I have a 2000 pontiac grand am that is constantly overheating. Replace the intake manifold gaskets, and thermostat
but the fans are not coming on when the motor gets hot only when the ac is on high. relays seem to be good what is the problem
Check the coolant sensor for the cooling fan relay, the fan relay itself and fan fuse 1 and fan fuse 2 in the under hood fuse compartment.
A scanner should be used to determine if the fans are commanded on. If they are commanded on and still doesn't work then the fan circuit should be checked out.
You can also check the proper operation of the gauge. The scanner will tell you the engine coolant temperature; you can compare the reading to the gauge reading.
Retrace your steps and check the connections on all the wires that were disconnected while replacing the intake manifold gasket like body grounds and ground wires going to the fans. Check fuses, relays and sensors.
Make sure the engine is not air bound. The system would have to be bled (burped).
You can easily do this by just jacking the front of the car and put on jack stands or use ramps and run the engine with the radiator cap off. Run the engine at idle and watch the coolant level, when it gets low keep refilling the radiator until it will not take anymore coolant. Test drive the car to make sure it is not overheating.
If you need more assistance let us know we will diagnosis the problem further if needed.