1990 subaru legacy mysterious overheating problem



I have a 1990 Subaru Legacy, 120k, overheats only after freeway travel over 60 miles. Get off the ramp, redline. Turn on the airconditioner, temp returns to normal. I have: changed the thermostat, then just punched it out. Add coolant about every 200 miles. No visible leak, no oil in water, no water in oil. Water pump pumps, radiator appears clean.

I check the water every time I go somewhere, and pay close attention to the temp gauge, but it is only a matter of time…

any ideas?


You should NEVER operate an engine without a thermostat. NEVER.

I think you may have a blown head gasket. The coolant you are replacing every 200 miles is going out the exhaust pipe as steam. That’s why there’s no visible leak.

Are you sure the cooling fan(s) are working? What happens if you let the engine idle for a while?


I agree with McP. This is a classic headgasket symptom set.

The reason you’re not seeing fluid exchance between the oil and the coolant is likely because the breech is between the water jacket and the cylinder with no breech in the oil paths.