I have a 2005 Grand Prix GTP with about 98k miles on it. I have been having a couple issues with it. First when it is cold out (even after the car is up to temp) the rpms going down the highway are higher than they should be. I set the cruise at 72 and normally the rpm would be about 2200 then it went to about 2400 and now its at about 2500 - 2600. It feels like the engine is having to work a lot harder to maintain speed. This only happens when it is cold out, things are fine when it is warmer out but it is almost always an issue after a cold night. Next, after riding for a while with the higher rpms, the cruise control will randomly kick off. Sometimes it can be reset right away, sometimes I have to wait a mile. The latest development is that after driving the car, and having the rpm issues the car idles very rough in park (not noticable when stopped in drive), the rpms will jump from arou 900 down to 500 and even inbetween the idle is rough enough to feel the engine shutter. I only notice this if I have had issues with the rmps.I thought it might be the gas, but I ran the tank low and refilled it a couple times at different places (always with premium). I had the rpm/cruise issue last winter and thought it might be the coolant temp sensor, but the mechanic hooked it to the computer and said that is working fine. I have done the following:
New plugs
New wires
New coils
New MAF
New brake pedal position sensor
New brakes
Oil changed regularly
Changed super charger oil
Radiator flush
I am looking for any suggestions here.
Thanks
J.
