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?

  • 2
    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. Commented Jul 11, 2014 at 15:39
  • what about the heater core or water pump? don't they have similar symptoms?
    – fred
    Commented Jul 11, 2014 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?
    – jscs
    Commented Jul 11, 2014 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
    Commented Jul 11, 2014 at 19:46
  • 1
    Head gaskets are a bit of a bummer on Subies, @Paulster2, since there's two of them.
    – jscs
    Commented Jul 12, 2014 at 8:06

7 Answers 7


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.

  • so much truth condensed in one sweet answer. props !!
    – amphibient
    Commented Sep 18, 2015 at 16:16

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.


Probably not the head gasket most of the time it might be but it would do it at all times worse and high rpm. Had the same issue thought it was the head gasket did the head gasket water pump and all that and the problem is still there, before someone says I messed up on the head gasket this isn’t my first time and I didn’t screw anything up. Start looking else where take the thermostat out and see if it helps any, I’m gonna look into my car and see what else it possibly can be. I would probably flush the system I think it doesn’t flow good and there might be a restriction that at one point doesn’t allow any flow and it ends up coming out of the reservoir


All your topics are good, but no one said anything about if the radiator has a hole in the suction side of the radiator that would cause a constant air bubble but in the system and you are going to constantly over heat yes than you would get a bad head and even badder things like water in the oil.

I would replace the water pump timing belt, radiator, and a cooler thermostat. Last but not least, 80$ blue devil works great for a bad head.


Before doing anything with the headgasket, check this first!

I may have an idea on what's going on here. It looks like the 98 Forester, like many Subarus, has two radiator caps. It has one on the radiator itself (which we would expect) but also one on the coolant expansion tank - which I believe is that big round thing towards the front of the engine bay, just left of the centerline of the car.

Subaru does a weird thing with this coolant setup, and if you have replaced any radiator caps recently then it could cause the system to fail - and the failure indicator is overheating. If you haven't replaced your caps recently, it's still possible one of them has failed and this is causing your issue.

Many people put new caps on and if they aren't careful, this will cause a problem. Here's why.

Subaru uses two different kinds of radiator caps: a 1-way and a 2-way. These caps will look almost identical, the difference is in how they function: the 1-way can only open in 1 direction (thus the name), to let pressure OUT. The 2-way...you guessed it, can open either way - to let pressure OUT, or can open under vacuum to pull air (or, in actuality, coolant) back in.

The easiest way to identify these different caps is to look at the bottom - the 1-way will look flat, the 2-way will look like it has a dome.

So here's the thing: the 2-way goes on the expansion tank ONLY. The 1-way goes on the radiator ONLY. It's designed this way so the radiator can vent excess pressure without sucking any air in. The expansion tank can vent pressure if necessary AS WELL AS let air in - this letting air in is what lets the coolant go from the expansion tank back into the radiator.

If you put a 2-way cap on the radiator (or accidently switch the caps), this will cause the system to malfunction, and coolant will not flow as expected. This can lead to overheating and thus too high pressure!

It's also important to point out that just about every single aftermarket radiator cap out there is a 2-way. This is because most coolant systems on cars only have the one cap on the radiator, thus it needs to work in both directions. Subaru has designed their system differently. If you bought an aftermarket cap and put it on the radiator, chances are very good that this cap is a 2-way, and could be causing you problems. Many people that mod their cars, especially by putting in new radiators, may not realize this and install the cap shipped with the radiator on the radiator, which is a no-no.

Check your caps. Find an aftermarket 1-way - or better yet, just buy a Subaru OEM 1-way from a dealer.

A note on headgaskets: typically when a headgasket blows, it doesn't force coolant OUT. Yes, it can cause overheating issues, which could then cause the coolant to blow. But it's a very easy check to see if you have a blown headgasket. Open up the expansion tank and/or radiator, start the car up, and shine a flashlight down in there. If you see any kind of bubbles, it's a good chance you have blown headgasket. If no bubbles, you may be okay. I would also check the oil. Do an oil change and look at the consistency of the oil. Coolant mixed in with oil will produce a milky, often light-colored oil mixture. It's called the milkshake of death.

So: before spending the money/hassle on a headgasket, check your radiator caps. Even if they are the original caps, I highly recommend buying new ones from a dealership (to ensure you get the correct ones, and be sure to put them in the correct spots) because caps can eventually fail. Try that first. It may save you a bunch of money. If that doesn't fix the issue, then at least you got some new caps out of it. Good luck!


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.


check coolant temperature sensor or if any blockage coolant on the radiator...

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