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Discussion Starter · #1 ·
Last night, I ran some errands. The car heated up normally; the temp. gauge reached nominal; took only about 10 operating minutes for the engine to warm.

After I had ran the errands, the car remained park for about two hours. Normally, the car would be completely cold (especially in the freezing weather recently) but the temp gauge was reading nominal right off the bat.

Strange, I thought. The car heated up normally, cold air was soon replaced by warm and then hot air.

After a short drive home, the car was parked overnight.

This morning, I started the car and the temp gauge was all the way down on the cold side. After just a couple of minutes the gauge read nominal while the air coming out of the heater was still ice cold.

I began my commute to work (about 25 mile drive) and after about a mile of surface-street driving the air coming from the vents began to get warm. Another mile of driving and I get on the freeway.

Once on the freeway, the temp gauge literally buried itself on the hot side. Very, very quickly at that-- the gauge jumped all the way from nominal to hot in about 10 seconds. Throughout this time, the air coming from the vents remained hot but not unusually hot.

I immediately pulled off the freeway, parked and popped the hood. Checked the coolant overflow-- fine. Popped the cap; no boil over. No steam, even. Felt the valve cover-- barely warm to the touch.

The car was definitely not overheating. After about 2 minutes of idling the car was back at nominal operating temp.

I decided that I would attempt to finish the remaining 20 or so miles to work. I drove the whole way with the gauge buried on the hot side. The air coming from the vents remained hot but not too hot the entire time.

I parked at work and popped the hood. Popped the overflow cap, no boiling. The valve cover was now hot, but not unusually hot. There was also not much excess heat coming from the engine bay. The car definitely was not overheating.

1. Last night, temp gauge stayed nominal after being parked for two hours in subfreezing temperatures.
2. This morning, gauge is cold. Gauge reaches nominal very quickly, air still cold coming from heater.
3. Drive on surface streets for a couple of miles, air from heater is warm, temp gauge still nominal
4. Get on freeway, temp gauge buries to the hot side, pull off and check. Car is not overheating. No boilover, no extreme heat coming from engine bay, heater air not unusually hot.
5. Finish 20 mile commute to work, check car again. No boil over, no extreme heat and the heater is still blowing normally hot air.
6. ...
7. wtf :(

My dad says it is the thermostat. This seems fishy to me though, as the thermostat being bad (therefore staying open) would result in extremely long warm-up times as well as the gauge reading below nominal; not above.

If the thermostat was stuck closed then I would imagine that my motor would have popped somewhere along those 20 miles of commuting, yeah?

So the gauge must be misreading? The first I've seen of this was last night, when we had record-for-the-season low temperatures of around 5F.

Bad temp sensor?

Thanks in advance

2,091 Posts
It sounds like the coolant/antifreeze froze up. Classic signs, engine will warm up, but no warm air and suddenly peg to hot - you froze up.
It could also be a combination of freeze up and bad thermostat.
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