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2004 Pontiac Grand Prix thermostat

My thermostat was taken out a few months ago and figured out water pump was bad. I just now replaced the water pump a week ago and had bought a thermostat but just figured after over 300 miles there was no thermostat since it was taken out as I let car sit for months before I fixed water pump. I guess miscommunication caused the new thermostat to be left out as i got the water pump put in and those the thermostat was put in but my mechanic didn’t know he was suppose to do that and I forgot the old one was taken out so that means no thermostat for over 300 miles. I just got the one I bought put in but temp gage only goes up to half way when idling otherwise it’s at the lowest point. What could be wrong and is my motor ok? It seems fine but maybe it’s taking some time for the damage to be really noticable. I think I hear a knock when driving down the highway. I hope it’s ok but I need advice so I don’t sit and stick money in it to find out it’s gonna go bad right away. I don’t know why the temp gage only goes to it’s normal half way up level when idling otherwise driving down road it goes down all the way to coldest. I appreciate any advice on what to expect now or what to do. Can I save my car is the main contact. Thanks in advance!

The only thing that will keep a vehicle too cool is a faulty thermostat or a missing one. You need to make sure you get the right thermostat and replace it. The clunking noise may be a separate issue, and would need more specific description to figure it out.

So the new thermostat that I just put in after realizing there was no thermostat is the wrong one? I got it new from the part store and they said it was the right one. To clear things up then is my motor going to be ok as far as the thermostat not being in
for over 300 miles? I’m reading that no thermostat is bad then I read it should be ok. Thank you for your help!

You should be fine as long as there is coolant to protect the engine.Its a good idea to put the right thermostat in there.

Running with no thermostat may prevent the engine from warming up fully; it will certainly increase the time it takes to warm up. This will reduce fuel economy somewhat, and may set an error code. But no permanent damage is likely to occur in 300 miles. Has the Check Engine light ever come on?


Even though your new thermostat is listed as the correct one, it may have been faulty from the factory. Some people will go to the effort of placing a thermostat in water, slowly heat the water, monitor the temperature with a thermometer, to observe at what temperature the thermostat opens.

Yes the check engine light came on. I had coolant in there since I replaced water pump and started driving it. The thermostat just got put in and I added more coolant. I have to drive it again after adding more coolant to see if the temperature gage goes in
the middle like it’s suppose to. I’m relieved to hear I didn’t wreck my motor by driving without the thermostat. I think that’s what you are saying. The right thermostat is now in there though and I’m curious as to why a car needs a thermostat if you can drive
without one and maybe that is a dumb question but I know coolant and oil are a must and the thermostat is with the is part of the coolant system. You have been very helpful and I greatly appreciate you!



The thermostat prevents the coolant from flowing into the radiator until the engine reaches the correct operating temperature. When the engine is at operating temperature the thermostat opens, the hot coolant flows into the radiator and cold coolant is drawn into the engine. The job of the thermostat is to maintain engine at the most efficient temperature range.
A cold running engine is primarily less efficient, however an engine overheating can have catastrophic damage.

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An engine is designed to run at a specific temperature ranging from 160 to 220 degrees depending on the car. That is called the normal operating temperature. AT that specific temperature the engine will do many things like mix fuel at the best rate and perform to it’s most efficient capacity. When an engine is not hot enough it has very poor efficiency. Too hot, it burns things up. To maintain the best temperature, the thermostat will open when the engine starts to get too hot and lets cool water (coolant) from the radiator into the engine too cool it down. As the water in the engine starts to get too cool, the thermostat will close off and stop the cool water from the radiator until it starts to get too hot again. This keeps happening over and over again keeping the engine at it’s operating temperature.


a mechanic sees a leaking waterpump and does not remove the thermostat normally. they are not connected. literally. if you motor was overheating due to a leaky waterpump than some shotgun mechanics remove the thermostat to make it run cooler. your motor has a bleed valve on the thermostat housing that needs to be cracked open to vent air. some mechanics might think the thermostat interferes with this venting. why? who knows