Which SUBARU Engines had Head Gasket Leaks?

Listed below are the SUBARU engines which had head gasket issues:

SUBARU H4 2.5L DOHC (phase I) Head Gasket Failure

This engine was used in various SUBARU models including Legacy and Outback from model year 1996 to model year 1999.

The SUBARU 2.5L DOHC Phase I engine sometimes suffered from head gasket failure. The head gasket would rupture between the exhaust passage and a coolant passage on the head. This failure allowed exhaust gases to enter the coolant. The pressure of the exhaust would force exhaust into the coolant.

Symptoms of SUBARU 2.5L DOHC Head Gasket Failure

Bubbles seen in coolant overflow reservoir

Sometimes people will say the coolant in the overflow tank is boiling. Typically the coolant is not actually boiling (in fact it would be quite a feat for it to manage to boil in the overflow tank), but rather it is the exhaust gases blowing bubbles into the coolant in the overflow tank through the reservoir hose.

The exhaust gases can easily pressurize the cooling system beyond the radiator cap relief pressure. Then the exhaust gases escape past the radiator cap and are released into the coolant overflow tank.

Temperature Gauge Spikes, then Drops Back to Normal

Another commonly reported symptom of the SUBARU 2.5L DOHC phase I head gasket failure is that the temperature gauge will suddenly spike to a high value, then drop right back to normal.

This temperature gauge spike is caused by hot exhaust gases collecting near the temperature sensor, thus spiking the temperature gauge. Then normal temperature coolant washes back over the temperature sensor, thus dropping the temperature gauge back to normal.

Often the temperature gauge spiking starts occuring very infrequently. Infrequently enough that the vehicle owner dismisses the problem as a fluke. Sometimes the frequency starts to increase, or it just suddenly fails.

Oily Film in Coolant Reservoir Tank

Often an oily, greyish murky mess will be observed in the coolant overflow tank. This mess is caused by the exhaust blowing into the coolant.

Coolant Level Appears Normal

The radiator level must be checked by removing the radiator cap with the engine cold. Simply looking into the reservoir tank is not sufficient.

SUBARU 2.5L H4 DOHC EJ25 Phase I Head Gasket Repair

To repair the head gasket leak, typically the engine must be removed from the vehicle. There is really not enough clearance to work in the engine bay to remove the heads without removing the engine. There have been reports that it can be done with the engine in place, that there are plugged holes in the body that are large enough to fit the head bolts through during removal.

SUBARU revised the head gasket design several times during this timespan. The final design was a MLS (multi-layer steel) head gasket design that seemed to fix the problem.

Engine Damage from Head Gasket Leak

The exhaust blowing into the coolant can cause hot spots in the engine. If the problem gets bad enough, it can result in warped engine heads. The heads may be able to be milled smooth again if they are not warped too badly. Otherwise replacement heads are required.

SUBARU 2.5L H4 SOHC EJ25 Phase II Head Gasket Failure

This SUBARU engine was used starting in model year 1999 Forester and Impreza. It was used starting in model year 2000 Outback and Legacy.

Up to around model year 2003, this engine had a risk of developing an external head gasket dripping leak. Typically this leak started at the left (driver's) rear of the engine. Some models and years had a plastic engine underdoor that may have hidden the leak from the owner.

There have been reports of this external head gasket leak on some 2004 and 2005 models, but much less it would seem than on the 1999-2003 models.

Often the leak started in cool or cold weather. The thought is that the engine and head shrink more when very cold, and then expand as it gets warm, eventually letting the head gasket develop the peeping leak.

SUBARU issued campaign WWP-99 about this particular external head gasket dripping leak for the phase II 2.5L SOHC. It basically specified adding SUBARU coolant conditioner to try to fix the dripping leak. In some cases it works. In other cases the head gaskets must be replaced.

Again, the coolant level must be checked by removing the radiator cap with the engine cold. The level in the overflow tank may appear normal while the level in the radiator is low.

The SUBARU 2.5L SOHC heads can be replaced without removing the engine from the vehicle, although it is generally easier to simply pull the engine.

Why Did the SUBARU Head Gaskets Have Failures?

This type of expensive to repair issue is not good for any vehicle manufacturer's history. SUBARU was not the only vehicle manufacturer to have head gasket issues in this timeframe. One gentlemen I was talking to mentioned manufacturers stopped using asbestos in head gaskets around 1996, and then it took a number of years to perfect a head gasket design that worked well without it.

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