Compression Testing

In order for an engine to run smoothly and efficiently, the combustion chamber must be free of leakage. An engine with low compression in one or more cylinders is inefficient and will run rough or lack in performance. Low compression may or may not cause the MIL to illuminate.

Low compression can be caused by the following:

• Leaking valves caused by burned valves or seats. The valve guide can also be worn causing the valve not to seat properly. Valves can also be bent from piston contact (from over-rev).
• Piston Rings which can be worn from high mileage or poor maintenance. Also, the rings can be damaged from foreign material or improper installation.
• Cracks in cylinder head or engine block. Cracks can be caused by overheating resulting in misfires or rough running.
• Defective cylinder head gasket. The cylinder head gasket can fail due to overheating which can cause cylinder leakage resulting in misfire, low compression and rough running.
• Bent connecting rod. A connecting rod can be bent from a defective fuel injector or water ingress into the combustion chamber causing hydrostatic lock.

Compression testing can be performed using a conventional compression gauge. There are some preliminary tasks and safety precautions that must be carried out before starting the compression test:

• Remove the fuel pump fuse and or relay, start the vehicle and allow vehicle to stall out on residual fuel
• Disable ignition by unplugging all ignition coils and remove ALL  sparkplugs.
• Connect battery charger to vehicle
• Ensure that the throttle is wide open during cranking
• Crank engine until compression gauge stops increasing. Be sure to crank engine equally between cylinders.
• Continue compression test on ALL cylinders so comparisons can be done. Record readings.
• If necessary, re-check cylinders with suspect readings.
• If some cylinder readings come up low, add a few drops of oil and re-check. This can differentiate between valves/rings.