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The radiator must have air moving through it to cool the colant. When your car is moving, you have air going through the radiator. When your car is still, the fans turn on when the temperature reaches a certain point to blow air through the radiator. If your fans don't blow, radiator is not cooled untill you move.
As for checking for power to the fans. Do you have a test light? Connect the ground strap of the testlight to a good ground (battery will do) and disconnect the fan plug. check for power at one terminal and then the other. One terminal is power and one is ground. If your light lights up at one of the posts, you have power. If you want specific diagrams, please give me the engine size and transmission (manual or automatic).
no, not a thermostat. Let me know when you are ready
I usually do this step by step with people, but I will give you this all in one. If you run into problem, please ask.
OK. Here's how you do this.
Warm up your car to operating temp (195deg)
Wait for your car to start overheating. (200+)
You must do this while the car is running and when fan should be on and isn't. Engine will be hot so be careful. Also, don;t let the car get too hot, keep the gage out of the red zone.
Check for power at the radiator fan motor for 12V. (red wire) If you have power, you have a bad motor.
If no power check power at relay. Remove the relay and see if you have 12V at terminal 1.
Got power, good. If not, check fuse 33 and 35 in under hood fuse box.
Check for power at terminal 3 for 12V. Got power, good. No power, check #13 fuse in dash fuse box.
Jump terminal 1 &2 with a suitable wire, paper clip, or anything that's metal. Fan should start.
remove jumper and Reinstall relay. Disconnect the temp switch. Engine coolant temperature (ECT) sensor. Jump the yellow/green to black wires together through the plug. if it fan turns on Replace the temp sensor.
If all fails, swap the radiator fan relay with the condenser fan relay and see if the fan works. If it does, you have a bad relay. If not, you have a wiring problem or a bad pcm.
If you have any questions, please let me know. Let me know with what you find.
First thing to do before you go any further is to do a compression test. This is to eliminate a blown head gasket. Test all 4 cylinders for 150 PSI. No cylinder can vary more than 15%. The lowest to highest cylinder can't be more than 25% off. You can also test for a blown head gasket with a chemical tester avaiable at most part stores.
Your thermostat is good, yet you have lack of flow. This can be because the thermostat is bad, the water pump is not cirulating, an air bubble in the system, or combustion gasess in the cooling system are building up. If your water pump would not be circulating, the overheating woult be more constant. An air bubble would have come out by now. That is why I would like you to test for a blown head.
Yes, a leaky cylinder head gasket will affect coolant flow. What happens is that combustion gasses will gather up at the top of the cylinder head and restrict coolant flow. It will cause the same effect as an air pocket in the system.
A thermostat is basically a coolant flow stop plug. What it does is restrict coolant flow when the car is cold to warm it up to operating temperature. (cars need to run at 195 deg for proper engine operation). When the engine heats up, the thermostat spring collapses due to the heat and opens the thermostat. A thermostat gasket is what seals the joint between the thermostat inlet and thermostat housing so no coolant leaks out. A good way to test the thermostat is to remove it, place it in hot boiling water and see if it opens. If it does, its good. If not, it needs to be replaced.