High Idle, Delayed Shifting - 03 Buick Lesabre 3.8L

[gah_not_again]

I've been out of town for a week. Issue still persists. Yesterday I installed a new IAC. This did not resolved the issue at all. I have noticed that the temp gauge fluctuates. I'm wondering if there is an intake gasket leak or something. The temp gauge will go up to 3/4 then drop down to 1/2. Temp varies between about 170 and 230. I believe this is related to my issue. Not sure how to proceed from here, waiting for the shop to call back so I can see if they will re-diagnose without having to pay more since they didn't properly diagnose the issue.

Today I got the full symptoms. High idle ( car will go from a full stop at a light to 40 mph without me even touching the gas), delayed shifting, temp gauge fluctuations, and then rattling like preignition when the temp gauge is fluctuating a lot. I'm sure it's overheating, at least partially. Smells hot too. Put into park and 3k rpm idle. This is driving me crazy.

[Brammer09]

I know this is an old thread, but I’ve got exactly this. Did you ever resolve it?

[Tech II]

Do you have the same year, model, and engine?

High idle?

Possibly overheating?

Coolant level ok?

[Brammer09]

Yes, exact same car, 03 Lesabre 3.8 engine. I have all the same symptoms described in this thread. I have been watching the coolant temperature for a couple weeks now, there may be some correlation with coolant temp going above 200 degrees, but I’m not certain of that. One other thing that I note is that even at highway speeds, my car is pushing forward without my foot on the gas. Normally if I coast on the highway the real time mileage goes to 70 mpg. When the car is acting up, it stays around 40 mpg. Something is making the engine push besides my foot on the throttle.
I also got a code ealier about the TPS. I replaced that, didn’t fix it.

[maxwedge]

I am sure you have cleaned the throttle body, and checked for any binding in the thottle linkage and throttle plates.

[Brammer09]

I finally traced this to a bad TPS signal getting to the ECM. Even without touching the throttle (and not sticking) the computer was seeing a 40% TPS signal. Must be some kind of stray voltage in the ECM somewhere. In short, I replaced the ECM, the signal is clean now, and it’s running perfectly.