Reply #46: I have the 2G 2.5L, and am aware of the problem.

[Xithras]

So far, no problems. My mechanic, who has many years of Subaru experience, explained the root of the problem to me once and said that it's entirely avoidable if you keep the engine compartment clean and flush your cooling system every 25k miles. Slipping a plastic tray under the battery also helps to eliminate the problem.

IIRC, he said that the problem is caused by the location of the ground strap, and the position of the battery next to the radiator. Induction caused by the position of the positive battery cable slightly energizes the side of the radiator, which then forms a complete circuit via the ground cable from the battery to the block. If your battery cables are unmaintained and corroded, the problem gets much worse. The circuit through the coolant causes mild electrolysis, which acidifies the water itself. The acidified water eats at the head gasket material until it fails.

Subaru has completely fixed the problem on the current generation of cars, but they've never really addressed it on the older ones because they consider it a maintenance issue.