Q: Overheats at idle, too cool while driving.

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The radiator fans only seem to kick on low when I turn on the A/C. Only kick on high when I disconnect the Cylinder Head Temp. Sensor. SOOO I know both speeds Physically work. (Two fans, both spin freely.) Also have checked, (with my ears and with feel) that both associated relays work, or at least make a click.

I've let the car idle up to temp, and neither low nor high speed seem to turn on. I've let it idle up to about 240F before I get scared and turn the a/c on to cool it manually. (just about boiling over by this point).

Another wierd thing, is that when driving consistently (i.e. Freeway), the car runs Cooler than normal.(Needle physically moves more toward cool on its own.) Temp is anywhere from 160f - 190f during that time. Before issues, its normal was like 210f.

I have Replaced: CHTS, Fan resistor and pig tail, (part that susposedly operates low speed), Thermostat and its housing. Coolant is in good shape and alwyas topped off.

Regards, Josh S.

My car has 160500 miles.
My car has an automatic transmission.

Hi there. What diagnostic tests were performed on those parts before replacement? What were the results vs. the specifications? It almost sounds like either a coolant temperature sensor fault (there should be more than one) or a controller/computer fault. If the sensor telling the controller is faulty then the high speed fans won’t engage. If the sensor to the gauge is faulty then the temperature reading is inaccurate; has the engine overheated? The reason it runs cooler while driving is that there is air flow moving through the radiator cooling the coolant and at idle, since there is no high speed fan, it runs warmer. I suggest having a qualified technician perform an inspection to avoid replacing unnecessary parts. Your Mechanic has several technicians available that can assist you with an overheating inspection.

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