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Is it the thermostate?

I just replaced my radiator less than a month ago and replaced the fan about 4 months ago. Just recently when i acclerate to get onto the highway my temperature gauge goes up and it doesn’t cool itself down untill i turn on the heater full blast. Then it usually stays down. I have about 8 lines on my temperature gauge in the dash board, and i get nervous if it goes over the 3rd line, about half way inbetween hot and cold. Am I harming my engine by turning on the heater full blast to cool it down? I’ve had problems with it overheating alot when i had the old radiator in my car. I think it’s the thermostate but i’m not sure. Is there any way to test the thermostate to see if i need to replace it?

Thermostats are inexpensive and are usually easy to replace. So just do it…

Also, in the morning, when the engine is cold, carefully remove the radiator cap (there should be no pressure) and check to be sure the RADIATOR is full to the brim with coolant. If the radiator is not full, you have a problem other than the thermostat…I’m assuming the coolant level in the overflow tank is at the normal level…

I will check it in the morning before i leave. Yes the coolant level is normal in the overflow tank.

You will have to push down to get the cap to turn…The thermostat is located at the engine end of the upper radiator hose…Under the housing the hose connects to…

Another possibility…The lower radiator hose has a spring in it to keep it from collapsing…Perhaps this spring was lost when the radiator was replaced. If the hose collapses, it starves the water pump…

The radiator isn’t all the way full, I have to add a little bit of coolant every morning. Now sometimes when i try to turn on the heater to cool down the engine, no heat comes out. It was doing that before I got the radiator fixed. What could it be? Would the thermostat make the heat not come on?