Q: Car stays cool while idling but once you start driving it it starts to overheat.

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Barely rebuilt the stock 305 in my car had a mechanic install it & everything car ran good for two weeks & then it started to overheat. I've replaced two thermostats, a jet temp cooling fan switch which kicks on at 190-195 & thats when I replaced the 1st one for one that opens at 195. Put in a brand new radiator flushed the system checked for air & inserted more coolant it seems to get not as hot anymore but it still is. Maybe should I try to replace the water pump? Thanks
My car has a manual transmission.

If the cooling fan is too small for the engine, then it may be enough to cool it at idle but not at higher engine speeds. Make sure you are not running under drive pulleys. You can check the coolant temperature difference between the top radiator hose to the bottom one using a laser type temperature tester.

This will tell you the temperature going in the radiator through the top hose and the temperature going in the engine through the bottom hose. If the temperature going in and out of the radiator with the fan on is more than a 20 degree difference, then the cooling system working okay and you may have a worn water pump causing reduced flow through engine. If you do not see the radiator temperature drop but by 5 or 10 degrees then you have a weak fan or clogged radiator. To have this issue inspected before making a repair, consider YourMechanic. They will be able to diagnose your overheating issue and let you know what should be done.

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