# thermostat only opens after i shut off the car



## breezesboy (Oct 18, 2020)

2014 1.4
replaced the water pump due to leak. After purging the system and running the car i took out my temp gun and shot a few things to check the function. My water pump hit a temp of 221, but the thermostat hose only went to 105 (ambient temp). I thought "well hell, i should have replaced the thermostat too" then shut off the car. Immediately the thermostat opened. Ive checked this the same way 3 times, every time is the same. Thermostat stays closed while running, opens when shut off. Im not crazy for thinking something is wrong right? My car is not overheating, no codes are thrown. anyone know if this is normal activity? is the computer keeping my thermostat closed instead of allowing it to open? My assumption is bad housing, but why only open after the car is off. I plan to replace the housing, but i want to understand whats going on here to cause it to stay closed during operation and open after shut off. Thanks!


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## snowwy66 (Nov 5, 2017)

Monitor the ecm and see what it says. 

Bluetooth dingle and torque pro app.

You might get an accurate story over a gun.


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## Taxman (Aug 10, 2017)

Isn't the thermostat somewhat computer controlled?
If so, does the computer open it or hold it shut?


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## plano-doug (Jul 1, 2015)

Taxman said:


> Isn't the thermostat somewhat computer controlled?
> If so, does the computer open it or hold it shut?


The computer controls a heating element in the thermostat housing.

By turning the heating element on, the computer can force the thermostat to open early, at a lower water temp than the thermostat would otherwise require.

Doug

.


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## breezesboy (Oct 18, 2020)

plano-doug said:


> The computer controls a heating element in the thermostat housing.
> 
> By turning the heating element on, the computer can force the thermostat to open early, at a lower water temp than the thermostat would otherwise require.
> 
> ...


Right but not the other way around? Will A faulty housing it hold it shut when it should otherwise open?


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## plano-doug (Jul 1, 2015)

breezesboy said:


> Right but not the other way around? Will A faulty housing it hold it shut when it should otherwise open?


Heat opens the thermostat. Either the hot coolant, or the heating element. If there is not enough hot coolant trying to get past the thermostat, it won't open. Or if it is somehow malfunctioning. But, If there's enough coolant in the system, and the water pump is working, it should open eventually. 



> Thermostat stays closed while running, opens when shut off.


How can you tell it's opening?

Doug

.


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## breezesboy (Oct 18, 2020)

plano-doug said:


> Heat opens the thermostat. Either the hot coolant, or the heating element. If there is not enough hot coolant trying to get past the thermostat, it won't open. Or if it is somehow malfunctioning. But, If there's enough coolant in the system, and the water pump is working, it should open eventually.
> 
> How can you tell it's opening?
> 
> ...


Hose off the thermostat just over 100 while running, instantly jumps to 200 when car is shut off


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## snowwy66 (Nov 5, 2017)

I'd be monitoring the pcm instead of the hose. Things get hotter when shut off and circulation stops.


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## Snipesy (Dec 7, 2015)

Complicated fluid dynamics. Honestly I’m not sure I even understand it all.

But no your thermostat is not opening. The water in the radiator is just sitting isolated from everything, unable to spread to everything else. Technically yes everything meets at the water outlet block buuut they barely touch each other.

When you turn off the water pump,thermal equilibrium takes over. All the hot water moves its energy to the colder water... And that makes its way to the water in the radiator. This gives you the illusion the thermostat opened. It’s not.


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## plano-doug (Jul 1, 2015)

breezesboy said:


> Hose off the thermostat just over 100 while running, instantly jumps to 200 when car is shut off


Here's a drawing I have for the 1.8's cooling system. It shows the flow from the head thru the thermostat to the radiator. 

I cannot explain why your thermostat only opens after engine shutoff. I know motors can heat soak after the engine is shut off, and hence the cooling is shut off. But in your case that would imply the cooling had been working before shutoff which means the thermostat had already been open.

Doug










.


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## snowwy66 (Nov 5, 2017)

Yep. He probably just happened to measure the temp at the exact moment the thermostat opened.


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## jblackburn (Apr 14, 2012)

The thermostat _conventionally_ fully opens (melts the wax ring) in the 219-225F range. Fan kicks in at 230F. In your case, it could be opening due to heat soaking around the cylinder head.

The heater circuit can request it to open basically whenever the ECM wants it to though. You'll see 219 a temp that they run at often.


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## Jhon (Aug 26, 2019)

breezesboy said:


> 2014 1.4
> replaced the water pump due to leak. After purging the system and running the car i took out my temp gun and shot a few things to check the function. My water pump hit a temp of 221, but the thermostat hose only went to 105 (ambient temp). I thought "well hell, i should have replaced the thermostat too" then shut off the car. Immediately the thermostat opened. Ive checked this the same way 3 times, every time is the same. Thermostat stays closed while running, opens when shut off. Im not crazy for thinking something is wrong right? My car is not overheating, no codes are thrown. anyone know if this is normal activity? is the computer keeping my thermostat closed instead of allowing it to open? My assumption is bad housing, but why only open after the car is off. I plan to replace the housing, but i want to understand whats going on here to cause it to stay closed during operation and open after shut off. Thanks!


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