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Sorry, this is going to be a bit wordy. I have lots of details...

A few weeks ago, the temperature was in the single digits with a negative wind chill. I started up my 96 Blazer and let it run for 10 to 15 minutes. I then drove around the corner, and by the time I got a few blocks away, the temperature was at 240 degrees. I immediately pulled off to my uncles house and let it cool down. After looking at it for a while, I drove it back home and then to the store. On the way to the store, it would get up to about 230 while idling at a stop light, but would immediately drop when I started moving. Anyway, the next day the temperature warmed up and I drove it from Chicago to Indianapolis with no problem.

Well yesterday it got to 15 below here with a -25 wind chill. I went to start it and it started OK. After warming up for 20 minutes, I decided to drive it around the parking lot. I got stuck in some snow briefly (even with 4WD), and by the time I got out of it, the temperature was 240 degrees. By the time I got back to my spot, it hit 260 and the radiator cap released and blew fluid everywhere. (No fluid in the oil.)

I waited until today, put some more anti-freeze in it and started it up. No problems at all. I drove around the block and it got up to 230 again. I let it idle for a few minutes and it stayed at 230. Finally, I hit the gas and got the RPMs up to about 3500. The temperature climbed to 250, and as soon as it 250, it immediately dropped to 150 (within seconds). Since no fluid was lost and I didn't crack the block, I'm assuming that it finally decided to cycle the fluid through the engine, which is why it dropped so quickly.

Anyway, the only hunch I have right now is maybe the thermostat is stuck, but given that this only happens in extremely cold weather, I'm not 100% sure. Does this sound like a problem with the thermostat?

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... or did the coolant freeze in one of the lines? –  Bob Cross Jan 8 at 2:53
    
not too wordy. well reported. +1 –  mac Jan 8 at 14:35
    
The coolant didn't freeze. While I usually don't measure exactly 50/50, I generally go on the side of too much antifreeze. That has problems of it's own, but they're not nearly as dangerous as freezing. :) –  GJK Jan 8 at 21:47

1 Answer 1

Single digit temperatures would not freeze your coolant assuming it has antifreeze mixed in appropriately and isn't just water.

Since you went from 250 to 150 within seconds I would assume something was blocking the flow of fluid. Thermostat staying shut would be the most likely culprit. It could be that the cold weather is pushing a failing thermostat to show more symptoms of the problem and less when the temperature is warm.

The fact that it dropped to 150 is also interesting because normal operating temperature for your blazer should be somewhere in the 180-190 range. That makes me think that the thermostat went from stuck closed to failing wide open. It wouldn't explain the overheating but there might also be a problem with your temperature sensor.

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Your answer seems very plausible and agree with your analysis. A couple of points I'd make, though: A drop of 250 to 150 could also be attributed to an air pocket in the coolant system. Also, it sounds like the radiator cap actually did what it was supposed to: relieve over pressure ... not sure replacing it would solve anything. Replacing it as a general maintenance item may be a good idea, though. –  Paulster2 Jan 8 at 12:19
    
@Paulster2, I agree that the radiator cap replacement is really just a general maintenance item to be replaced. I will edit the answer. –  jzd Jan 8 at 13:00
    
Coolant temp sensor alone wouldn't explain all the symptoms, since at one point while the gauge was reading high the pressure limiting radiator cap was triggered. At that point the car was actually running hot, not just reading hot. –  mac Jan 8 at 14:34
    
@mac, I might not have been clear enough in the answer, the "or there might also be a problem with the temperature sensor" is in reference to just the rapid change in temperature not the overheating issue. I will clarify. –  jzd Jan 8 at 14:55
    
Thank you for the answer. Since a thermostat is only a few bucks, I'm thinking that it would be a worthwhile effort to replace it. I have a feeling, as you said, that the thermostat isn't the only problem, but it's likely one of them. –  GJK Jan 8 at 21:49

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