Mechanical Inspection: Compression

The results of a compression test will fall into the following categories:

• Normal Compression builds up quickly and evenly to specified compression on each cylinder.
• Piston Rings Compression is low on the first stroke, tends to build up on following strokes, but does not reach normal. Compression improves considerably with the addition of oil.
• Valves Compression is low on the first stroke, does not tend to build up on the following strokes, and does not improve much with the addition of oil. Use approximately three squirts from a plunger-type oiler.

1. Conduct the following steps in order to check cylinder compression:
   • Engine should be at room temperature.
   • Disconnect wiring from the ignition module.
   • Remove the spark plugs.
   • Throttle body valve should be wide open.
   • Battery should be at or near full charge.
2. For each cylinder, crank engine through four compression strokes.
3. The lowest reading cylinder should not be less than 70 percent of the highest.
4. No cylinder reading should be less than 100 psi (689 kPa).