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Thermostat question

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S

SAWYER

Lifer
My car has been overheating and I decided to check take the thermostat out. I have not put it in boiling water yet but at cool temperature it is fully expanded and I can press down on the end to make it go down. I thought when it gets hot is when it is supposed to be fully expanded?


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Thermostats are designed to fail open, not closed. Sounds like yours is stuck open.

That's a problem and it should be replaced, but you might have something else wrong. A stuck open thermostat would be too much cooling, not overheating.
 
what car do you have, how many miles? Did you just the car recently?


If you haven't replaced the radiator cap (if it's older than 5 years old), replace it. You'll obviously have to bleed the air out of the cooling system. If the thermostat is over 10 years old, replace it.
 
When cold it should be closed (the plunger is seated fully in the hole). Hot it will open up.

I'm not sure what you mean by expanded.
 
DeviousTrap said:
That's a problem and it should be replaced, but you might have something else wrong. A stuck open thermostat would be too much cooling, not overheating.
Click to expand...
You know, I've wondered about this. If the coolant is circulating freely with no restriction from a thermostat, does the coolant stay in the radiator long enough for adequate heat transfer to occur? It's a rhetorical question that I don't really expect an answer to.

You can look at it two ways. Is a thermostat restricting the flow of coolant from the motor or too the motor? Just something I've pondered about from time to time.
 
Coolant flow FROM the motor. In a state less than the degree rating it should be closed. It should start to open at the degree rating ie 190. It should be fully open about 25 to 30 degrees above the degree rating
 
boomerang said:
You know, I've wondered about this. If the coolant is circulating freely with no restriction from a thermostat, does the coolant stay in the radiator long enough for adequate heat transfer to occur? It's a rhetorical question that I don't really expect an answer to.

You can look at it two ways. Is a thermostat restricting the flow of coolant from the motor or too the motor? Just something I've pondered about from time to time.
Click to expand...

The idea that coolant needs to "stay in the radiator" for a bit is an old wives' tale. It's based on a complete lack of understanding of heat transfer.

Even with no thermostat at all, there will be plenty of time for the coolant to lose heat in the radiator. In fact, the problem will be that it takes the engine far too long to warm up and will run too cold.

ZV
 
DeviousTrap said:
Thermostats are designed to fail open, not closed. Sounds like yours is stuck open.

That's a problem and it should be replaced, but you might have something else wrong. A stuck open thermostat would be too much cooling, not overheating.
Click to expand...

Nope. Wrong, wrong, and wrong.

You sure live up to your name.

To OP: Since you already have the thermostat out, replace it with a new one. Then do a compression test to see if you have a leaky head gasket. If the compression #s aren't even across the board then you've found your problem. If they are even, then the other possibly culprits are the water pump, leaky radiator, or clutch/electric fan.

You will also need to bleed the system of any air bubbles as that could cause issues too.
 
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