Procedure
- Warm the engine to operating temperature, then remove all four plugs and the fuel pump fuse. Battery on a charger - cranking speed changes readings.
- Compression test each cylinder with the throttle held wide open, cranking the same count (5-6 puffs) per cylinder. Record every value.
- Read the pattern, not the absolute number: a healthy EJ shows all cylinders within about 10 percent of each other. Cylinder 4 reading 15-20 percent low is the classic ringland signature.
- Any low cylinder: a teaspoon of oil down the hole and retest. Reading jumps up = rings/ringlands. No change = valves or head gasket.
- Leakdown adds precision: cylinder at TDC compression, meter reading under 10 percent is strong, over 20 percent needs attention.
- Listen for WHERE the leak exits: hiss at the oil filler is rings, at the intake is an intake valve, at the exhaust is an exhaust valve, bubbles in the coolant is head gasket.
- Verdict for buyers: even compression buys confidence; one weak cylinder prices the car as needing a rebuild - because it does.
Work at your own risk. Torque values and procedures vary by vehicle — always confirm against the factory service manual for your exact model.