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Why would a bad head gasket cause the coolant reservoir to fill up?

I noticed my car was overheated today, so I pulled over. It only took maybe a minute (with the car off, hood open, and fan on) for the temperature gauge to start falling, so I figured that I probably hadn’t done any major damage. I opened the hood and looked in the coolant reservoir, which was almost overflowing! The next time I looked in the reservoir, it was almost empty.

I waited an hour for the car to cool, then added water to both the radiator and the overflow tank-- over a liter. I started it again, and the temperature was normal, so I drove it to my mechanic.

He took one look at the oily residue around the coolant tank, diagnosed a bad head gasket, said it would be $2-3K to fix, and that the dual overhead cam Subarus tend to have that problem.

He advised me to keep the radiator (not the overflow tank) topped up with water regularly, and drive it until it dies (which will be soon).

My questions:

1) Why was the overflow tank near overflowing when I first opened it up? Did all the fluid somehow get pumped out of the radiator and into the tank? Could this have been caused by a blockage in the system somewhere, which caused the overheating, which caused the gasket to fail?

2) Why will the coolant no longer circulate properly, just because the head gasket is leaking? Why do I have to refill the radiator directly, and not just keep the overflow tank filled up?

3) Is there anything else besides a blown gasket that could be causing the oil in the coolant? (I don’t see any smoke from the tailpipe.)

This is a 97 Subaru Legacy Outback, 125K miles.

I wonder if the problem is the water pump. If it is not circulating the water the car over heats and coolant overflows into the contain like it is supposed to and then gets sucked back in as the car cools. Is the oil level in the engine going down?

I supposed it could be a small amount of oil might go through the gasket into the coolant but I agree with you, why would that cause over heating?

Maybe it is good I am not an mechanic by trade.

Your head gasket is allowing high pressure combustion gasses into the low pressure cooling system. These gasses are displacing the water in the cooling system. There isn’t room for the gasses and the water, so the water gets pushed out. It’s as simple as that.
Since air doesn’t cool nearly as well as water does, the engine overheats. When it overheats, the remaining coolant boils and that just adds to the problem.

First have someone run a cooling system pressure test. Follow this procedure. Install the pressure tester. Pump it up to 5 psi and see if it holds that pressure. It might. Then start the engine and watch the pressure gauge. Goose the throttle. Does the gauge jump? If so, you have a blown head gasket…Many big shops have a hydrocarbon sniffer that can tell FOR SURE.

Oh. That makes sense now. Thank you.

When you pull the dipstick, what does the oil look like? Chocolate milk?

If you’ve got water in the oil, you’re in for a pricey fix, though maybe not 2-3K. It’s not out of the question at all @ 125K that you have a blown head gasket.

You need to fill the radiator because (as has been explained) gases from the engine are getting into the coolant system, which’ll cause bubbles.

The breech in the headgasket allows hot combustion gasses to be blown directly into the water jacket, adding more heat than the cooling system can dissipate, adding volume by adding gasses, and adversely affecting the ability of the cooling system to disspate heat. Much as a hot pan will cool far slower in air than in water, your engine’s heat will dissipate slower if the coolant is aerated.

The headgasket seals the oil passages between the block and the head also, and if an oil passage is involved in the breech it can allow movement of oil into the coolant with the combustion gasses.

If you do not repair the headgasket, your engine’s life is going to be very short. The breech will erode quickly and grow, the head will inevitably warp, and you rengine will be history.

The reason you need to refill the radiator directly is because the gasses being blown into the water jacket are displacing the coolant. When the engine heats up the air pushes cooolant to the reservoire, and when it cools after you shut it off, it retains the air content, thus not drawing all the coolant that it should back into the engine. Opening the radiator cap and fillling it there allows the air out and fluid in to replace it.