Hate to say so, but it sounds like you've got the dreaded Phase 1 headgasket problem. Take a close look at the overflow tank looking for bubbles right after shut-off when warm and/or a black, oily residue in the coolant. You should also have the coolant checked by a shop for hydrocarbons(exhaust gases) present in the coolant. These are all signs of a leaking headgasket, very common in these motors. What's happening is the exhaust gases leak into the coolant passage, forcing the coolant out until there isn't enough in the engine to cool it, and it overheats.
Subaru has redesigned the gaskets at least twice to improve life, but most of us have experienced this by the time mileage hits 120K-miles and many far earlier than that. If you don't take care of it you risk anything from warped heads to catastrophic engine failure, as overheating the aluminum components significantly shortens their life. Also, be aware the radiator has a "burp" screw that needs to be removed when refilling the radiator to let all the air out. It's on top of the radiator on the passenger side and has a Phillips type head. Replace it when coolant flows out the hole. Best of luck, and be sure your mechanic uses the latest gasket design; PN 11044AA610

Glenn O

 

Your motor, of opposed 4 cylinder design, has a head on each side of the motor. The headgaskets are sandwiched between each head and the engine block. This is a major repair and especially with the Phase1 DOHC motors, not to be undertaken by the backyard mechanic, unless one has a lot of time, patience and more than average mechanical skills & tools. To do both heads, any possible repair to the heads, like resurfacing, guides, valves, springs, possibly resurfacing the block mating surface if damaged/warped, waterpump and cam-belts if contaminated will run into the $2K plus range. Take a look here for some excellent articles on just what I'm saying. Look in the archives, and you'll need Adobe Acrobat to view them. http://www.endwrench.com/

Glenn O

 

I had the same cooling problem. Replaced my thermostat, and still a problem. I figured it was an air-lock, and it was (can never trust some mechanics, eh). Still had the over-fill of reservior problem. Then changed the radiator cap. Seemed to stop the problem somewhat. But still filled the reservior over time.

Thought perhaps the cooling system is not sucking the coolant back in properly due to a hole in the overflow hose or something, but seems to be OK.

I guess a compression check is needed. What else can be investigated to try pin-point the problem before going to a shop?

I am gettin' scared 'bout the head gasket.

 

A compression check won't necessarily pinpoint the problem. I believe a cylinder leak-down test usually will, but the easiest test is an exhaust gas "sniffer" test of the coolant, looking for exhaust gas traces in the coolant. A visual check will sometimes reveal these traces as well, as black "gunk" lining the walls of the overflow tank. A quick check of the overflow after shut-down for bubbles rising to the top is also a good indicator of same. I don't get any joy from pointing these things out guys, as I went through it, sweating bullets nursing my GT Wagon home from Sis-in-Law's wedding on 350 mile trip carrying 2 gallons of pre-mixed coolant and praying I wouldn't need it. It is critical that you diagnose this problem as early as possible though to prevent any real damage to the engine's bottom end or catastrophic head warpage or cracking. The latest generation of Head-Gasket is touted to prevent recurrence. Good luck guys!

Glenn O

 

I did a compression check on my 98 GT, with the whole overheating problem and the highest compression readings were on the right side cylinders by about 3 PSI. But all were well within specs. I decided that rather than paying some mechanic to change the head gaskets, that I would try it myself, saving close to $2000 CDN.

For those of you with the suspect head gaskets, and who are mechanically inclined (which I am sure most of you) then take a long weekend, or bike/bus to work for an extra day or two, but consider trying it yourself. I am glad I did, because it turned out to be not that difficult, just more time than anything. All the steps that perhaps can be taken to make the job easier, I may post (including some pictures) if people want to see them.

As for the leakdown test, I am making an educated guess that in my case, it would not have clearly shown the problem. Why? After removing the right cylinder head, the comprimised area of the gasket was on the bottom of both cylinders. I know this because the coolant had cleaned off the carbon deposits on the bottom bit of both cylinders. I suspect the coolant would leak in under the pressure of a hot engine after shutdown. Therefore a leakdown test, would be not work due to the coolant inside the cylinder hold a 'seal' for the test. Bubbles in the overflow, and blank gunk as mentioned were my symptoms too.

Driving with this problem is not good, which I realized only after I removed the head. The flat engine design would allow for half of the internal passages to be filled with 'air' and only the bottom (or lowest half of each cylinder) to have circulating coolant. Again, not good. I drove this way for two months, keeping revs below 3000 rpm. Glad I didn't leave it for much longer, but should have done it sooner.

Damned hindsight, eh?!!

Old and new gasket designs differ significantly. Hope the problems are solved for most with this redesigned gasket.

Sparrow