Most of the vehicles we work on are Subaru’s and that means we have lots of experience replacing Subaru headgaskets. I get a lot of questions about gaskets used and repair procedures that I hope to clarify here. First off the 2.5 SOHC engine is the most common to have cylinder headgasket failure and this article will be primarily about Subaru 2.5L SOHC headgasket failure. The Subaru 2.5L DOHC engine had headgasket failure also which was an internal combustion leak causing different symptoms and problems. The SOHC engines have a problem with oil and coolant leaking externally and eventually the gasket failure will cause oil and coolant to mix. I have seen the SOHC engine also have internal combustion leaking problems but usually after headgaskets have been replaced but not done correctly (cylinder head not resurfaced or warped, block surface not prepared correctly and incorrect head bolt torque procedure) cheaper gaskets used or just way overdue for the headgasket replacement which also eventually leads to coolant contamination in the engine oil.
Now the Subaru 2.5L SOHC engines have first-generation and second-generation engines, which have differences in the valvetrain. The first-generation 2.5L does not have variable cam timing and the second-generation has variable valve timing or VVT. We use a different torque procedure for these engines and the headgaskets are different also. All gaskets used are genuine OEM Subaru gaskets but for the first-generation engine there is a revised part number for the headgasket we use on that engine which is the multi-layer steel or MLS with imprinted gasket not the fiber gasket that deteriorates. If you were to go to Subaru and order the headgasket(s) it would be the same as the ones you’re taking off. The second-generation SOHC engines use the same fiber gasket but have an updated head torque procedure and if done correctly you should not have future headgasket problems.
When replacing the headgasket(s) I almost always re-surface the cylinder head(s). If the headgaskets have been done before make sure to check the cylinder head height incase they we’re already machined. Spending the money to have them pressure tested is not necessary if your only dealing with an external oil leak but it cannot hurt and is always a good idea. Just make sure to remove the cam tower and camshaft then make sure there are no metal shavings after you get them back from the machine shop. You also need to remove the valves and replace the valve seals and inspect the valve guides for signs of movement. These cylinder heads are known to drop exhaust valve guides causing misfire or even worse engine damage. If the valve guide has been moving you can usually tell, as they are lower than the others. If the valve guide is not worn out and loose you can fix it by pressing the guide up and knurl outer edge after pressing it in place, if done right it will not move anymore preventing future problems.
You definitely want to clean all the hard carbon deposits out of the valve ports and clean up the valve seats. You can wire brush and lap the valve to the seats but I almost always recommend to at least having the exhaust valves re-faced. The exhaust valves usually have pitting at 100K and are start to mushroom out when closer to 200K. The valve seats are usually okay but check them out anyways. The intake valves and intake valve seats almost always look good unless the valve seals have been leaking oil which burns up the valve and will eventually cause it not to seal. When working on the VVT second-generation 2.5L SOHC engines valvetrain inspect the rocker arms closely, especially if they are close to 200K. I often find the exhaust rocker arms to fail. It is usually a worn roller bearing or the roller is wearing a groove into the rocker arm, which will eventually cause it to stop turning and flatten a cam lobe in extreme cases. Below is a picture example of what to look for.
The exhaust valve guides need to be checked for signs of movement. They have been known to drop into the cylinder causing either a misfire or complete engine failure as a worse case scenario. Makes sure they are all at the same height and not dropping into the cylinder as shown in the picture below. In this situation the valve guides can be replaced or you can press the guide back up and ream edges of the guide at the cylinder head and it will stay put. Take a look at the before and after valve guide repair pictures to better understand what I’m trying to explain.
It may be hard to determine when you already have oil leaks but be aware of the engines oil consumption rate. Some of these engines also have oil consumption problems that can be corrected when replacing the headgaskets. The oil consumption problem is most likely caused from stuck piston rings and plugged oil drain back holes in the lower piston ring groove. Unlike most engines the pistons can easily be removed when the engine is removed and the cylinder heads off. It is not always necessary to remove the engine to replace the cylinder headgaskets but if the engine is removed keep this in mind in-case you have oil consumption problem. If you want to know more about replacing/repairing piston rings read this article here: Subaru Oil Consumption Problem & Repair
How do you replace the cylinder headgaskets without removing the engine, there is not enough room to remove the cylinder heads or is there? The engine is a lot easier to work on if it’s on an engine stand but it can be done without removing the engine. First off, the head bolts will not come out if done this way. They must be left in the cylinder head and make sure not to mix up the center two bolts, they are different and will not fit the outer cylinder head bolt holes. To gain the extra clearance you need remove the transmission dogbone mount (located on top transmission to firewall) and the two engine mount nuts on the bottom side. The exhaust y-pipe will need removed or un-bolted and lowered. If you do not completely remove the exhaust then plug the ends with rags or something to prevent coolant and oil from getting in when removing the cylinder heads. Last you have to remove both left and right side axle nuts and make sure both sides slide freely in their splines. This will prevent you from damaging axle when moving the engine from either side to gain clearance. Raise the engine with a floor jack enough to clear the engine mount bolt from cross member slots then move the engine to whichever side you decide to remove first. I will usually use a ratchet tie down to help pull engine to either side for clearance, connect it to the AC compressor bracket then something stable in the shop.
Here is a complete list of the most common parts used when replacing Subaru cylinder headgaskets:
– Headgasket (2)
– Intake gasket (2)
– Exhaust manifold gasket (2)
– Cam seal (2), Front crankshaft seal (1)
– Valve seals (8 intake) (8 exhaust)
– Valve cover gasket (2)
– Sparkplug tube seal (4)
– Valve cover bolt grommet (10 | first-generation SOHC engines only)
– Timing belt
– Water pump
– Cogged idler pulley (inspect, replace if needed)
– Smooth upper idler pulley (inspect, replace if needed)
– Smooth lower idler pulley (inspect, replace if needed)
– Hydraulic tensioner pulley assembly (inspect, replace if needed)
– Upper and lower radiator hoses
– Drive belts (alt/ps belt) (ac belt)
Now if your either planning to replace gaskets yourself or your just looking to educate yourself about the procedure before having the repair done I hope this article helps. Please leave any questions or comments below and thanks for reading.