I had a couple subarus before and digging around their forums before I found this.


"How does a battery effect head gaskets you ask? Without getting to far into the scientific end of it, the battery is located very close to the radiator. The electrical system in the car is grounded to the engine block on the left side of the engine, as the ground circuit resistance increases (from corroded battery cables), the voltage found in the cooling system will also increase, this is what causes electrolysis. Coolant can become very corrosive as a result."

"A battery that is covered in battery acid and corrosion will add to the level of corrosion in the cooling system, by increasing the resistance in the vehicle ground circuit which can lead to higher levels of electrolysis. This corrosion can eat away at metal gaskets, seals and metal that it comes in contact with. On a 2nd generation 2.2 and 2.5l it is almost always the left side head gasket that leaks coolant externally and it is also the cylinder head gasket that is the closest to the battery. Odd, no? A properly serviced and healthy battery will decrease the possibility of the battery adding to the corrosion level of the cooling system.

As the battery vents out acid and the cooling fans come on some of the vented acid can make its way into the coolant overflow bottle, not a lot in most cases but how much is too much? The overflow bottle catches coolant from the cooling system as heat and pressure cause expansion of the coolant from the radiator into the overflow bottle as the engine cools the coolant is then pulled back into the cooling system form the “expansion tank” or “overflow bottle”. In some cases you can look at the inside of the hood of your Subaru and see white acid all over the hood liner, if that is your car you are pulling a tiny amount of acid into the overflow bottle past the tube and from there into the radiator, remember as the engine cools, coolant is pulled back into the radiator via the vacuum that is created as pressure decreases, so there is a small vacuum pulling at particles surrounding the coolant overflow bottle.

The health of the electrical system is a contributing factor. We know that corrosive coolant is part of the problem, we know that electrolysis is a result of increased voltage levels in the cooling system as a result of poor grounding , we know that a poor ground can be caused by resistance in the primary electrical circuit. We know the coolant is the same in the entire engine, and the gaskets the same left to right the only difference is the fact that the ground is at the left side of the engine and that if there is voltage present in the cooling system it will always travel the shortest path to ground and the ground is on the same side of the engine as the gasket that always fails the most via external coolant leaks."


http://www.rs25.com/forums/f190/t100250-head-gasket-problems-explained.html

I know this is for a subaru but just came to mind while I was reading the posts.