Expert: Rob Painter - 6/20/2011

Question
My question is why would my car over heat while i am driving slow in town or at a stop light? It doesn't have water leaks and fans are working.Could it be a sensor or the water pump? The thermostat has been replaced and radiator has been checked and the mechanic does not know what else it could be other then the water pump

Answer
Hello,

Every time you are overheating that aluminum engine, yo are doing damage internally to it. You state it doesn't have any leaks. Maybe it didn't the last time yo checked it, but maybe it does now. These engines are real problems to diagnose for over heating. It could be a broken impeller on the water pump. It could be a circulation issue where there is a restriction in one of the water jackets. These engines are so sensitive to restrictions, that even a small piece of RTV from replacing a water pump or a thermostat will block the flow of the coolant.

as uncomfortable as it is-it can be 110 degrees outside, just temporarily run full heat and you will see the temp gauge go down and that is just to prevent you from blowing the head gaskets!
Bottom line, whether is is a water pump or something else, yo are not getting the circulation yo need.

You said the mechanic checked the radiator. For what? Leaks or circulation? Did he pressure test the system to make darn sure yo didn't have a leak? The radiator cap to see if it holds pressure?

Most mechanics treat this aluminum engine like any other engine and yet these are the ones that that are easily destroyed by issues like yo are having.

Yo need to make sure that this system holds 15 pounds under a pressure check. If not, you do in fact have a leak somewhere in the engine. A cracked head, cracked block a bad head gasket. Who knows until you find it.

These engines are powerful and fuel efficient, but a nightmare to diagnose over heating issues. In other words, it is an engineering marvel or the biggest piece of junk a person can own.

As for a sensor issue for being bad, no. If the fans are coming on, which operate in the city or slow movement, that rules that out.

It's ashame that you have to be the Guinea pig on the mechanic's guess of a bad water pump, but I am not the one working on the car. If the water pump has a broken impeller, the coolant will not circulate, but I would expect to see this not only under the driving conditions you state, but overheating at highway driving too.

What happens when the water pump gets replaced and you have the same issue? Then what?

Before replacing the pump, I might recommend checking the system with a pressure tester first. Then doing a back flush on the cooling system to see if there is some kind of crap in the cooling system restricting it, that maybe you can get it out. Yo have nothing to lose because that takes doubt out. Might fix the problem entirely withot the replacement of a water pump.

I have been very successful with my answers involving this style engine I have dealt with since it premiered in 1982.

Please do me a favor and let me know what happens. I am very curious.

http://www.autotheftexpert.com