A compression test measures how well each cylinder seals during the compression stroke. It is one of the most informative diagnostic tests you can perform on an engine and reveals worn rings, leaking valves, blown head gaskets, and worn cylinders. Understanding how to perform the test and interpret the results helps you diagnose poor performance, oil consumption, and hard starting.
How to Perform a Compression Test
Warm the engine to operating temperature, then disable the ignition (disconnect coil pack power or pull the fuel pump fuse to prevent the engine starting) and remove all spark plugs. Thread the compression tester adapter into the spark plug hole of cylinder one. Have an assistant hold the throttle wide open while you crank the engine for 5 to 8 compression strokes (you should see the gauge needle climb with each stroke and then stabilise). Record the reading. Release the pressure and repeat for each cylinder. Always test with the throttle wide open – a closed throttle restricts intake airflow and gives artificially low readings.
Interpreting Results
Most petrol engines should produce 130 to 180 psi when cranking. Diesel engines produce much higher readings (typically 300 to 500 psi). The exact specification for your engine is in your workshop manual. The most important number is not the absolute value but the variation between cylinders. All cylinders should be within 10% of each other. A cylinder that is 20% or more below the others has a problem.
The Wet Test
If a cylinder reads low, perform a wet test to narrow down the cause. Squirt a small amount of engine oil (about a teaspoon) into the cylinder through the spark plug hole, then repeat the compression test. If the reading rises significantly, the rings are worn or broken (the oil temporarily seals them). If the reading does not change, the problem is with the valves (a valve is not sealing) or the head gasket. This single test eliminates half the possible causes.
Common Compression Problems
Two adjacent cylinders both reading low usually indicates a blown head gasket between them (the gasket has failed and they are sharing compression). One cylinder reading zero with the others normal indicates a severely burnt valve, broken piston, or cracked head. All cylinders reading low indicates worn rings throughout the engine (a tired engine that needs a rebuild) or a badly stretched timing chain causing valve timing issues. Slightly low readings on all cylinders may simply indicate a high-mileage engine that is still serviceable but past its prime.
Leak-Down Testing
A leak-down test is a more advanced version that pinpoints exactly where compression is escaping. With the cylinder at top dead centre on the compression stroke, regulated air is fed into the cylinder through the spark plug hole. Listening at the exhaust pipe (exhaust valve leak), throttle body (intake valve leak), oil filler cap (ring leak), or radiator (head gasket leak) reveals the leak path. Combined with a compression test, leak-down testing tells you exactly what is wrong inside the engine.
Compression specifications and acceptable variation limits are vehicle-specific. MechanicMate offers PDF workshop manuals for over 960 models at mechanicmate.net/shop.
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