Hey guys, i have a 1990 subaru legacy wagon that i bought about 10 months ago. it was running great up until about a month ago when it overheated the first time, i drained the radiator and put new radiator fluid in it and it was fine. well two days ago the heater started blowing cold air on the hottest highest setting, and the engine overheats. I'm not sure what it is i checked the trouble codes and 21(coolant temperature sensor) is being thrown but i previously switched out that sensor three months ago.

Any suggestions would be helpful. also i am new to mechanics so please dumb it down a bit. Thank you

Make sure that your fans are turning on and off. Unfortunately its very possible its a head gasket, that's what it sounds like. When it starts overheating pop the coolant resiviour and see if you see bubbles coming up. If there are bubbles you have a bad head gasket.

I suggest you search head gasket and read up on how these things blow gaskets.

It could be an air bubble in the cooling system.  Put the car on some ramps and burp the radiator to see if you have air in the cooling system.  Certainly worth a try.  Replace the temp sensor if it keeps throwing the code also.

fans blowing cold air, not hot means.....

the cooling system is low on coolant.

it may be bad head gaskets,

but i would bet on a simple cooling leak.

either one could cause a loss of coolant and overheating.

 

1st,

refill the system correctly,

remove the vent plug,

fill slowly, at rad cap,

install vent plug,

idle until the temp is normal,

top off rad and then cap radiator.

and drive it.

 

2nd,

when / if it over heats,

open the overflow bottle for the rad,

and look for ''bubbles'' in the coolant.

this would be a ''bad head gasket'' indicator.

 

3rd,

when you come home after work,

park and let the engine idle for a while.

and place a large clean piece of cardboard under the engine.

look for stains and drips .

 

 

given the age of the car / coolant hoses,

my first guess would be a slow leak.

Edited November 18, 201510 yr by johnceggleston

Thermostat stuck closed.

I had the same problem. Turned out there was very little coolant in the system. If you do a search on this forum you should be able to find the correct way to re-fill the cooling system.