Pontiac Problems? Ask a Mechanic for Answers ASAP
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Have you checked the coolant level to be sure its full?Is the cooling fan coming on?
If the fans are not coming on then that is likely your problem. That would explain why it cools down when you start driving as your pushing air across the radiator. Has the system been scanned to see if there are any fault code setting?
It could be any of the above. Are you able to do some testing?
That means the fans are not the problem, one thing you can try is to disconnect the engine coolant temp sensor with the engine running and see if the fan comes on. Check this and let me know.
Its in the engine just below the thermostat housing . See the diagram in the link below for location.https://dl.dropboxusercontent.com/u/104923897/ECT%20Sept%209.pdf
Are you able to see the thermostat housing? It right below it.
This is the 3.8 engine right?
Okay thats a different location, on the 3.1 its on the block just below the fuel pressure regulator located on the fuel rail. See this link.https://dl.dropboxusercontent.com/u/104923897/ECT%20Sept%209.pdf
The fan should have come on with the sensor unplugged. That usually means the PCM (computer) is not seeing the signal from the ECT. If your sure your fans run with the AC on and the cooling system is full then that usually means the PCM is at fault.
You would need to replace the PCM and programming would need to be done. I think before you jump to this step you would need to have some hands on professional testing done to be sure the PCM is at fault.
This is not something your likely to do with out a good scan tool. Most shops are going to charge about 1.5 hours in labor to diagnose this for you.