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Discussion Starter #1
2003 Impala 3.4

So...I replaced the radiator cap last April, the radiator in May or June, and the thermostat on Wednesday (with a no-fail) and I go to work this morning and the car hits the red zone twice on my 18 mile journey.

I had heat when the engine warmed up this morning but none when it overheated. I think when I last checked the engine, the top hose was hot and the bottom was warm.

Not sure where else to go. Could it be the water pump? The heater core ( I do not see leaks on the passenger side)? Or is it definitely a head gasket?
 

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Okay, so on the drive home I figured out the problem.

My thermostat failed. Luckily it was the fail-safe so it stuck open, but the car now drives at a lower temperature - anywhere from 155-170 on the highway and closer to 185 in the city.

I've heard this can decrease gas mileage, so that will cost me a lot in the long run if I don't figure out the problem. Can anyone tell me why my thermostats are failing? I'd replace this one but if I do I may have to keep doing it week after week and that'll be worse than having to pay more for gas.

Also, I noticed I placed this in the wrong section. I was on my phone and I couldn't remember which section was correct.
 

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There is a reason the car is running hot.
It may be that the thermostats are made of Chineseium, or, you have a cooling system problem with the car. Head gasket, water pump, etc.
Did you properly burp the air out of the system ?
Are the cooling fans starting and running?
You say you don't have leaks on the "passenger side" I assume that you don't have any leaks at all ?
Is the coolant leaving the system ?
If your having to add coolant and it's not dripping on the floor then it's either going into the crank case or out the tail pipe which brings us back to head gasket.
A car that gets to 180+ degrees with the stat stuck open is pretty warm.
How does the oil look ?
How does the coolant ( if any is left ) look in the recovery/expansion tank ?
 

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Discussion Starter #4
Car no longer runs hot. It runs cold and I'm not losing coolant. Fans don't kick on til a certain temp as designed, so engine gets warmer in city traffic but does not exceed 200 degrees at any point. On the way to work this morning, car stayed around 140 for 15 minutes until I hit a few small towns. Then it hovered around 170 until I hit the city where it stayed at 195.

It will not overheat. The failsafe is stuck open.
 

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Discussion Starter #6

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I think I would start by removing the current thermostat and confirming that it has in fact failed. If a visual inspection does not show it open, then you can check it's operation with a juice can of water, and a thermometer on the stove and a small piece of wire around the stat so you can lift it up to look.
As the water heats up to 195 you should see it start to open and by 200-210 it should be fully open. If it opens before 195 then I would suggest it's defective.

The "jiggle pin" allows for any trapped air below the stat to escape and into the upper rad hose where it will eventually make its way out.
Kind of like a self clearing vent of sorts. Some of the thermostats will have a small notch in them to achieve the same thing.
 

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I think I would start by removing the current thermostat and confirming that it has in fact failed. If a visual inspection does not show it open, then you can check it's operation with a juice can of water, and a thermometer on the stove and a small piece of wire around the stat so you can lift it up to look.
As the water heats up to 195 you should see it start to open and by 200-210 it should be fully open. If it opens before 195 then I would suggest it's defective.

The "jiggle pin" allows for any trapped air below the stat to escape and into the upper rad hose where it will eventually make its way out.
Kind of like a self clearing vent of sorts. Some of the thermostats will have a small notch in them to achieve the same thing.
Okay, so let's say it has failed (which I am almost certain).

My plan is to take this one out and flush the system, then replace it with a different thermostat.

Are there any thermostats you would personally recommend to avoid this in the future?
 
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