Thread: Need Help with Surging Idle Problem on GTP

I have been struggling with a surging idle on my 98 GTP. I have replaced the MAF, IAC, PCV valve, Caniser purge valve, TPS and cleaned the throttle body. I have had occasional codes set for idle higher than expected and Evap system flow during non purge. The only code that still comes back is the idle higher than expected but the idle surges, mainly on cold start until it warms up. If I wing the throttle a little (up to 2K rpm or so, when it comes back down it drops way below the normal 750 rpm and oscillates between near stall and 1300 rpm. Once warmed up a little it smooths out but idles at about 850 rpm in gear and the idle higher than expected code comes back eventually.

Is there actually some kind of idle learn process that I have read about or is that urban myth?

Any suggestions?

FIXED!!!! New MAF Sensor Was Bad!! See last Post....

Checked for vacuum leaks???

idle air control valve?

x2 on the vac leaks. check around the blower casing and check all the plastic vac lines. are you still getting problems with your evap purging? it definitely shouldn't be purging on start up, and that would be an "internal" vac leak. do you have access to a scan tool that would tell you purge flow rate or fuel trims?

and yes, there is an idle relearn process. i recently replaced a throttle body on a newer tahoe and before it relearned position it would idle at almost 2000rpms. no fluctuation though, but its worth a shot. you can command it with a tech2 or other scan tool, or you might be able to let it idle for a few minutes and the pcm will relearn on its own

still sounds like it's getting too much air... or not reading sensors right. check your connections around all of your air and fuel sensors - ECT, MAF, MAP, TPS, IAC, etc. what's the P0--- code? you might be able to look up enable criteria or conditions to set the DTC and that could be useful in further diagnosis

I did some searching and this is a common problem but it seems no one has a straight answer for a solution.. Everything I did read related to a vacuum leak or a IAC.. Another one mentioned O2 sensors not working correctly until they are warmed up. I do know O2 sensors give better accurate readings once up to working temp. Perhaps yous is working enough to not throw a code, but could need a cleaning. I know the O2 behind the cat, if its not warming up is usually a sign of a bad cat, but in this case, I wouldn't say its your 2nd sensor if any. Just throwing it out there.

there's one pre-cat and one post cat. pre-cat is used to adjust fuel trims once the car is in open loop, post cat is to check for catalyst efficiency

You mention the throttle plate was changed. Just the plate or the whole throttle body? Have you taken the Intake tube or airfilter off the throttle body to view the inlet? For the throttle adjustment screw, I just adjusted it a hair after the throttle plate is loose, but I'm sure there's a more correct procedure to make it work correctly with the TPS.

You have to be sure that the pintle in the new IAC isn't out too far before installing it (28mm from the face of the mounting flange to the tip of the pintle cone). Then theres a short procedure to reset the pintle once the IAC in in place turn ign on, then off for 20 seconds, then start the engine). Make sure the passages for the IAC are clean.

Sounds more like an unmetered air problem to me and the throttle plate needs to be put back at stock.

With the throttle plate back at stock and the engine idling... if you unplug the IAC, does the engine stall?

Originally Posted by markeheiden

Regal GS98- I meant that the stop screw for the throttle plate has been adjusted only. After cleaning, the plate closed too far and was sticking a little so I adjusted it off bottom a bit. (Maybe too much as it turns out?)

Rocknachos- I checked the pintle on the new IAC after I installed it and fought with it a couple more days. It was past the 28mm distance indicating it was trying to close off the air too much. This led me to believe my throttle plate adjustment must be letting too much air past the plate and the IAC is not able to compensate. Never the less I returned it and got another new one and the pintle was way in, maybe 1/2" shorter and way under the 28mm max. I did the procedure you mentioned each time I had the IAC out but no help. I would really like someone to tell me how to "Put the throttle plate back at stock". This information seems to not exist anyplace in my manuals or on the internets! I am not sure yet If I unplug the IAC wether the engine stalls or not right now. I set the throttle plate stop lower today and during cold start, I just manually keep the rpm steady for a couple minutes until it can run on its own and so far so good.

I did find that if I disconnect the MAF it does not surge at all. It runs fine, and sets a code of course.

If you have to hold the throttle a little now to keep the engine idling, sounds like it's an issue between your throttle body and the IAC then. Check the wiring to the IAC and try unplugging the IAC to see if the engine stalls like Rocknachos said. Always check what you've recently worked on. Hopefully someone will chime in on setting the throttle plate correctly. I've always been told to leave it alone, but when I adjusted it, I just set it a hair after it gets stuck like you did.

Originally Posted by rocknnachos

I would adjust the throttle plate stop so that my TPS was .75-.8 volts, or so. Also, sweep the TPS up n down to make sure your voltage flows up and down with it, smoothly. From under 1 volt at stop to 4 v at WOT. After sitting over night, check that the IAT & ECT are at the same temperature.

Had to do a similar procedure with my Honda when changing the TPS. I read the ohms for that sensor when replacing it. Didn't know the voltages for this engine though.

I've only come across a high idle that set the code once. FSM's pointed us toward the IAC wiring etc and testing the connections from IAC valve to PCM. That resulted in nothing but teaching the guy how to check wiring. Then I took the cruise control and throttle cables off the TB. Fired up and it idled perfectly. This led me to look closer at the cables and how they are coiled on the strut tower. Wiggled them a little to ensure they were sitting right. Hooked them back up and magic, car has idled perfectly since.

I'm not conviced that is your issue though since yours tries to stall and catches itself etc. That does sound like the IAC is fighting to keep the engine running.