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My '98 Subaru Forester spills all its coolant with very high pressure and overheats. I changed the radiator cap and did a flush, but still have the same problem. I added stop leak, but that did nothing -- maybe added an hour of driving time. I still have no idea what it is, and all the mechanics tell me is the head gasket, but that's a big chunk of change for me.

Is there anything else I can try before I make that big investment?

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Head gasket was exactly what I was thinking. The only thing you can do to make it cheaper is to do it yourself. I'll wait for @BobCross to chime in, though, as he's the Subie guy. –  Paulster2 Jul 11 at 15:39
    
what about the heater core or water pump? don't they have similar symptoms? –  fred Jul 11 at 16:10
    
Your title says "reservoir tank" -- can you confirm that you're talking about the plastic bottle hanging off the back of the radiator? –  Josh Caswell Jul 11 at 19:05
    
If it's the head gasket, you'll probably have coolant in your oil. If so, then you need to fix it sooner rather than later, as diluting the oil will lead to bearing failure and other expensive things. –  TMN Jul 11 at 19:46
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Head gaskets are a bit of a bummer on Subies, @Paulster2, since there's two of them. –  Josh Caswell Jul 12 at 8:06

3 Answers 3

It's most likely a headgasket. When they fail in a certain way, they allow exhaust gases to get into the coolant circuit instead of escaping out the exhaust valves like the Lord intended. Only way to be sure is to have a compression test done by a workshop. If you're lucky, they'll do it for free, otherwise it's not an expensive or long procedure. They just stick a gauge into each each sparkplug hole and turn the engine over to measure how much "compression" each cylinder makes.

If the compression test says you have bad compression on one or more cylinders, it's your head gasket and there's no way around having it swapped ASAP. Seeing as you flushed the system and replaced the cap, it is unlikely that you have air trapped in the coolant circuit (I just have a feeling you know about "burping" a car when you replace the coolant).

If you leave it as-is, you're going to warp you cylinder head(s). And if you think replacing a headgasket is expensive, wait til you have to have the head skimmed AND have the headgasket replaced.

I was in the same position last year with my Astra Coupe. The only thing to do is to find out what's wrong and deal with it. Even if you have to borrow the wife's car or take the train while you save up.

As an aside, don't use StopLeak. It has a nasty habit of also clogging things that shouldn't be clogged.

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In direct answer to your question... no, there's probably nothing you can try before you invest in a head gasket.

There are two possibilities here. No, there are three.

First is that your motor oil is entering the water jacket through a breach in the head gasket - the motor oil is under higher pressure than the coolant should ever be, so it'd pressurize the coolant. That, though, would also give your coolant an obvious "milkshake" appearance & consistency.

Second is that your fuel/air mix and exhaust gases are entering the water jacket through a breach in the head gasket. During the compression stroke, and especially during the power stroke, the fuel/air and exhaust are under VERY high pressure, far more than oil pressure. This sort of failure doesn't necessarily alter the appearance & consistency of the coolant, but may cause a very dramatic overpressure condition within the cooling system.

Third is that fuel/air and exhaust are entering the water jacket through a crack in the head, with results nearly identical to head gasket failure. Cracked heads are much less common than failed head gaskets, though.

Cooling system leak sealers will have no effect in any of these cases, short-term or long-term. It may work to stop minor external leakages from the cooling system to the outdoors, but your leak is INTO the cooling system.

If you drive the car any further before getting the problem fixed, you should count on it getting worse - quickly. Even if the leak doesn't get worse fast (which it will), it WILL blow your coolant out, and your car will overheat, and the highest point (the head) will be coolant-starved and suffer from it. Warpage may occur as early as TOMORROW.

If your head is allowed to warp, the car'll never run right again. A machine shop may be able to plane the head, but if they do... every combustion chamber will thereafter be a different size so you'll have different compression (and different power) on every cylinder.

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From the above and your description I would change the coolant thermostat first, and secondly remove and check out the water pump for a displaced/broken impeller.

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