Hello first time here, just looking for a little help or a lot, well here is my issues. I purchased a 1998 Pontiac Grand Prix gt 171k. It had a blown headgasket, so did all repairs, valve job, new water pump, ect sensor, t-stat, upper and lower hoses, replaced radiator not new but good. Also all new gaskets. Car fired right up. Sounded good got to running temperature and fans never turned on. I topped of radiator. I can feel when the thermostat opens up cause the radiator goes from cool to warm, but my gauge reads max temp, but the car never boils over. I thought it might of had air in system, so i removed the thermostat, and tried like that. I can see water flow in radiator, and still my gauge max at temp, and fans never turn on, but if i unplug the ect sensor they both turn on , so that tells me they and the relays are good.
So i borrowed a temperature sensing gun, i put the tstat back on and ran the car till what i thought was operating temp. Started taking readings, here is what i got, tstat housing 195-205’, radiator 205-210’, water pump 185’, upper and lower hoses 185-190’, i have to have a bad gauge and there has to be something electrically going bad that doesnt allow my fans to turn on. Also this is not my first car I have done this kind of repairs but it has been a pain in the butt, so any help please and thanks.
If your gauge is reading full hot but the coolant is not, you may have 2 sensors. One is for the gauge and the other is for the computer to control the radiator fans. The one for the gauge or its wiring would be faulty. If the car only has one sensor, then i would suspect the gauge is bad or the sensor is skewed. Since everything works and the engine is not overheating, you do not have an engine problem. Unplugging the sensor causes the computer to see -40 degrees F, so as a precaution, it turns on the fans.
All of the reading you are getting of the coolant in different areas are correct for a normal running engine. Replace the ECT first and see what happens. This is a very inexpensive repair and easy to do. Also tap on the dash gauges cluster when while it is acting up to see if maybe it is just reading incorrectly. If it bounces around, drops or anything like that, it would need to be replaced.
You cannot fix just the temperature gauge, you would need to replace the whole unit with a factory cluster. The part number is on the back. There are some aftermarket companies that can repair these, but sometimes you get what you pay for. Just do a search for GM cluster repair services.
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