Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
Engine Compression
Check engine compression pressure with engine at normal operating temperature.
On gasoline engines, remove all spark plugs. Ensure throttle plates are wide open and battery is fully charged. Remove fuel pump or ECM fuse. Disconnect battery terminal from HEI distributor. Crank engine through at least 4 compression strokes.
On diesel engines, remove air cleaner. Install Intake Manifold Cover (J-29664-2) over intake manifold opening. Disconnect wire from injection pump shutoff solenoid. Disconnect glow plug wiring and remove all glow plugs. Use Compression Gauge (J-26999-10) to test individual cylinders. Crank engine through at least 6 compression strokes.
On gasoline engines, the lowest cylinder reading should NOT be less than 70 percent (80 percent on diesel engines) of the highest cylinder reading. No cylinder reading should be less than 100 psi (7 kg/cm2 ).
| Application | Specification | |
|---|---|---|
| Compression Ratio | ||
| 2.5L (VIN E) | 8.3:1 | |
| 2.8L (VIN R) | 8.9:1 | |
| 3.1L (VIN D) | 8.9:1 | |
| 4.3L (VIN Z) | 9.3:1 | |
| 5.0L (VIN H) | 9.3:1 | |
| 5.7L (VIN K) | 8.2:1 | |
| 6.2L Diesel (VIN C & J) | 21.3:1 | |
| 7.4L (VIN N) | 7.9:1 | |
| Min. Compression Pressure | ||
| Gasoline Engines | 100 psi (7 kg/cm2 ) | |
| Diesel Engines | 380-400 psi (26.7-28 kg/cm2 ) | |
| Maximum Variation Between Cylinders | ||
| Gasoline Engines | Less than 30% | |
| Diesel Engines | 20% | |
Use the Manual With the Right Hardware
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
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