Q: Top radiator hose is cold while the lower is hot

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I believe it may be the new thermostat that installed

My car has 107000 miles.
My car has an automatic transmission.

You may be correct that the thermostat is stuck closed. One way to test a thermostat is to put it in a pot of water and bring the water temperature up to the thermostat opening temperature. This is imprinted on the thermostat. If the thermostat does not open or opens more than 5 degrees late, then have the thermostat replaced. If the thermostat is open as it should, then you have another issue of no or low circulation in the system.

This can be a restricted radiator or a water pump with worn off impeller fins. If you remove and test the thermostat to be okay, then flush the radiator with both hoses disconnected to see if a lot of scale or debris is blocking it using a garden hose. It is harder to test the water pump. You will need to remove it to check it, at which point you may as well replace the pump. The radiator can be checked for hot and cold spots when it is running to see if it has plugged passages and needs replacement. If you want to have this issue diagnosed before moving forward with any repairs, a technician from YourMechanic can come to your home or office to inspect your car and let you know what adjustments should be made.

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