Why is my van overheating only at high speeds and cools down if heat is cranked up?

Question

I have a 2004 Mazda mpv. I was rear ended over the summer and all of a sudden it started over heating. ( I had just purchased two months prior) I put it in the shop and was told it was air in the lines from the water pump being replaced. They fixed it and two days later it's over heating again and leaking everywhere. I take it back again and they tell me there's a small hole in the radiator but will take a week or so to get the new one. In the mean time I used stop leak which worked for all of one day before overheating again. My friend replaced the radiator, thermostat and quite a few hoses. It drove great for a day. Then same thing over heating while idling or driving even for a few minutes. He then replaced some other part that regulates the fan or something. It was fine after that for a few days until I used the air or drive at highway speeds. Then it was cold so no need for air. I stayed off the freeway until I got a flush. Got the flush, air filter and oil change and it was driving great even on highway until yesterday and now it's getting hot again if I do over 60mph but if I blast the heat it cools down right away. Also it smelt like oil burning while idling but started to go away once driving. Any ideas? Could I need another flush due to the stop leak or is it possible that thermostat was bad too? Or the wrong one? If that's possible.

Answer 1

The symptoms you describe are classic of an underperforming cooling system - when you switch on the heater, you're using heat from the engine to warm the cabin, which helps to cool down the engine.

I'd check that the radiator you have is the correct one for the car, and that it's clear (nothing blocking the airflow through it), and that the system has been properly bled. It's probably worth doing another full flush to get rid of the stop-leak.

You can check the thermostat by putting it in an old saucepan full of water, and heating it up - as it gets to the marked temperature, you should be able to see it open - also check that the marked temperature is correct for the car.