Cooling System Puzzle

I don’t understand what I’m observing in my cooling system. When driving without stopping, the temperature gauge is exactly where it belongs; however, on stopping to inspect the radiator, I find that the expansion tank is filled to overflowing, with air bubbles bubbling from the overflow hose.

If I shut down and let the engine cool down fully, I expect the coolant to get sucked back into the radiator, but it doesn’t: I’m left with a filled-to-the-brim expansion tank, and a radiator that wants more coolant.

Also, if I drive in a stop-and-go situation (i.e.: traffic lights and stop signs), then the temperature gauge will climb into the red until I get going at a regular, steady speed.

I replaced the thermostat, but that changed nothing. The fans work perfectly.

Does anybody have any idea what this could be?

Oh oh, this sounds like the common Subaru owner complaint we hear here, failing head gasket. Make sure the coolant system is full, all the air has been removed, and the engine cooling fans are working properly. If all that’s ok, a cooling system pressure check is next.

fyi, OP. you can use the forum search feature, icon upper right this page, to see what other Subaru owners here have said about cooling system problems. Which engine option do you have?


Head gasket! I hadn’t thought of that. Wow, that really sucks if it’s the head gasket.

My DIY cylinder head jobs are decades behind me. I can’t face doing that work anymore. This is going to cost me some bucks.

I would think that, if the engine is pumping air into the cooling system through a failed head gasket, that the idle would be a little rough, but it’s not. I did get a p-code 0303 (cylinder 3 misfire), though. Maybe that’s consistent with the head gasket theory.

1 Like

Yeah, common Subaru problem. What year/miles?

Double oh oh … lol … sorry, not intending to poke fun of your problem, but the possibility it isn’t the head gasket at this point it isn’t looking good.

Which engine option do you have. Some engine have this problem more than others. Are you loosing any coolant?

Very likely head gasket failure; check for combustion products in the radiator space to prove. If that test fails to show blown head gasket, it could be water pump sucking air and blowing coolant into expansion tank. Good luck.

Watch this video.

I’ve seen this occur a few times over the past fifty years.


Expensive to fix, too. Big downside of the boxer engine design, there’s no other way to get the heads off than to pull the whole engine. At that point you might as well just have your mechanic go over the whole thing with a fine-toothed comb and fix anything else they find wrong with it: I had that done on my '01 Forester at about 160k.

I wasn’t getting coolant in the oil or exhaust in the coolant, but I sucked coolant into the cylinder at least once and got a code about a cylinder misfire like you did.

2005 Forester
≈ 180,000 miles

That’s a prime year for head gaskets problems. Find a Subaru specialist and get a quote.

I don’t see how the water pump could be pumping air into the cooling system.

Even when the engine cools down, the cooling system still has big air pressure in it, with coolant level below the radiator neck. This suggests to me that air is being pumped into the cooling system on the compression stroke.

When I had a head gasket issue on my Olds, it would over-heat a low speed and be ok at high speed when there was enough circulation to overcome the air bubble. Toward the end, I had to take the radiator cap off to make it home.

Just for the record though, when the coolant heats up and expands into the overflow tank, if it doesn’t get sucked back into the radiator, that indicates a leak somewhere, like the cap. Likewise if you see bubbles, that indicates air in the system from either not bleeding the air out or from the combustion gases being forced into the coolant. Never fun to see this stuff.

It’s possible it’s just the radiator cap not holding pressure.
From a cold start the radiator hose should get stiff after just a couple minutes of idling.

Won’t this be covered by that Sub warrantee?

2005 Forester
≈ 180,000 miles

Long past the warranty.

1 Like