Weird Overheating

[jdaxford]

Good afternoon -

 

my happy 2000 Forester was running perfectly, until yesterday, when the heater started blowing cool air while at a red light. Once moving, it warmed up. I got on the interstate, and soon noticed the heater air cooled off if I coasted - so, 50 mph, engine barely more than idling. But for two miles at 60 mph, the gage was normal. Then the gage pegged to hot.

 

I pulled over, let it cool thirty minutes, started up, made it a block before the gage pegged. I waited an hour, then tried again and made it about home - about ten miles. It would overheat and the heater would blow cold at red lights, but once moving, it immediately cooled down. The overheating at stops is as if the fans didn't work, but I don't get the heater being cold.

 

I bet on a sticking thermostat, thinking higher engine speeds spun more water and opened it -  so I replaced the t-stat and both rad hoses, then did a chemical clean-and-flush. The system seemed clean, and is now cleaner.

 

Also did an engine flush and changed the oil and filter.

 

Then I drained all the water out and added in a gallon of antifreeze and topped it up with water.

 

It warms up normally and the fans operate normally while parked and idling.

 

There is a bit of steam coming from the fill-neck area, but from just below the tank bottom seam. I imagine there's a minor leak, but I watched it carefully and wasn't really losing water.

 

With the engine at normal operating temperature, I removed the rad cap. At idle, it didn't look like water was moving through the top tank. If I revved the engine, I could see water coming in fast from the top rad hose - on the passenger side - but it seemed to come from a small orifice - not a big hose.

 

The overflow tank doesn't seem to do anything but sit there and bubble.

 

Still, it idled for twenty minutes, and the heater worked and I couldn't see any leaks, besides the steam leak

 

So, I drove a two-mile loop: the gage was pegged to hot when I got back and the heater was blowing cold. I shut it off, confirmed water was visible under the cap, and started it up. It immediately returned to operating temperature - the gage dropped to normal operating temp in about ten seconds, where it stayed, idling for twenty minutes, fans cycling normally.

 

The system does not seem pressurized when I remove the cap. I put a heavy rag over the cap, expecting the usual dangerous whoosh of steam, but there was nothing, not even a hiss, as if the system was not pressurizing. I don't think the steam leak is preventing pressurizing; the steam wafts out like from a boiling pot. It doesn't jet out, like from a pressure vessel. 

 

I think I need a new cap to ensure it seals, and a new radiator because of the steaming leak - but I'll try the cap first and see if the steam stops.

 

But why does the heater blow hot and cold? It's as if when the engine is hot, the water quits moving.

 

Past t-stat failures have been with them refusing to open, and I have vague memories of having one fail open, but I've never had one act sticky. And water pump failures have been noisy and leaky at the pump.

 

But that's been on Chevy, Jeep, and Toyota...is Subaru different?

[keith3267]

Subaru's are notoriously difficult to bleed all the air out of the cooling system.  Your radiator is about the age when the rubber seals between the tanks and the core begin to fail.  What appears to be a steam leak is really a full on leak.  It can also be responsible for the system failing to pressurize.

 

Since the Subaru engine is a horizontal configuration, that puts the heater core as high or higher than the engine, so an air bubble will migrate to this area, hence the cold air.  The air bubble plus the lack of pressurization is responsible for the overheating under  load.

[jdaxford]

Thanks, Keith - sounds like a new radiator, and a more careful filling and bleeding. Any idea how large the "bubbles" would be? Seems they'd have to displace a lot of water to get the heater to blow cold.

 

Does air entrainment/entrapment account for the inconsistency? For the ten-mile ride home before fixing it, it overheated at redlights, with cold heater air, but operated normally when moving. In my only test drive, it was the opposite.

 

Still - I'd forgotten they needed careful bleeding....like a VW water box!

[montana tom]

When you refill , remove upper hose from the radiator and pour anti freeze directly into the block,after filling the block reconnect upper hose and finish filling from the radiator.As you fill the rad gently squeeze upper hose to get the last of the air out. You wont have an air pocket if you do this.  

[1 Lucky Texan]

hopefully, you installed an OEM style t'stat.

 

was coolant pushed out of the overflow?

 

bubbling in the overflow or radiator is often a sign of a bad headgasket.

 

there is a chemical test for bad HGs that a few people have found helpful.

 

sometimes a leakdown test can find the bad cylinder.

 

always check the fluid level in the radiator (safely, when cool enough of course) don't trust the level in the o'flow.

[Fairtax4me]

Constant bubbling in the overflow container generally is a symptom of a failed head gasket.

The original cause of the overheat was likely just a low coolant level. The overheating has compromised the head gaskets.

[DaveT]

+1 to what fairtax4me wrote. It has been my experience with EA82 engines.