Subaru Headgasket Failure

Subaru head gasket failure
Two types of engines, two types of problemsThis site contains  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: W-99WP, 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 problemsPhase 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.

SUBARU Front camshaft oil seals oil leak

Note: The front camshaft oil seal leak information applies only to Subaru engines with a timing belt such as the 1.8L EJ18, 2.2L EJ22, 2.5L EJ25.  Does not apply to the H6 EZ30 which uses a timing chain).

The front camshaft seals are behind the camshaft sprockets.  The camshaft sprockets are behind the timing belt cover.  These oil seals can leak with age and mileage, especially the earlier black colored seals.  The later brown colored seals are more resilient.

The smart time to replace the front cam seals is when the timing belt it is changed, since much of the labor involved in replacing these oil seals involves the R&R (removal&reinstallation) of the timing belt.

On the SOHC (single overhead cam) engines, there are two if these front camshaft oil seals.  On DOHC (double overhead cam) engines, there are four of these oil seals.  The front cam seals live behind the camshaft sprockets.  The sprockets must be removed to replace the seals.  The photos below are of the SOHC variety (specifically the EJ22):

SUBARU Front crankshaft ‘crank’ seal oil leak

The front crankshaft seal is mounted on the oil pump.  The oil pump is turned by the crankshaft.  The front crank oil seal is smart to replace when replacing the timing belt.  Resealing the oil pump is the wise way go since there are other things about the oil pump that can leak, namely the rear rotor case cover screws backing out.

SUBARU Camshaft ‘cam’ cap o-rings oil leak

These o-rings are located behind the LH (left-hand) cam cap (on the sohc 2.2L and 2.5L anyway).  This o-ring can get squashed and brittle with time and potentially allow a leak.  The smart time to replace this o-ring is when replacing the timing belt.  To replace it, the camshaft sprocket must be removed.

Also, EJ22 engines have a cam cap at the RH (right-hand) side rear of the engine.  This cap also has an o-ring that can leak oil.  (On SUBARU EJ22T (turbocharged) engines, the turbo oil return goes into this area).

SUBARU Valve cover gaskets oil leak

These gaskets seal the valves covers against the mating surface cylinder head.  They can shrink or crack with age and then result in an oil leak.  The oil in this area is not under pressure as it is on its way to draining back to the oil pan.

Sometimes the valve cover bolts loosen slightly and can allow oil to leak past the gaskets.  There are also bolt sealing washers that should be replaced if replacing the valve cover gaskets.  A kit is available from Subaru dealers containing the needed valves cover gaskets and bolt hole seals.

If the spark plug tubes on your soob engine penetrate the valve cover (such as on the EJ25 sohc), you should also replace the spark plug tube seals.

SUBARU Spark plug tube seals oil leak

Spark plug tube seals Subaru oil leak[/h3] If the spark plug tubes penetrate the valve covers on your soob engine (such as EJ25 sohc phase II), these seals can shrink with time and allow oil to seep past and down into the spark plug tubes.

If you see an excessive amount of oil in the spark plug tubes when replacing the spark plugs, then these spark plug tube seals probably need to be replaced.  If too much oil accumulates in the spark plug tubes, it can result in cylinder misfire from the oil diverting the spark energy.

Cylinder misfire typically shows up as a flashing CEL (check engine light) (aka MIL – malfunction indicator lamp).  If the codes are read typically P0301, P0302, P0303 and/or P0304 are read.

SUBARU Oil separator plate oil leak

This plate is at the rear of the engine.  It provides a hand hole for access during certain engine repair jobs.  Many SUBARU engines came from the factory with a plastic oil separator plate.  These plates can then start to leak with age and mileage.

The solution is an upgraded metal oil separator plate with new screws.  Unfortunately, the engine or transmission must be removed to gain access to replace this plate.  So unless the engine is really gushing oil it may not be practicable to replace the oil separator plate.

However, if the engine or transmission is pulled for some other reason (clutch replacement, head gasket replacement, etc), then it is definitely prudent to replace the separator plate with the upgraded metal version at that time

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • Andrea says:

    I have had a new radiator replaced in my Subaru Impreza 2000 model. I am having problems when the motor car get hot it push the water out of the over flow pipe and back into the reserved tank. I’ ve had my dad have a look he thought it was the thermistor on the bottom of the engine but it works fine. It has a boxer engine.

    Also does a Subaru Impreza 2000 model have two thermostats?

  • caleb says:

    Brown sludge in coolant around cap in 04 wrx sti
    and in resevor on walls

    • Martin Hand Martin Hand says:

      Subaru has what they call a coolant system conditioner that they add to help with the headgasket problem they’re having with these engines. This is most likely what your seeing.

  • Edek says:

    Hi, i have just replaced a burnt head gasket, on the drivers side, of my legacy 94 ej22 enginbe……the engine works much better…but some presure is present in the cooling sistem; the ruber hoses are inflated. Idle is not soft, like going on three cilinders……May be the other head gasket is bad too ???
    Other question…when i replaced the timing belt, positioned all three pulleys marks algned with the nothes on the oil pump and timing cover…. having 40,5 teeth on the belt to the driver side…not the sane on the other side…instead of 44 teeth as reference there are 46 to the notch !! all three pulleys are 12 o´clock facing..
    Thanks in advance for your help…

    Edek–Patagonia Argentina..

    • Martin Hand Martin Hand says:

      I always replace both cylinder headgaskets, never just one side. When setting the timing belt you should be able to line everything up without counting the teeth. Make sure to turn the engine by hand 2 revolutions and re-check your timing marks before firing the engine.

  • […] Here is some more good info about the EJ25 headgasket problems, and also talks more about the DOHC and SOHC versions of the motor. Subaru Headgasket Failure | MDH MOTORS […]

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