Healthy 4.0L: 120-150 psi across all six cylinders, within 10% of each other. Below 100 psi or large spread indicates head, rings, or valves.
A compression test is the fastest way to assess 4.0L health. You need a screw-in compression gauge (the rubber-tip ones leak on cylinders 5/6 because of plug position). Procedure: engine at operating temp, all six plugs removed, throttle blocked open, fuel pump relay pulled (prevent flooding/fire), crank engine 5-7 revolutions on each cylinder and record peak pressure. Spec for stock 4.0L is 120-150 psi with all cylinders within 10% of each other (about 15 psi spread). Below 100 psi on any cylinder indicates a problem. Wet test: squirt a teaspoon of motor oil into the spark plug hole and retest - if compression rises significantly, the rings are worn; if compression stays low, valves or head gasket are the cause. Adjacent low cylinders point to a head gasket between them. Across-the-board low compression on a fresh engine is more often timing chain stretch (a known issue at 200k+).
| Part | Vendor | Est. price |
|---|---|---|
| Compression tester kit | Harbor Freight | ~$35 |
Written and maintained by an AZ wheeler and driveway wrencher. Always cross-reference your factory service manual — modifications affect vehicle safety and warranty. Work at your own risk.