So my car keeps overheating if I drive 30-40 minutes on the highway or 10-20 (depending on how hot the day is) minutes city driving with the a.c. on. I'm not sure what the problem is I have replaced the thermostat and the water pump and I've had it checked out by my mechanic 3 times and we can't find the problem. should I take it to a different mechanic?

Some background on the car is this helps. it has 350,000 miles on it it's from Oregon but just moved to Las Vegas. Could it also be the weather change since it is its first summer in this type of heat?

I really have no idea and I love this car so I'm trying to figure it out.

 

Replacing the radiator cap is pretty easy. The cap might not be holding pressure, which will cause the coolant to boil at a lower temperature. Cheap and easy. Do it cold.

If that doesn't solve the problem, the radiator is next. The radiator might have biological fouling or hard water deposits that keep it from transferring heat as well. It may have plugged passages that essentially make it a much smaller radiator. You never know what the previous owner might have used for coolant.

 

Just a thought. You car was built before the switch to R124a refrigerant. It originally used the R12. I converted a car once and never got it to cool properly. That could possibly have something to do with cooling when the AC is on.

 

Do the cooling fans turn on? If not you might have a dead sensor or fan controller. If you do buy a new radiator, make sure it has a thick core, not some cheap 1/2" one.