Stuck thermostat, or just winter? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 12-23-2009, 12:19 AM   #1
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Question Stuck thermostat, or just winter?

I've been noticing that ever since winter came around, my engine has a hard time staying warm when the car is moving. If the car is idling (say, rush hour on I-95), the temperature gauge will run about 205. However, if I'm moving (especially at highway speeds), after a few minutes, the temperature gauge drops to about 160-170 (130-150 with the heater on).

Gas mileage is down to about 18 mpg highway (used to be ~23 last summer), but that says little on its own as it'* about tune-up time.

Is my thermostat stuck open, or is this just normal in cold temperatures? (For the life of me, I can't remember last winter.) It'* running the stock, and probably original, 195.
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Old 12-23-2009, 03:21 AM   #2
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I would say theres a chance its stuck open, but depends on how cold it is where ur at. its been in the teens here in michigan and my car barely ever makes it to 200F
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Old 12-23-2009, 08:25 AM   #3
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I agree with Ray, start with your thermostat and also test your antifreez for cold weather temps


Have you ever replaced a thermostat?
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Old 12-23-2009, 09:42 AM   #4
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The temperatures around here are in the teens in the morning, but around or above freezing in the daytime.

I've never replaced a thermostat, but I have a general idea of how to do it.
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Old 12-23-2009, 09:56 AM   #5
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That'* a stuck open thermostat. Easy to fix. less the $10, and about 20 mins of your time.
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Old 12-23-2009, 10:29 AM   #6
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Dan do we have a tech-how-to to reference here for him?
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Old 12-23-2009, 11:14 AM   #7
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Ya, there'* a thermostat change in tech info. https://www.gmforum.com/mechanical-134/installing-new-thermostat-278803/
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Old 12-23-2009, 11:26 AM   #8
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Yeah, Dan, I saw that. I just have to get my hands on one of those cable pliers.

One thing that the write-up didn't mention, though: do I need to replace the thermostat housing or O-ring when I take it out?

If so, RockAuto has three different housings (one round, one kind of diamond-shaped, one with no image) and three different O-rings (under "Thermostat Seal"); which one(*) would I need?
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Old 12-23-2009, 11:45 AM   #9
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I see no need to even remove the hose from the housing. Unless you are replacing the housing.

Once you remove the 2 bolts, simply pull the hose back and out of the way.

You can get a new gasket when you pickup your thermostat from autozone. I think I paid a total of $12 or so for both parts. I didn't even need to replace my gasket.

The diamond shaped gasket is the housing to intake manifold gasket. The round black one is the thermostat to housing gasket.

Last edited by Mike; 12-23-2009 at 11:46 AM. Reason: Forgot additional info
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Old 12-24-2009, 07:06 PM   #10
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All right, after an oil change, I pulled off all of the vacuum lines and connectors surrounding the housing, pulled it off, switched thermostats, scraped off the old diamond-shaped gasket, put on the new one, closed it up.

The old thermostat was not stuck open. A little orange in color, but I can push on it and it opens just fine. Haven't yet tossed it into boiling water, but it seems like it was fine.

I also snapped off the vacuum line going to the fuel pressure regulator. Nothing a little electrical tape couldn't fix, but I see myself replacing those six inches of vacuum line pretty soon.
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