What a compression check tells you about your H6 engine

A compression check measures how much pressure builds up in each cylinder when the piston moves up on the intake stroke. On your 2007 Outback's H6 engine—the 3.0-litre six-cylinder—this test reveals whether the rings, valves, and head gasket are still sealing properly. Low compression in one or more cylinders points to worn piston rings, a burned valve, or a leaking head gasket, all of which will eventually cost you power, fuel economy, and reliability.

The H6 is a horizontally-opposed engine, which means the cylinders sit flat on either side of the crankshaft rather than in a vertical line. This layout affects how you access the spark plugs and where you'll work, but the compression test itself follows the same principle as any other engine. You're looking for a baseline number and then checking whether all six cylinders are within about 10 percent of each other. A single weak cylinder is often more telling than an overall low reading.

Key Takeaways

  • Compression on a healthy 2007 Outback H6 typically reads between 180 and 200 psi per cylinder, with all six within 10 percent of each other.
  • You will need a compression gauge, a spark plug socket, a ratchet, and a helper to crank the engine while you watch the gauge.
  • Remove all six spark plugs before testing; leaving them in place will give false readings and make cranking harder.
  • If one cylinder reads 20 psi or more below the others, suspect a burned valve, worn rings, or head gasket failure in that area.
  • A wet compression test (adding a small amount of oil to a low-reading cylinder) can help you tell whether the problem is the rings or the valves.

Gathering the right tools and preparing the engine

You will need a compression gauge with a range of 0 to 300 psi—most auto parts stores rent these for a small deposit, or you can buy one for $20 to $40. You'll also need a spark plug socket (usually 5/8 inch for this engine), a ratchet, a spark plug gap tool or wire brush to clean the plugs, and a helper to turn the key while you read the gauge. Make sure the engine is cold before you start; a warm engine will give slightly higher readings and is harder to crank by hand.

Park on level ground, set the parking brake, and disconnect the negative battery terminal to prevent accidental starting. Remove the engine cover if your Outback has one—it's usually held by a few clips or bolts. On the H6, the spark plugs sit on top of each cylinder head, three on the driver's side and three on the passenger's side. Use a spark plug socket and ratchet to unscrew all six plugs and set them aside in order so you know which cylinder each one came from. Do not skip this step; the engine will not crank hard enough to build proper compression with the plugs still in place.

Running the compression test on each cylinder

Screw the compression gauge into the first spark plug hole—usually cylinder 1, which is the front cylinder on the driver's side of the H6. Have your helper sit in the driver's seat with the key in the ignition. Tell them to crank the engine (turn the key to the start position) for about 5 to 8 seconds without letting it actually start. Watch the gauge needle rise with each stroke of the piston. The highest number it reaches is your compression reading for that cylinder.

Write down the number, then remove the gauge and move to the next cylinder. Repeat this process for all six cylinders, cranking for the same amount of time each time so the readings are consistent. A healthy 2007 Outback H6 typically shows 180 to 200 psi per cylinder. If all six are within 10 psi of each other, your rings, valves, and gaskets are in good shape. If one or two cylinders read 20 psi or more below the others, you have a localized problem in that area.

Understanding what low compression in one cylinder means

If cylinder 3 reads 165 psi while the others read 190, the problem is in that cylinder alone. The most common causes are a burned intake or exhaust valve, worn piston rings, or a head gasket leak between that cylinder and the water jacket. A burned valve will not seal, so pressure escapes during the compression stroke. Worn rings allow pressure to leak past the piston into the crankcase. A head gasket leak lets pressure escape into the cooling system or between cylinders.

To narrow it down, perform a wet compression test on the low cylinder. Remove the gauge, squirt about a tablespoon of clean engine oil into that spark plug hole, then screw the gauge back in and crank again. If the reading jumps up significantly—say from 165 to 185 psi—the oil temporarily sealed the worn rings, which means your piston rings are the problem. If the reading stays low, the problem is a valve or head gasket, and you will need a more detailed inspection or a borescope camera to confirm which one.

When all cylinders read low but equal

If all six cylinders read 150 to 160 psi and they are all within a few psi of each other, the engine has uniform wear across all cylinders. This usually means the piston rings are worn evenly throughout the engine, or the engine has straightforward accumulated high mileage and lost some compression naturally. A 2007 Outback with 150,000 miles or more may show this pattern without any when ready failure.

Uniform low compression does not always mean the engine is failing right now. Many engines run reliably with 150 psi per cylinder, though they will use more oil, start harder in cold weather, and produce less power. If your Outback starts easily, does not burn oil noticeably, and runs smoothly, uniform low compression is often just a sign of age rather than an urgent repair. However, if compression is below 140 psi across the board, or if the engine is hard to start or misfires, a top-end rebuild or engine replacement may be necessary within the next few thousand miles.

Head gasket failure and the H6 layout

The 2007 Outback H6 is known for head gasket issues, particularly around 100,000 to 150,000 miles. A failing head gasket often shows up as low compression in two adjacent cylinders on the same side of the engine—for example, cylinders 2 and 3 both reading 20 psi below the others. This pattern suggests the gasket between those two cylinders is leaking. You may also notice coolant in the oil, a sweet smell from the exhaust, or white steam from under the hood.

If your compression test points to a head gasket, do not ignore it. A leaking gasket will allow coolant into the cylinders, which thins the oil and accelerates wear on the rings and bearings. The longer you drive with a failed gasket, the more expensive the eventual repair becomes. A head gasket replacement on the H6 is a significant job—the engine must be partially disassembled—but catching it early prevents catastrophic engine damage.

Reinstalling the spark plugs and next steps

Once you have recorded all six readings, reinstall the spark plugs in the order you removed them. Use a spark plug gap tool to check the gap on each plug (the 2007 Outback H6 uses a 0.028 to 0.032 inch gap) and clean any corrosion with a wire brush. Screw each plug back in by hand first to avoid cross-threading, then tighten with the socket and ratchet. Reconnect the negative battery terminal and reinstall the engine cover.

If your compression readings are healthy and even, your engine is in good mechanical condition and you can move on to other diagnostics if the vehicle has a specific complaint. If you found low compression in one or more cylinders, the next step depends on the pattern. A single weak cylinder may warrant a borescope inspection to see the valve and piston condition before committing to a repair. Uniform low compression or a head gasket pattern usually means you need to plan for a more involved repair in the near term.

Frequently Asked Questions

What if the gauge will not screw in or keeps leaking?

The spark plug hole threads may be dirty or damaged. Clean the hole with a wire brush and try again. If the gauge still leaks, the threads may be stripped, which is rare but requires professional repair. Do not force the gauge in; you can damage the threads further.

Can I do this compression test without removing the spark plugs?

No. The spark plugs must be out or the engine will not crank hard enough to build real compression. The starter motor will struggle against the resistance of six cylinders firing, and your readings will be falsely low and inconsistent.

Is 170 psi per cylinder acceptable on a 2007 Outback H6?

If all six cylinders read 170 psi and are within 5 psi of each other, the engine is acceptable, though slightly below the ideal 180 to 200 range. This is common on higher-mileage engines. Watch for oil consumption and cold-start difficulty; if neither is a problem, the engine is still reliable.

What does it mean if one cylinder reads 80 psi?

A reading that low indicates a major problem in that cylinder—likely a burned valve, a cracked piston, or a severe head gasket leak. The engine will misfire on that cylinder and run rough. This requires professional diagnosis and repair; the engine should not be driven hard until the problem is identified.

Should I repeat the test if the readings seem odd?

Yes. Make sure the engine is cold, the battery is fully charged so the starter cranks at full speed, and you crank for the same duration on each cylinder. Inconsistent cranking speed or a weak battery will give false readings. If the second test matches the first, your readings are reliable.