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Old 11-09-2009, 12:45 PM   #1
2GOFAST
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Default What are the symptoms of a faulty thermostat?

I think my thermostat is failing to open as my coolant temp is getting very high. It started all of a sudden about a week ago and my coolant temp is getting up to 240-245F on a regular basis while it used to average around 210-220F. Note - it is fall/winter here with outside temps typically at a high of 50F. Also, my fans are running and the coolant level appears good (about 1/2 of overflow tank is full when the system is cold).

So, what are the symptoms of a faulty thermostat?

Thanks

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Old 11-09-2009, 12:50 PM   #2
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More than likely your problem is road dirt, leaves, grass, and small stones and like things plugging the front side of the radiator. When is the last time it was cleaned?
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Old 11-09-2009, 01:11 PM   #3
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Quote:
Originally Posted by Eric D View Post
More than likely your problem is road dirt, leaves, grass, and small stones and like things plugging the front side of the radiator. When is the last time it was cleaned?
Radiator is clean - put the car on ramps and looked at it yesterday.

Thanks for the suggestion though.
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Old 11-09-2009, 01:27 PM   #4
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Pull the t-stat and put it on the stove in water with a thermometer, you'll know in less than 5 minutes if it opens when it's supposed to. Factory T-stats FAIL CLOSED, you'll know quickly if it's failed.
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Old 11-09-2009, 01:30 PM   #5
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Has the coolant system been open recently? Having entrapped air will cause what you described.
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Old 11-09-2009, 02:19 PM   #6
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Here is some more information. The coolant system was not opened prior to the problem ocurring - in fact it had not been opened since my H/C install 1-1/2 years ago. I drained the radiator and pulled the thermostat yesterday, played around with it by hand and then put it in boiling water and got it to cycle open and closed. I put it back in the car and followed the service manual for refilling the coolant and running at 2500+rpms for awhile follwed by 3 minutes of idling. It worked great and my max temp was about 215F during a 15 minute drive at 2500+rpm.

This morning, however, the temp has again spiked to higher than normal (240-245F) and it appears that the same thing is happening again.

So, I am thinking that maybe the thermostat is sticking closed and that my playing with it only temporarily fixed it.


But, back to my original question - What are the symptoms of a bad thermostat???
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Old 11-09-2009, 04:34 PM   #7
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AGAIN, they fail CLOSED-you would OVERHEAT QUICKLY. The only REAL way to tell is the hot water test with a thermometer. You can also use an IR thermometer to see how hot the hoses are. Are the Fans coming on correctly?
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Old 11-09-2009, 05:28 PM   #8
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Quote:
Originally Posted by 2GOFAST View Post
But, back to my original question - What are the symptoms of a bad thermostat???
You overheat.

Why would you stick an old $10 thermo back in your car? That's one of those parts that I don't even dick with diagnosing. If it's torn down to that point and drained, then put a new one in.

If the fans are coming on when they're supposed to, then you've probably got a clogged radiator. I know you say you looked at it, but you know what? I looked at mine too before I blasted it out, and it looked fine but I bet I got 2 cups of sand out of that damned thing.

Last edited by wcsinx; 11-09-2009 at 05:30 PM.
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Old 11-10-2009, 11:20 AM   #9
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Quote:
Originally Posted by 2GOFAST View Post
But, back to my original question - What are the symptoms of a bad thermostat???
Symptoms of a bad t-stat are:
1. Overheating due to t-stat not opening.
2. Coolant not coming up to proper temp due to t-stat stuck open.

If you checked it and it opened/closed then I would suspect the fans aren't coming on
OR
Your radiator is clogged.
OR
Your coolant level is too low.
OR
Radiator hose is collapsed.

Does the temp rise higher then normal while you are driving or when you are at idle (not moving)?

Next time you go to the trouble to remove and check a t-stat, recommend you just put in a new one.
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Old 11-10-2009, 12:19 PM   #10
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Thanks for the responses. FYI - I have a 160F thermostat and have to order one to get a replacement - thus the reason I did not change it out when I opened up the system.

I will work through the list. Maybe my fans are only operating on low and not high speed.

As for the temp - it is higher during idle and only drops a little bit when driving - I suppose it is a fight between greater heat production due to combustion and the higher airflow over the block due to driving. The service manual indicates that the cooling system is only designed to remove some (about 1/3 - I forget the exact #) of the heat of combustion.
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Old 11-10-2009, 03:39 PM   #11
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Quote:
Originally Posted by 2GOFAST View Post
Thanks for the responses. FYI - I have a 160F thermostat and have to order one to get a replacement - thus the reason I did not change it out when I opened up the system.

I will work through the list. Maybe my fans are only operating on low and not high speed.

As for the temp - it is higher during idle and only drops a little bit when driving - I suppose it is a fight between greater heat production due to combustion and the higher airflow over the block due to driving. The service manual indicates that the cooling system is only designed to remove some (about 1/3 - I forget the exact #) of the heat of combustion.
Lots of debate over the advantages and disadvantages of not using a stock (187 deg) t-stat. Personally, that's one of the things I'd keep stock. Good luck chasing down your problem.
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Old 11-10-2011, 09:17 PM   #12
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Please Help!!
Have a 2000 C5, had some coolant loss because of a cut hose. I had the hose replaced but I was told that the Dexicool causes crystalization in the radiator. The engine is still running hotter than usual. it was running about 199 to 240 until I started driving then it would cool back down. Now it seems to be running hotter at highway speeds and will cool down as I slow the car down until I come to an idle then back up. Is this the thermostat? The radiator? or something else? Any advise would be appreciated due to mechanics just wanting to replace everything.
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Old 11-10-2011, 09:17 PM
 
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