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I have had my 91 Legacy Non Turbo giving me sporadic overheating issues the past couple months and after troubleshooting day after day I have come to a few conclusions.

I can run the car daily with no overheating problems as long as the heater control is in the off position and the slider is all the way to the cool side.

I do notice however, after a couple days driving with the above conditions, the cooling system will slowly purge into the overflow tank and air will begin to enter the system. This makes me thing there is a leak in the system.

Now, If I run my car with the heater or defrost in the on position, I will get about 10-20 minutes of driving with no overheating and good heat from the vents. Usually after the 10-20 minutes, suddenly the car will overheat and the vents will blow cold air. If I stop the car and let it cool, I check overflow and see that it has filled to the top and the overflow itself has overflowed. And I check the radiator and it is nearly empty.

My theory on that is that turning the heater on is allowing air into the system, the air gets stuck at the thermostat blocking coolant from flowing causing the coolant to spill into the overflow. This also causes the car to overheat due to lack of cool water to the block.

Yes I have an aftermarket thermostat and I plan on changing over to the Subaru t-stat very soon because of the "jiggle valve". The "jiggle valve" allows air to pass through the T-stat and allows the coolant to still flow even when there is air in the system.

My questions are:

1) Does coolant constantly flow through the heater core while the engine is running even if the heater control is set at OFF? Or does the ON/OFF switch control a valve between the engine and the heater core.

2) Because I can run my car just fine with the heater turned off does that mean I should suspect my heater core of being faulty?

I have reasons I have not suspected the heater core already. I have no sweet anti-freeze smell coming from my vents, nor do I have problems with film on the inside of the windows and I have not noticed any wetness or the floors in the front seat.

Thanks for your help

Coolant always flows through the heater core irregardless of the lever position.

You need to get that aftermarket t-stat out. Troubleshooting before you do that is pretty much useless.

Second, when you change the tstat, make sure you remove the bleeder screw from the passenger side of the radiator to allow the system to properly bleed the air out.

Once you do those things, see how the car runs, and if it reacts any differently.

Thanks, I will get that aftermarket Tstat out soon!

Any idea as to why my car overheats almost immediately after turning the heater on? I have no doubt they are directly related, I have been troubleshooting this for months and it happens everytime, and its getting hard to drive without heat!

Thanks

MALCHICK wrote:Any idea as to why my car overheats almost immediately after turning the heater on?

This dosent make sence to me because the heater core is just like another radiator only its in the car, so if you turn the heat on and the blower motor is pulling air past the heater core it should actually reduce the coolant temp if anything at all, when you turn the heat on do you have the HVAC contorl set to Defrost in any way?

Don't have a clue....the aftermarket t-stats make the engine do very weird things. I experienced similar things as to what you described back in 95 or 96 when I put in an autoparts store tstat.

The heater vavle could be sticking at that could help cause it to act differently when running the heater. I have had this issue in other cars but am not sure of the exact heater core and vavle set-up in these cars

There's no heater valve in our cars, so that's not an issue.

have you replaced your t-stat? any improvements?

Legacy777 wrote:Don't have a clue....the aftermarket t-stats make the engine do very weird things. I experienced similar things as to what you described back in 95 or 96 when I put in an autoparts store tstat.

I asume that the problem with turning on the heat is that somehow, it allows more air into the system than when the heat is turned off (exactly how that is, I am not sure) and because of the excess air, and the non-OEM t-stat, the air gets caught at the the t-stat and blocks flow of fresh coolant to the block, thus causing overheating.

I figure I have a leak somewhere, no doubt, the trouble is trying to find it. No visible stains on the driveway when I park it and no signs of a BHG (at least not a terribly blown HG, could be a very tiny HG failure?).

I pride myself in taking care of my own cars and doing all the work myself but I'm contemplating taking it to a Suby shop and asking them to locate the leak, then I can replace what is needed.

Still haven't got the tstat replaced but I really havent had a day off lately that has been warm enough to lay on the driveway... soon hopefully.

That's not really the case about the air.....it happened with mine when I had my issues.

Replace the tstat before you spend too much money on anything.

When was the last time your coolant was changed and your radiator flushed? Are your electric fans kicking on at the proper engine temp?

This is an old thread, but MALCHUCK's problem matches mine exactly. I didn't replace my Tstat because it seemed it was working very well - passed the stovetop test & everything. We even drove 70 mi round trip w/AC on Sat after I got the water pump replaced & all the front end all buttoned up. I had the car on a good incline (about 8" over the 100" wheelbase) and kept adding fluid w/the bleeder off until no air bubbled to the cap. Didn't get fluid to come out of the bleeder, though. That was before the trip.
Then Sat eve, I made a small trip & tested the heat - ICE cold in vent, bi-lev & heat, then the temp gauge rocketed up. Playing with the climate controls kept the temp lower, but well above the "just below half" that the car always runs at. So I parked on a greater incline with the left side up & the bleeder spewed geyser-like for a good 5 min. I added fluid with the engine running and also w/it off. squeezing hoses, & the like. I've only driven it 2 miles since then, w/o testing the climate, but I fear a HG. Besides the dealer, how would I find a good shop here in Tucson to do the compression test?

I'm having the same problem right now. I did a comp test and found that 2 and 4 have a slow leak. I was planning on having the heads shaved anyway. Now I just need to find a spare motor for cheap.

i had this same issue and it was the headgasket as well

If you have the money consider a new radiator or at least getting the old one pressure tested. The end tanks crack OFTEN and the cracks seal themselves up at low pressures making them invisible.

Yup filling up the overflow bottle could be 1 of 2 things

1 Bad rad cap - this happened to me basically the rad cap would allow fluid into the overflow bottle but would not let it back into the rad. where it should go once the car cooled down
2 Head gasket

You could try the Subaru coolant conditioner before you dive into a HG job as that can seal small HG leaks & its just $5.

I'll test the radiator tonight and see. The cap is good. I've basically replaced everything but the HG's. Trying to make it to summer when I'll have Fridays off.