it up yet.

Any ideas would be greatly appreciated.

[richtazz]

with the key off, unplug the MAF sensor then attempt to start. If it stays running, the MAF is bad.

[tblake]

Also almost sounds like an IAC issue....

[hagglerock]

Unplugged the MAF, no change in symptoms. Hooked up my code reader, no issues found.
IAC Idle Air Control? could you elaborate a little?

Other thought is Throttle Position Sensor. Is there any way to ohm this out to check if it is good?

Thanks

[BNaylor]

The IAC controls the idle via the PCM module. Low or high IAC counts will cause a flaky idle either low or high. Remove it and clean the spring loaded pintle with throttle body spray cleaner and lube with silicone lube. Also, clean out the seat where it mounts to the throttle body. Carbon builds up pretty bad over the years or if it has never been serviced. Make sure you clean the bowl at the end of the pintle real good with a rag. See pic below.

A bad TPS will normally throw a DTC or cause a noticeable shifting problem but you can check the input and output voltages with a DMM. But try the IAC as suggested first.

[hagglerock]

Ding, Ding, Ding... We have a winner!

Took out the IAC, it was original, to say there was a little carbon build up is a understatement. I did not have any FI cleaner so I went to the local parts store. The new IAC was only $30 so I went ahead and got one. Installed the new one and it runs great!

Thanks for the help...again!

[Chinook507]

Had the same problem and just a few shots of cleaner, then cleaned the larger deposits with a brass brush. Put it back in and started up and idled just fine. Thanks for the help!