Jim Grant's Tech Tips

'90 Subaru Legacy, Overheating Problems

Q: My '90 Subaru Legacy won't stop overheating. I have replaced the water pump and thermostat. I've corrected the engine timing and put on a new timing belt. I have replaced the radiator, the car has not lost any power and there is no water found in the oil. The car will idle for hours without overheating, but as soon as you're on the road and accelerating, the engine overheats within 2-4 minutes. Any ideas on what else to check? The local dealerships and garages are stumped and frankly so am I.

- Zack

A: I don't mean to be the bearer of bad news but you have either a headgasket problem or a crack in a cylinder head. You've covered the bases. For an engine to heat that quickly under load there is combustion gases entering the cooling system. Here are a couple of tests you can try. There is a chemical test for the cooling system that detects the presence of CO2. This chemical test is really good at proving that there are combustion gases entering the cooling system. To pin down which cylinder is the problem remove all the spark plugs and pressurize the cooling system while the engine is hot (We use regulated shop air for this test) and allow the test to run for up to 24 hours. At the end of the test period crank the engine over. If there is a problem you'll see coolant splashing out of one of the spark plug holes. Failing either test in a Subaru means the engine has to be extracted from the vehicle and the cylinder heads removed for inspection.