Thermostat closing when starting to drive, please help

I’m having this problem for almost 2 months now and literally no one can figure this out, I really need help with this…
The car is Mitsubishi Lancer 2011 1.5L.

So the thermostat wont open at all when the car is on a drive and when the car is still, its working as it should most of the time.
Problem is that the car gets hot everytime it happens, the fan starts working and doesnt stop becuase it gets hot becuase the thermostat isnt opening…

The engine has been replaced twice, thermostat has been replaced dozen of times, radiator is new, water pump new, basically evrything is new but the problem still exist.

I have never seen such a problem, I cant understand this, thermostat is a mechanical item, what makes it to be close when driving and letting the car to get too hot?

Thanks in advance to everyone…

Time to replace this vehicle with something new in your budget or at least a much newer vehicle.

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I have already spent too much money in this, I won’t give up now… but I won’t replace more stuff before having a close enough direction to the solution.

Remove the transmission dip stick and look at the color of the transmission fluid.

If the color is black and the fluid has a burnt smell, the transmission is overheating causing the engine to overheat.



I don’t really understand how its related to the thermostat not opening, but the transmission oil is not dark and with no smell, its brown ish

There’s a transmission fluid cooler inside the radiator.

If the transmission fluid overheats, it causes the coolant in the radiator to overheat, which causes the engine to overheat.

And some might think there’s a problem with the thermostat when there isn’t.



Where are you located at, USA, UK etc etc?? RockAuto doesn’t show a 1.5L and Napa shows one but no info on the t-stat (unless I am missing something)… Are you sure you have the correct t-stat installed and installed correctly??

You mean with a different used engine, yours rebuilt or bought a rebuilt engine, new engine (not sure one is even available)?? If rebuilt, who rebuilt it??

How do you know the T-stat is not opening?? Possible air pocket in the cooling system…

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If you’ve replaced everything else (and some things twice), and the problem still remains, clearly you’re missing something.

Thermostats are pretty simple items. It’s hard for me to see how multiple thermostats could just work the total opposite of what they’re designed to do.

As Tester said… I don’t know that the thermostat is your problem. And on a 15 year old Mitsubishi… personally, I’d question how much more money to put into it. The money you’ve spent so far is gone, so it shouldn’t factor into your decision going forward.

Good luck.

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What makes you think the thermostat is closing while driving but opening when idling?

An old-fashioned thermostat is pretty easy to test with a pan of water on the stove.


The last thing I would expect is a thermostat to close on its own, after opening as it should, especially after it has been repeatedly replaced. The problem is somewhere else. Fans? Transmission? Wiring?

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im not from the US this type of car sold only in europe, im sure 100% the thermostat is fine after all of those replacements and its the correct type and installed correctly.
2 of the engine were used, but they have been checked to be working fine…
I know the thermostat isnt opening becuase its hose is cold unlike the other hoses…

I didn’t know there is a transmission fluid, but I will surly check that.

Plugged radiator?

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radiator is new

Have you tried removing the thermostat?

you mean driving without one? yeah but the engine is cold all the time

Have you installed a lower temp t-stat?? like a 160f degree instead of a 195f temp t-stat.

As a temporary test to prove that coolant will circulate. Your stated cause (the thermostat closes) doesn’t make sense, I need proof that is actually happening.


actually there are two types of thermostat to this car, one is 82c and other 87c, i chose the 82…

i have another question… just trying to understand, if the transmission makes the coolant to overheat, why the thermostat still wont open? i mean its job is to open when the coolant is hot isnt it?

If the transmission fluid gets too hot, the cooling system can’ t handle the higher temperatures and the engine overheats.

Engine Temperature Warning Light: While the engine and transmission are separate components, they are interconnected, and an overheating transmission can potentially lead to engine overheating. The transmission relies on the engine’s cooling system to regulate its temperature, and if the transmission overheats, it can strain the engine’s cooling system. The excessive heat from the transmission can cause the engine coolant to heat up as well, potentially overwhelming the cooling system and resulting in engine overheating. Therefore, promptly addressing any signs of an overheating transmission is crucial to prevent increased heat transfer to the engine. Having both the transmission and engine inspected by a qualified technician is advisable to diagnose and resolve any underlying issues, ensuring the proper functioning of both systems and preventing potential damage.