Subaru Head Gasket Logo

Two types of engines, two types of problems

This site contains links to information and hopefully some helpful explanations regarding head gasket problems with Subaru 2.5 liter normally aspirated (non turbo) engines. There are two types of engines, Phase 1 and Phase 2. Both variants can have head gasket problems associated with them, but the problems, how to diagnose them, and how to fix them, differ between the two phases.

PHASE 1 (Dual Overhead Cam):
Used in:
1996 to 1999 Legacy Outback
1996 to 1999 Legacy GT
1998 Impreza RS
1998 Forester

Typical failure mode:
Internal leak, not externally visible. May see bubbles in overflow reservoir, sludgy residue on walls of overflow reservoir, random overheating of engine.

Covered by service bulletin: No

Cost to repair:
DIY: $200 parts
Independent shop: $1000-$1500
Dealership: $2000

PHASE 2 (Single Overhead Cam):
Used in:
2000 to present (non-turbo) Legacy Outback
2000 to 2004 Legacy GT
2000 to present Legacy
1999 to present non-turbo Impreza 2.5 (RS, TS, Outback Sport)
1999 to present non-turbo Forester

Typical failure mode:
External leak, sweet smell, coolant visible on engine, slowly dropping coolant level.

Covered by service bulletin: WWP-99, for 1999 to 2002 model years, some models. Note this is only for Phase 2 engines, and some 1999 models have the Phase 1 engine and are NOT covered by this bulletin.

Cost to repair*:
* Subaru has a "coolant conditioner" that is added to the coolant. This is required to be added as part of the service bulletin and the cost is nominal. Subaru contends that this is all that is required to "repair" the leaks. 2003 and newer models are not covered under WWP-99 and supposedly not prone to head gasket leakage.

More about diagnosing problems

Phase 1 engines:

Phase 1 engines typically experience an "internal" head gasket failure if failure happens. This is hard for a lot of mechanics to diagnose, is difficult to reliably duplicate, and often has resulted in the owners throwing money and parts at the problem. New thermostats, flushing coolant, new water pumps and radiators are examples which do not fix the problem if it is indeed head gasket failure. If the engine overheats too many times or too severely, it can result in warped heads and the need for a new engine. As of this date Subaru has never acknowledged the existence of any problem, and are unlikely to do so. They have redesigned the head gaskets and most people that have had the new gaskets properly installed have had good success. The "coolant conditioner" described in WWP-99 DOES NOT fix this internal leak, nor does it give you an extended warranty against head gasket failure. Headgasket replacement is your only option besides engine replacement.

Common symptoms:
Overheating, often when slowing or stopped after extended high load driving. The overheating can be seemingly random and sporadic.
Bubbles in coolant overflow reservoir, immediately after running.
Sludgy residue in coolant overflow tank.
Hydrocarbons in coolant overflow tank, this is tested by a mechanic with specialized equipment and is not evident visually.

Phase 2 engines:

* Phase 2 engines are SOHC (Single Overhead Cam), were used from 1999 or 2000 to present depending on model, and typically experience an "external" head gasket leakage if it happens. This problem, while pretty widespread, does not seem to be as damaging in nature to the engine. It has been acknowledged by Subaru in the form of a "Service Campaign" #WWP-99 which applies to certain '99 to '02 cars. It involves adding a "Coolant Conditioner" to the coolant and if this is done the owner's head gaskets are covered against external coolant leaks for 8 years or 100K miles.

Common symptoms:
"Sweet" smell after operating engine and then stopping.
External fluid leakage (green) visible on engine or below car.
Slowly dropping coolant level due to leakage.

  HG discussion on Nasioc., North American Subaru Impreza Owners Club. Highlights of this link include several people detailing their head gasket experiences, and some good technical information about the updated gaskets. It's supposed to be about the Phase 1 engines, but there is some Phase 2 discussion mixed in there too.
  HG discussion on Nasioc, North American Subaru Impreza Owners Club. This one is specifically about the part numbers of old and new gaskets on the Phase 1 DOHC 2.5 liter engine.
  HG replacement instructions. Skip's writeup on a replacement of head gaskets. Includes a link to a "guestbook" where he invites you to sign up if you have experienced head gasket failure. This thorough writeup also has information on telltale signs of HG problems, and links to other good Subaru information. Again, this is supposed to be specific to the Phase 1 DOHC 2.5l, but there are references in the guest log to the Phase 2 SOHC. Skip mentions the bad head gaskets in 2.5l's from "1996-present", and I personally feel that Phase 1's and Phase 2's should not be lumped together. As mentioned elsewhere on this page, there are two different versions of the engine and they differ in how the head gaskets fail. Never the less, kudos to Skip for such a thorough writeup and maintaining the guest log even though he hasn't owned a Subaru for quite a while now.
  Autobild article (German)

Autobild article in English
Translated by Google

The results of a Subaru Legacy owners survey from, this article is in German. The second link is to Google's translation from German to English. The article illustrates the discrepancy between the reliability of the different sizes of engines. The 2.0 has very few problems, the 2.2 perhaps a few more, but the 2.5 definitely has the most. While the tone of the article is quite favorable, one thing that stands out is the number of respondents with the 2.5l engine who had head gasket problems: a sizable 21 percent. If one includes all engine configurations (2.0, 2.2 and 2.5), the percentage reporting HG problems drops to eight. My German friend who helped translate the article did raise a point which should be considered: In Germany, cars tend to be driven much harder than in the U.S. Consequently, they break more often.

WWP-99 Campaign thread

Replacement Phase 1 engine source:

Engine replacement alternative: Replacing the Phase 1 2.5l engine with a 2.2l engine.

Another engine swap link (2.2): Swapped EJ22 for EJ25

Link to a Subaru service shop in WA with a good writeup of the problem and procedures:Why's my car overheating?