What you need to know before testing compression on your 2007 Outback H6
A compression check on your 2007 Subaru Outback L.L.Bean with the H6 engine tells you whether each cylinder is holding pressure the way it should. Low compression in one or more cylinders points to worn piston rings, a leaking valve, or a head gasket problem — all expensive repairs if ignored. The test itself takes about an hour and costs $100 to $150 at a shop, or you can do it yourself with a compression tester (around $30 to $50) and basic hand tools.
The H6 is a horizontally-opposed six-cylinder engine, which means the cylinders sit flat on either side of the crankshaft. This layout makes accessing all six spark plugs straightforward, though you will need to work around the intake manifold and air filter housing. The test requires the engine to be cold, the battery fully charged, and the throttle held wide open while you crank the engine — so you will need a helper or a way to prop the throttle.
Key Takeaways
- Compression testing requires a cold engine, a fully charged battery, and the throttle held at full open while cranking — do not start the engine.
- The 2007 Outback H6 has six cylinders arranged horizontally, so you access spark plugs from both sides of the engine bay.
- Healthy compression for this engine typically falls between 150 and 180 psi per cylinder, with no more than 15 psi variation between any two cylinders.
- If one cylinder reads significantly lower than the others, a wet test (adding oil to that cylinder and retesting) helps determine whether the problem is worn rings or a valve leak.
- You can rent or buy a compression tester at most auto parts stores, and the test itself requires only basic wrenches and patience.
Gathering the right tools and equipment
You will need a compression tester — the screw-in type works best for this engine because it seats firmly in the spark plug hole. Most auto parts stores rent them for $20 to $30 or sell them for $30 to $50. You will also need a socket set (typically 10mm and 14mm sockets), a ratchet, a spark plug socket, and a helper to crank the engine while you watch the gauge.
Have a notebook ready to write down each cylinder's reading as you test. The cylinders on the 2007 Outback H6 are numbered 1 through 6, with cylinders 1, 3, and 5 on the driver's side and cylinders 2, 4, and 6 on the passenger's side. Knowing which cylinder is which before you start prevents confusion and ensures you catch a weak spot accurately.
If you find low compression in a single cylinder, you may want to have a wet compression test kit on hand — this is just a small bottle of oil and a syringe. Adding a small amount of oil to a low-reading cylinder and retesting tells you whether worn piston rings or a leaking valve is the culprit. If compression jumps after adding oil, the rings are worn; if it stays low, the valve is likely the problem.
Preparing the engine and removing spark plugs
Park the Outback on level ground and let the engine cool completely — compression testing on a warm engine gives false readings. Disconnect the negative battery terminal to prevent accidental starting. Open the hood and locate the spark plug wires or coil packs; on the 2007 H6, you will see three coil packs on the driver's side and three on the passenger's side, each sitting directly over a spark plug.
Gently twist and pull each coil pack or spark plug wire straight up and away from the spark plug — do not yank sideways, as you can damage the connector. Once all six are removed, use a 14mm spark plug socket and ratchet to unscrew each spark plug. Keep them in order so you know which cylinder each one came from. Lay them out in a row labeled 1 through 6.
Before you insert the compression tester, blow compressed air into each spark plug hole to clear out any debris or carbon buildup. This step prevents dirt from entering the cylinder and affecting your reading.
Running the compression test on each cylinder
Screw the compression tester firmly into the first spark plug hole until it seats snugly — do not over-tighten. Have your helper sit in the driver's seat with the key in the ignition. Tell them to hold the throttle pedal all the way to the floor (or prop it open with a block of wood if you are working alone) and crank the engine for about 5 to 10 seconds without starting it. Watch the gauge needle; it will climb with each engine rotation and then settle at the highest reading.
Write down the reading for cylinder 1, then release the pressure valve on the tester (usually a small button or lever) to reset the gauge to zero. Move to cylinder 2 and repeat the process. Continue through all six cylinders, recording each reading. The entire test takes about 10 to 15 minutes once you develop a rhythm.
Healthy compression on the 2007 Outback H6 typically ranges from 150 to 180 psi per cylinder. If all six cylinders read within 15 psi of each other — for example, all between 160 and 175 psi — the engine is in good shape. If one or more cylinders read 20 psi or more below the others, that cylinder has a problem.
Interpreting low compression and running a wet test
If a single cylinder reads low, the next step is a wet compression test to narrow down the cause. Add about a teaspoon of engine oil directly into that cylinder through the spark plug hole using a syringe or oil can. Screw the compression tester back in and crank the engine again for 5 to 10 seconds, watching the gauge.
If compression jumps significantly — say from 120 psi to 160 psi — the piston rings in that cylinder are worn and no longer sealing properly. The oil temporarily fills the gap and restores compression. If compression stays low even after adding oil, one of the intake or exhaust valves in that cylinder is leaking and not sealing against its seat. A leaking valve is typically a more serious problem than worn rings and usually requires cylinder head work.
If multiple cylinders read low and the wet test shows improvement in all of them, the problem is likely overall ring wear across the engine rather than a single cylinder failure. If multiple cylinders stay low even after the wet test, you may have a head gasket leak or a cracked head — both require professional diagnosis.
Reinstalling spark plugs and reconnecting the engine
Once you have recorded all your readings, remove the compression tester and reinstall each spark plug in its original hole. Hand-thread each one first to avoid cross-threading, then tighten with the spark plug socket and ratchet. Do not over-tighten; snug is enough. Reconnect each coil pack or spark plug wire by pushing straight down until you hear or feel a click.
Reconnect the negative battery terminal. Start the engine and listen for any changes in idle quality or rough running — if the engine sounds different than before, double-check that all coil packs and wires are seated properly. Let the engine run for a minute to confirm everything is working normally, then turn it off.
What to do with your compression results
If all cylinders read between 150 and 180 psi with no more than 15 psi variation, your engine compression is normal and you can rule out major internal wear as the cause of any running issues you are experiencing. If you are troubleshooting a specific problem — rough idle, misfiring, or poor fuel economy — the compression test confirms the engine itself is sound and points you toward ignition, fuel, or sensor issues instead.
If compression is low in one or more cylinders, the wet test results guide your next step. Worn rings typically develop gradually and may not require when ready repair if the engine is not burning excessive oil. A leaking valve usually worsens over time and should be addressed sooner rather than later. Either way, a professional mechanic can inspect the cylinder head and advise whether repair is urgent or can wait.
Frequently Asked Questions
Can I do a compression test on a warm engine?
You can, but the readings will be artificially high and unreliable. A cold engine gives consistent, accurate results. If the engine is warm when you start, wait at least 30 minutes for it to cool completely before testing.
What if the compression tester leaks or does not hold a reading?
Make sure the tester is screwed in firmly and the spark plug hole is clean. If it still leaks, the tester may be defective — try a different one. A poor seal gives false low readings and wastes your time.
Do I need to disconnect the fuel injectors or ignition to prevent starting?
Disconnecting the negative battery terminal is sufficient and safest. You can also remove the fuel pump relay if you want extra insurance, but it is not necessary as long as the battery is disconnected.
How much variation between cylinders is normal?
Up to 15 psi difference between the highest and lowest cylinder is acceptable. Anything more than 20 psi difference suggests a problem in the low-reading cylinder or cylinders.
If compression is low, does the engine need to be rebuilt?
Not necessarily. Worn piston rings may not require when ready repair if oil consumption is acceptable. A leaking valve may be repairable by removing and resurfacing the cylinder head. A professional mechanic can inspect the engine and advise whether repair is worth the cost given the vehicle's age and condition.