Overheating & coolant loss in 1998 Subaru Outback

Question

I'm having trouble with this vehicle overheating, presumably from losing coolant. The problem is very similar to what's described here:

http://community.cartalk.com/discussion/2285268/98-subi-coolant-overflow-tank-keeps-overflowing

The overflow tank fills up, and coolant spews out around where the host goes into the overflow tank. I'm well aware that this issue is usually caused by a blown head gasket, but I've run a test for combustion gasses in the cooling system and it turned up negative. I'm going to repeat it at least one more time in case the leak is sporadic, but I'd like input on whether there are other causes for this problem, and how to diagnose them.

The usual details for overheating issues:

• fans work fine
• no visible leaks elsewhere, but some of the hoses do look like they're in bad condition
• thermostat seems to be opening (coolant is clearly flowing, and I think the vehicle would overheat before the coolant level drops dangerously low if it weren't opening)
• coolant is "clean" (no oil/sludge/etc.) but does seem to have a small amount of shiny metallic particles in it, suggesting maybe a stop-leak product was used by the previous owner

Could it be something as simple as a bad radiator cap or hose to the overflow tank not allowing coolant to return properly? One thing I notice is that, after the vehicle has cooled down, there's a lot of suction (internal pressure lower than atmosphere) when I go to remove the radiator cap, but perhaps that's just because the coolant is low and the air gap between the coolant and overflow prevents the cooling of the fluid from pulling back the overflow...

Ideas?

Update: I tried replacing the radiator cap and cleaning the hose to the overflow tank and the problem remains the same.

One odd thing I noticed is that that overflow tank is not sealed/pressurized at all. On every other vehicle I've seen, the host into the overflow tank went through the cap, and everything was (at least loosely) sealed so that some pressure would build up in the tank, which presumably aids return flow. On this vehicle, the hose just fits loosely through a hole in the overflow tank, and it's completely unpressurized. Is this normal?

Also, I should mention that after the system cools, the radiator cap is very tight and difficult to remove. When I do remove it, I hear gurgling from the overflow tank. It feels like there's negative pressure (lower than atmospheric pressure) in the cooling system before opening it, so my first guess would be a bad cap, but replacing it didn't help. Is there a good way I can test and make sure it's not actually still holding positive pressure in the system? That would definitely explain the return failure, but I don't have any good answer for why there would be positive pressure in the system after it cools. The obvious answer would be combustion gasses getting into the system, but I've run the Block Chek test again two more times and turned up nothing.

Answer 1

some of the hoses do look like they're in bad condition ... coolant is "clean" (no oil/sludge/etc.) but does seem to have a small amount of shiny metallic particles in it, suggesting maybe a stop-leak product was used by the previous owner

I think you've already called it: this sounds like a partially (or totally) clogged return from the overflow tank.

I'm basing this assessment on your check for possible head gasket problems (so far, so good) and your remark about negative pressure in the radiator (so we'd expect coolant to return from the tank).

You could try to rinse out that hose but, since you've already noticed old hoses, I'd recommend pulling the whole expansion tank and replacing the connected hoses.

EDIT to adapt to additional information in the question and the comment:

If the hose and tank seem clean then it sounds as if there is a clog further downstream. I'm basing that assessment on:

... the radiator cap is very tight and difficult to remove. When I do remove it, I hear gurgling from the overflow tank. It feels like there's negative pressure ...

We expect negative pressure in the cooling radiator. That's what should pull coolant back in from the expansion tank. Unfortunately, it sounds like the negative pressure is fighting against something that's preventing that flow.

I'm assuming that you've confirmed that the flow path is clear from the expansion tank back to the radiator (it sounds like you have). If so, it sounds like it's time for a coolant system flush. I'm expecting that you'll see a bunch of yuck in the flushed out coolant.

Answer 2

The pressure ended up being positive, not negative - it was not very extreme with the vehicle cool and hard to tell the direction, and I just observed the hissing/gargling sounds. So I took it to a radiator shop I trust and they redid the combustion leak tests, apparently better than I did, and found that the problem is combustion gasses in the cooling system.

I'm trying to seal the system with Blue Devil, and not too hopeful about it working, but in any case I opened a new question about using it:

Getting the best results sealing head gasket leak with Blue Devil