Reasons for car overheating while idling or using a/c

Knock sensor may be bad causing idle to retard to stop spark knock . Carbon buildup in the cylinders on piston heads or around the valves can cause retardation as well as overheating .

Did it run out of coolant? If so, you have internal engine damage? I'd check cylinder compression.

Also, check proper oil pressure. You can always check the basics, for a rough idle, check for a leak of some kind, egr--fuel--vacuum leak. Any applicable trouble codes?
What is year--make--model, It didn't post for some reason?

Low on coolant? Often the cause of poor heater performance. Also a reason for overheating. Be careful, overheating can lead to expensive repairs, engine wise. I would not be driving the car until the overheating is solved.

For car running hot and overheating issues,i suggest to you go through this help links. Click this link below and go through the troubleshooting procedure to take care of the problems :----Car runs hot? http://howtobyme.blogspot.com/2011/11/car-runs-hot-in-traffic.html
-----------All types of car overheating problems and troubleshooting?

http://technoanswers.blogspot.com/2011/11/all-types-of-car-overheating-problems.html
Car freeze overnight and started overheating while driving?http://technoanswers.blogspot.com/2012/01/car-freeze-overnight-and-started.html-----------Engine Overheats and heater blowing cold?

http://howtobyme.blogspot.com/2011/11/engine-overheats-and-heater-blowing.html

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Car overheats and Air blows hot at idle?
http://howtobyme.blogspot.com/2011/11/car-overheats-and-air-blows-hot-at-idle.html
--------overheating? http://schematicsdiagram.blogspot.com/2011/12/buick-century-overheating.html
----------Car engine Overheats and transmission starts slipping?http://schematicsdiagram.blogspot.com/2011/12/car-engine-overheats-and-transmission.html
----------This details will help.Thanks.

REPLACE THE THERMOSTAT AND RADIATOR PRESSURE CAP.IF COOLANT LOW, ADD MORE COOLANT THROUGH COOLANT OVERFLOW UNTIL COOLANT LEVEL IN THE COOLANT OVERFLOW STAY AT FULL COLD MARK.REASON POUR COOLANT THROUGH COOLANT OVERFLOW MOST GM CARS DONT HAVE RADIATOR CAP.WHEN YOU GET COOLANT LEVEL OKAY, CRANK CAR LET IT IDLE UNTIL ENGINE GET HOT BUILD UP PRESSURE THEN TURN OFF CAR.CHECK FOR COOLANTS LEAKS AT THE RADIATOR HOSES,CHECK FOR LEAKS AT THE RADIATOR, CHECK FOR LEAKS AT WATER PUMP WEEP HOLE IF YOU SEE COOLANT LEAKING FROM WEEP HOLE REPLACE WATER PUMP. IF CAR OVERHEATING WHILE IDLING YOUR COOLING FANS NOT WORKING CHECK COOLING FANS FUSE AND RELAYS.IF FUSE AND RELAYS OKAY THE ENGINECOOLANT TEMPERATURE SENSOR BAD.

When does it overheat? At idle, or at highway speed? Is the coolant level where it should be? Overheating at idle or highway speed may be a faulty thermostat. If it overheats only at idle then it is likely a cooling fan fault. To determine if the fan is working start the engine and switch the A/C on, the fan should come on, if it does not, then you need to have the computer "scanned" to see if it can detect the reason for the fan not coming on. If the fan is switching on, then you need to have the water pump checked to see if in fact it is pumping coolant (this could also be the cause of overheating at highway speed) If you are overheating only at highway speed you may want to have the enigine checked for a blown head-gasket, or faulty radiator.

Spray wd40 around the head sounds like it did not seal right

The radiator cooling fans are not coming on for some reason...It is probably a fuse...check all of your fuses...there should be a fuse that just runs the cooling fans...look around the drivers side near the hood release for the fuse box

more than likely the thermostat is bad (not opening ) need more info ..... only when idling ? do the fans kick on ?