Pontiac Repair: Overheating, pontiac gran prix, coolant level

Question
I have a 2001 Pontiac Gran Prix GT with a 3.8L. We had the radiator flushed in December and replaced the thermostat in January. When driving in town the heat gauge creeps up and soon it is almost at the redlight warning level. The fans are working and no obstructions of the radiator. If I am driving on the highway, the problem does not happen. Do you have any suggestions about what to look for?

Answer
Hi
First I would check the coolant level, making sure it is topped off.
I would check the actual flow through the radiator.
I would also check on the upper intake (plenum) close tho the throttle body, check REALLY close for a coolant leak at that location.
Repair ANY leaks.
If nothing found externally I would pressure test the coolant system.
I would also check for exhaust in the cooling system, a sign of a faulty head gasket.(not common on the 3.8, BUT it is possible)
If that checked out OK, I would then make sure the fan(s) are coming on and coming on at proper SPEEDS, eg.. with A/C on and driving through town the PCM SHOULD command the fan(s) on HIGH speed.
USUALLY if the vehicle is cooling properly at highway speeds but not in city driving it is USUALLY a fan speed problem.
Does the A/C stop cooling well as this is happening?
You also MAY want to take it to a repair shop, have them hook a scan tool up to it, check for codes, and ALSO check and see what the ACTUAL temperature is from the PCM data.
If it is within specs, you may just have a faulty gauge.
I hope this helps.
JUST advice from experience.
Good luck
Pawl