fast idle problem

First of all, I have searched... endlessly, there is a ton of information here but nothing found with my exact problem.
I have a 98 GTP 156K miles on the clock, 49K on engine.
The Problem:
Day one, start the car and it idles a little faster than normal, I blip the gas and it goes to normal. The rest of the day it's fine.
Day two, get the same faster than normal idle and it wont go down to normal until after driving for a while.
Day three, get the same faster than normal idle but it never goes to normal, and eventually I get a DTC P0507, (Idle Control System RPM Higher Than Expected)
the car is pulling itself without the gas being pressed! (RPM's 2000+)

I replaced the TPS and the IAC and PCM, nothing changed. To control the idle, I self adjusted the IAC to get the idle at 900 rpms in park and unplugged it.
Could it be the coolant temp sender?

Anything is possible but I would check for a vacuum leak. The fact that it goes away when the engine heated up indicates that heat was sealing the leak. It can be hard to find but could also be the intake manifold gasket that is allowing increased air flow into the engine.

Makes me wonder when you have to "blip" the gas if you have either a sticky throttle plate, or one of your throttle/cruise control cables are binding.

mark

Just had a similar symptom on my 3.4L 92. I also replaced the IAC a $30.00 waste of money! Found a bad intake gasket. Replace it and all is well. Symptoms, erratic idle. Sometimes it would be high then drop to normal. Also, ran hotter then normal. I am told that sometimes there is also a coolant leak associated with this...

It's a brand new stage 2 ported tb from zzp...
checked everywhere with carb cleaner around all gaskets and vacuum lines...
I disconnected the CC...
it doesnt go away any more, it did in day 2, day three to present it idles at 2K+ (with IAC plugged in)

it never overheats, I have a 180* and electric WP, barely gets over 190* (in city traffic.)

HAve you chenged/removed the battery lately or noticed the HVAC system not switching modes (from floor to vent to defrost)? A common issue is the vacuum line running to it is near the batter and gets piched/broken in the process. This will cause a vacuum leak no where near the engine and slow/no switching of modes.

If that all checks out, I would suspect a bad/misadjusted TPS or bad MAF sensor.

It's a brand new stage 2 ported tb from zzp...


Did you have the problem before installing the Stage 2 ZZP TB? All ZZP throttle bodies are re-worked cores sent in. Maybe there is a problem with it. Other possibility is an incompatibility with the MAF modification ZZP does to Stage 2 TBs. I have the ZZP Stage 1 and never had any problem with the idle but that was with the stock MAF sensor. Now I use an external 85mm GM MAF sensor but the idle is still good.

Also, make sure the PCM 5 volt reference signal at the IAC connector plug is within specs.

Did you have the problem before installing the Stage 2 ZZP TB? All ZZP throttle bodies are re-worked cores sent in. Maybe there is a problem with it. Other possibility is an incompatibility with the MAF modification ZZP does to Stage 2 TBs. I have the ZZP Stage 1 and never had any problem with the idle but that was with the stock MAF sensor. Now I use an external 85mm GM MAF sensor but the idle is still good.

Also, make sure the PCM 5 volt reference signal at the IAC connector plug is within specs. I have had the TB for over a year, no problems until this. All vents work, I have checked all vacuum hoses for leaks.