Engine overheating

[dormirnoir]

I am haveing a problem with my 2000 outback overheating. Before i replaced the radiator I had problems with not getting any AC or Heat in the cabin. It wasn't until about 4 weeks ago I was haveing the overheating problem. So I replaced the radiator and the hoses and the cap. I filled it up with fluid and and went on my way. the car still continued to over heat. Now for the conditions. It's more prone to overheat when the heater in the cabin is running (yes it is blowing hot air and AC) I noticed there wasn't any fluid in the resivoir when I had filled it initially, so I kept topping the radiator off only to have it flow into the resivoir and pour our of the pinhole at the top. I also noticed that the coolent hadn't been recycleing back into the radiator from the resivoir for 4 days and then sundenly decide to suck back in. Both conensor fans are blowing. The top hose does get warm to the touch, but I can still grab hold of it.. There is no milkyness to my oil ( I hear that is a synmptom of HG). I put it up on ramps today cause a friend who rebuilds engines says I might have an air pocket. Idled the engine an noticed bubbles in my resivoir. could these problems be a symptom for air in the system?

[sinisterplaydough2]

So, are you saying the blower motor is working but you're not getting heat in the cabin? Or you are getting heat in the cabin? Have you tried replacing the thermostat? If you do, make sure you get one from Subaru. Aftermarket ones don't work well in these vehicles.

[dormirnoir]

yes, i do get heat in the cabin. as well as the cabin fan is working

I have an update. I took the car down of the ramps and took it for a drive. the car didn't overheat until I turned on the cabin heater. When I saw the emp gauge rise, i switched off the heater and the temp returned to normal. My mechanic friend is also of the opinion that the thermostat is faulty.

[plain OM]

Yes, perhaps the thermostat is not functioning properly, or, maybe, the heater core is partially blocked.

See this post: cooling system "theory"

[grossgary]

turning the heat on causes it to overheat? that is so bizarre, never heard of that before.

oil/coolant mixing is not a headgasket failure symptom for this motor, it can be for others but doesn't happen with subarus or that particular engine. each engine can have it's own headgasket failure symtpoms.

yours typically leaks externally. but if the headgaskets have ever been replaced before then that opens up alternate failure modes.

[subiesailor]

Failed heater core dumping coolant or a failed heater line in the engine bay dumping coolant would be suspect if this happens when the heater is turned on given the heater is just another radiator shedding heat. You actually use full heat to cool an engine thats over heating so to have the opposite happen something is happening with the heater system and its impacting the coolant flow.

[plain OM]

turning the heat on causes it to overheat? that is so bizarre, never heard of that before.

Yes, it seems counter-intuitive, but the way the heater circuit is set up, it could happen. See my link in post #4 above.

As long as the heater fan is off, the hot coolant coming into the heater core from the crossover pipe goes out from the core and back to the water pump inlet (and thermostat sensor) at almost the same temperature. This helps keep the t'stat open according to the coolant temperature, and the "balance" mentioned in the "theory" post.

If the heater core is partially blocked, as long as the heater fan is OFF, the hot coolant going back to the pump will still help to regulate the t'stat. But when the heater fan is turned on, the coolant returning to the pump is cooled, and this could cause the t'stat to narrow it's opening, reducing flow through the radiator.

At that point, given that the heater can sometimes be used to cool an overheating engine, we might think that the same would apply with the partially blocked core, but it might not. The partially blocked core will cool the returning coolant, but only the amount that is able to flow through the core, which is, presumably, far less than a properly flowing core.

As noted, it's a "theory" and seems to explain what happens, but it hasn't been reliably tested.

[DavidPeab]

Could an air bubble in the system cause this? Did you "burp" the system after refilling the coolant?