Subaru EJ25 Head Gasket: The Definitive DIY Entry — Tools, Steps, and Why Fel-Pro MLS is the Answer

Symptom

⚠️ The EJ25 head gasket failure is not a question of "if" — it's a question of "when." Subaru's original single-layer gaskets were flawed from the factory. If your EJ25 has over 100,000 miles and hasn't had the head gaskets replaced, they will fail. The only variable is when and how spectacularly.

One more thing: The EJ25 is an interference engine. If the timing belt breaks, the pistons will hit the valves. When you're doing head gaskets, you're also doing a timing belt and water pump. It's not optional — it's part of the job.

Diagnosis — Confirming What You Probably Already Know

Step 1: External inspection. Park the car, let it cool, and get under it. Look for oil seepage along the cylinder head-to-block seam, especially on the driver's side at the rear. If you see oil dripping from the bottom of the bellhousing, the head gasket is leaking.

Step 2: Check the coolant overflow tank. With the engine cold, look at the coolant level. If it's low, top it up. Then start the engine and watch the overflow tank. If bubbles appear (a steady stream, not just a few), combustion gases are entering the cooling system.

Step 3: Block test (chemical test for exhaust gases in coolant). Buy a combustion leak test kit (usually a fluid that changes color when exposed to CO₂). With the engine warm, draw air from the coolant reservoir through the tester. If the fluid changes from blue to green/yellow, you have combustion gases in the coolant. This is a definitive test.

Step 4: Compression test. Low compression in one cylinder (especially adjacent cylinders, like 2 and 4, or 1 and 3) indicates a head gasket failure between cylinders or a valve issue.

Step 5: Check for coolant in the oil. Pull the dipstick. If the oil looks like a milkshake (or is a suspiciously dark chocolate color), coolant has mixed with the oil. This is serious — it means the internal leak has gotten bad enough to contaminate the entire lubrication system.

Pro tip: If you have bubbles in the overflow tank, stop driving the car immediately. The combustion gases are pressurizing the cooling system, which will eventually cause a radiator or heater core failure — or catastrophic overheating.

Why Fel-Pro MLS Is the Answer

The original Subaru head gasket on the EJ25 is a single-layer composite gasket. It works fine — until it doesn't. The problem is that the EJ25 has an open-deck design (the cylinders are not fully supported by the block), and the aluminum block and head expand and contract at different rates. Over time, the single-layer gasket can't handle the movement and fails.

Fel-Pro's MLS (Multi-Layer Steel) gasket is the solution.

The Fel-Pro MLS gasket is the standard for EJ25 rebuilds. It's not the cheapest option, but it's the only option that doesn't eventually fail. If you're doing this job once, do it right. Buy Fel-Pro. Skip the cheap aftermarket and skip even the Subaru OEM replacement (which is still a single-layer gasket in most cases).

Tools Required

Specialty Tools

General Tools

Parts Required

Pro tip: If you're doing the job and the car has over 100,000 miles, consider replacing the valve seals and checking the valve clearance while the heads are off. It's cheap insurance against future issues.

Procedure — Step-by-Step

Preparation

1. Disconnect the battery (negative terminal).

2. Drain the engine oil — remove the drain plug and let it drain completely.

3. Drain the coolant — remove the radiator drain plug (or lower radiator hose) and collect the coolant.

4. Remove the radiator and cooling fans — this gives you access to the front of the engine.

5. Remove the accessory belts — alternator, power steering, and A/C belts.

6. Remove the timing belt covers — both upper and lower covers.

Set Engine to Top Dead Center (TDC)

1. Rotate the crankshaft pulley clockwise (as viewed from the front) until the mark on the pulley lines up with the 0° mark on the timing cover.

2. Check the camshaft marks. The marks on the camshaft sprockets should align with the marks on the timing covers. If they don't, rotate the crankshaft one full revolution and re-check.

3. Install the crank lock pin (if you have one) to keep the crank from moving.

Remove the Timing Belt and Components

1. Release the timing belt tensioner — use a 10mm hex bit to compress the tensioner and insert a pin to hold it.

2. Remove the timing belt. Mark the direction of rotation if you plan to reuse it (but you shouldn't — you're replacing it).

3. Remove the timing belt tensioner and idler pulleys. Note which bolts go where.

4. Remove the camshaft sprockets. You'll need to hold the sprockets with a tool or a strap wrench.

5. Remove the water pump (if you're replacing it, which you should).

Remove the Intake and Exhaust Manifolds

1. Remove the intake manifold. Disconnect all electrical connectors, fuel lines, and vacuum lines. Label everything. The intake manifold has 10mm bolts — keep them organized.

2. Remove the exhaust manifold. The nuts will likely be rusted — use penetrating oil and patience.

3. Remove the spark plugs — you'll want to inspect them for signs of coolant or oil contamination.

Remove the Cylinder Heads

1. Remove the valve cover gaskets. Clean all the old RTV from the cam cap surfaces.

2. Remove the camshaft caps. Use a 12mm hex bit socket. Keep the caps in order — they must go back in the same position.

3. Remove the camshafts. Be careful not to drop them on the floor. Store them in a safe place, oriented as they were removed.

4. Remove the head bolts. Use a 12-point 14mm socket. The head bolts are torque-to-yield — they will feel tight the whole way out. Do not reuse them.

5. Lift the cylinder heads off the block. They may be stuck — use a plastic or rubber mallet to gently tap them free. Do not pry between the head and the block with a metal tool — you'll damage the sealing surface.

6. Remove the old gaskets. They will likely be stuck. Use a plastic scraper to remove the gasket material. Do not use a metal scraper or a wire brush on the aluminum surface.

Clean Everything

This is the most critical step for a leak-free seal.

1. Clean the cylinder head surface. Use a plastic scraper, brake cleaner, and a Scotch-Brite pad (the fine kind, not the abrasive kind). Remove all old gasket material. The surface should be clean and smooth.

2. Clean the engine block surface. Same process — all old gasket material must be removed. Any leftover material will create a gap that the new gasket can't seal.

3. Clean the head bolt holes in the block. Use a tap or a thread chaser to clean the threads. Brake cleaner and compressed air to remove debris. This is critical — old debris in the holes will cause false torque readings and uneven clamping.

4. Check the heads for warpage. Use a straightedge and feeler gauges. The maximum warpage is 0.0015 inches (0.04mm). If the head is warped, take it to a machine shop for resurfacing.

Install the New Head Gaskets

1. Check the Fel-Pro MLS gaskets. They have a "Top" marking — make sure you install them with the correct side facing up. The orientation matters.

2. Install the gaskets dry. MLS gaskets do not require sealant or gasket spray. They seal with clamping force. Do not apply any sealant to the gasket surface.

3. Position the gaskets on the block. The head bolts will hold them in place temporarily.

Install the Timing Belt and Water Pump

1. Install the water pump (if you removed it) — torque the bolts to 14 ft-lb (19 N·m) in a crisscross pattern.

2. Install the new timing belt tensioner and idler pulleys. Torque to 30 ft-lb (40 N·m).

3. Install the timing belt. Follow the routing diagram:

• Start with the crankshaft at TDC.

• Route the belt over the crankshaft sprocket.

• Route it over the driver's side camshaft sprocket, then the passenger's side camshaft sprocket.

• Ensure the belt tension is correct — the tensioner will set the tension.

1. Release the timing belt tensioner. Remove the pin that's holding it compressed. The tensioner will take up the slack.

2. Verify timing. Rotate the crankshaft two full revolutions clockwise. Check that all timing marks align. If they don't, the belt is off a tooth — repeat the routing.

Reassemble Everything

1. Install the intake manifold — torque the bolts to 14 ft-lb (19 N·m) in a crisscross pattern.

2. Reconnect all electrical connectors, fuel lines, and vacuum lines. Check everything against your photos and labels.

3. Install the exhaust manifold — torque the nuts to 30 ft-lb (40 N·m).

4. Install the radiator and cooling fans.

5. Install the accessory belts.

6. Refill the engine oil with 5W-30 synthetic (5 quarts).

7. Refill the coolant with Subaru Super Coolant (premix).

First Start — Critical Steps

1. Prime the oil system: Crank the engine with the fuel pump fuse removed for 10 seconds. This builds oil pressure before the first start.

2. Start the engine. It will run rough for a few seconds. Listen for any abnormal noises — if you hear tapping or rattling, shut it off and re-check timing.

3. Check for leaks. Oil and coolant leaks around the head gasket area, valve covers, and water pump.

4. Bleed the cooling system. The EJ25 is notorious for air pockets. Use a vacuum filler if you have one — otherwise, run the engine with the radiator cap off and heater on full hot, topping off the coolant as the thermostat opens.

5. Check the cooling fans. Ensure they come on when the engine reaches temperature.

6. Check the oil and coolant levels after the engine has cooled. Top off as needed.

Torque Table — Fast Summary

Critical: Always verify torque specs against your vehicle's specific factory service manual. Different model years may have different specifications.

Common Pitfalls

🚫 Pitfall 1: Using a single-layer or cheap gasket

This is the mistake that creates repeat customers. A single-layer gasket or a cheap aftermarket gasket will fail. The Fel-Pro MLS is the answer. Spend the extra money.

🚫 Pitfall 2: Not cleaning the head bolt holes properly

Old debris in the head bolt holes will cause false torque readings. The bolts won't clamp properly, and the gasket will leak. Run a thread chaser through every head bolt hole. Use brake cleaner and compressed air.

🚫 Pitfall 3: Not replacing the head bolts

The head bolts are torque-to-yield. They stretch when torqued. Reusing them means you don't get the correct clamping force. Replace them. OEM or ARP — but replace them.

🚫 Pitfall 4: Reusing the old timing belt

Don't do this. If the timing belt fails, the engine is destroyed. The timing belt is cheap compared to a rebuild. Replace the belt, tensioner, and idlers.

🚫 Pitfall 5: Not using the correct coolant

Use Subaru Super Coolant. The EJ25's aluminum components are sensitive to improper coolant. Generic coolant can cause corrosion and contamination.

🚫 Pitfall 6: Not bleeding the cooling system properly

The EJ25 is prone to air pockets. If you don't bleed the system thoroughly, the engine will overheat and damage the new gaskets. Use a vacuum filler if you have one — or be prepared to spend a long time burping the system.

🚫 Pitfall 7: Overtightening the valve cover bolts

The valve cover bolts are small and strip easily. Use an inch-lb torque wrench (84 in-lb). If you tighten them with a 3/8" ratchet, you will strip the threads.

🚫 Pitfall 8: Not checking the valves and valve seals

If the heads are off, this is the time to check valve clearance and replace the valve seals. It's a few hours of work and adds maybe $50 in parts. It saves you from doing this job again.

🚫 Pitfall 9: Using RTV on the head gasket surface

Do not do this. Fel-Pro MLS gaskets seal with clamping force. RTV or sealant will create an uneven surface and cause the gasket to fail. The gasket goes on dry.

🚫 Pitfall 10: Not verifying timing before reassembly

Rotate the crankshaft two full revolutions by hand before you put everything back together. If the timing is off, the engine will run poorly or the pistons will hit the valves. Always verify timing.

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