Compression Testing

A compression test directly measures the mechanical ability of each cylinder to seal and build pressure. It answers one of the most fundamental diagnostic questions — can this cylinder hold the compression it needs to fire? No amount of fuel or spark fixes a cylinder that cannot build compression.

Disable the fuel system — pull the fuel pump relay or fuse. Disable the ignition system. Remove all spark plugs — all of them, not just the one you are testing. Removing all plugs equalizes cranking resistance and gives accurate readings. Hold the throttle wide open. Install the compression gauge in the first cylinder. Crank the engine for about five revolutions. Record the reading. Repeat for each cylinder. All cylinders should be within 15 percent of each other and above the manufacturer's minimum specification — typically 120 to 180 PSI depending on the engine.

If a cylinder reads significantly low, squirt a small amount of clean engine oil into the cylinder through the spark plug hole. Retest immediately. If the reading jumps up significantly — the oil temporarily sealed the piston rings and the low compression is caused by worn rings or cylinder wall wear. If the reading does not change — the leak is at the valves or head gasket. The oil cannot seal a valve or gasket leak.

A leakdown test goes one step further than a compression test. Compressed air is fed into the cylinder through the spark plug hole with the piston at top dead center on the compression stroke — both valves closed. The tester measures what percentage of air pressure leaks out. Then you listen. Air hissing from the intake — the intake valve is leaking. Air hissing from the tailpipe — exhaust valve is leaking. Air bubbling in the coolant — head gasket is leaking into the cooling passage. Air hissing from the oil fill cap or dipstick tube — the rings are leaking past. The leakdown test not only tells you there is a leak but tells you exactly where it is.