I have a 2001 Pontiac Montana 3.4L ~140,000 miles, that exhibits a strange behavior. It runs absolutely normal most of the time. However, after running on really HOT days here in Texas, or on a long roadtrip, it acts as if the butterfly valve or pedal is getting stuck open just a tad to where it idles around 3000 rpm, and is enough power to keep the vehicle speeding along at 70mph. It is scary how much you have to ride the brakes to slow. And the transmission pulls hard at a stop.

I have idled the thing with hood open and verified that the pedal, cable and even the butterfly action is operating correctly while it is running high. It does not appear to be carbon buildup.

After a high idle return home commute Friday, I turned the thing off, came back to it Monday morning, and it was still idling high. I put it in gear and it was pulling hard. A minute or two on the road and it calmed down and ran fine the rest of the day.

To me it "feels" as if the issue is artificial, as if a sensor result and action is actuating a valve somewhere that is causing this behavior. And it took a minute after starting cool to "rest" the scenario to put it back to normal.

I do not expect a vacuum leak.

I haven't had time to do much troubleshooting yet. I'm at work hoping I will get some ideas here.

I plan on hooking up my odb laptop cable to watch where it thinks the pedal is depressed and to look at other sensors. I have not had the intake open yet. Maybe the butterfly is messed up somehow but I really cannot fathom what would "display" and then correct itself upon a cool start.

This almost sounds like the IAC is actuating open. I will plan to remove and clean the basic throttle body parts and see what I see. The fact that it isnt always is what gets me here.

I have seen this before hot and cold of all things it was the water temp switch telling the pc its cold so give it a high idle.

I'll have a look at the output of that when I get to it.

A code isn't set?

Sure no vac leak, or binding throttle plate or sticking cable?

If not the IAC itself, it could be the harness to it......

If it was the coolant sensor, his gauge would be reading cold.......

As far as the temp gage reading wrong actual no. As most temp senders send reading to the gauge and the ECM.
I have had 5 cases the gauge read normal and the engine would idle fast for a while like 5-10 mins.
That'* were the ECM knows the eng is cold correct? So it will give it a fast idle if the temp sender is sending the wrong reading to the ECM
It would not surprise me if most have never seen this but after seeing it 5 times what can I say just my 2 cents worth

On older vehicles, there were two temp sensors....one for the ecm and one for the gauge.....

This has one temp sensor....it goes directly to the PCM, and then the PCM sends the temp signal to the IPC....it will send the signal to the cluster what it ,itself, is seeing.....

OK and my 2001 Regal had only 1 and the temp gauge read normal and it would idle at 2000 rpm until the it got full warmed up.
Half way on the temp gauge then it would drop to 800.
Replacing the temp sending unit fixed it.
And I have seen 4 other cars do the same thing some hot some cold fast idle.
Replacing the temp sending unit fixed all of them.
So I guess the sending unit telling the ECU the engine was cold when it was not right??

Well, all can say is, I have never seen this happen...it just doesn't make sense....

The night before I was going to run for parts the throttle "stuck" again. I managed to watch the TPS readings and they always stopped at 3.14 not zero. So I immediately realized that the TPS must be bad in the low end and it is probably heat agitated. The 3.14 may well be an "error" result when the PCm cant see the correct value.

I swapped both the TPS and the IAC for good measure. It has been near a month and the problem hasnt come back. And I swear that the transmission is shifting better and the entire system is better. I suppose the TPS was sending wrong info and the PCM was doing confusing things with the transmission at times.

Problem solved.