See Figure 1
- Warm the engine to normal operating temperatures.
- Label and disconnect all spark plug wires at the plugs. Remove all spark plugs from the engine.
- Disconnect the high tension lead from the ignition coil.
- On carbureted vehicles, fully open the throttle either by operating the carburetor throttle linkage by hand or by having an assistant floor the accelerator pedal.
- Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.
Be careful not to crossthread the plug hole. On aluminum cylinder heads use extra care, as the threads in these heads are easily ruined.
- Ask an assistant to depress the accelerator pedal fully. Then, while you read the compression gauge, ask the assistant to crank the engine two or three times in short bursts using the ignition switch.
- Read the compression gauge at the end of each series of cranks and record the highest of these readings. Repeat this procedure for each of the engine's cylinders. Compare the highest reading of each cylinder to the compression pressure specifications in the Tune-Up Specifications'' chart in Engine Performance and Tune-up . The specs in this chart are maximum values.
A cylinder's compression pressure is usually acceptable if it is not less than 80% of maximum. The difference between each cylinder should be no more than 12-14 pounds.
- If a cylinder is unusually low, pour a tablespoon of clean engine oil into the cylinder through the spark plug hole and repeat the compression test. If the compression comes up after adding the oil, it appears that the cylinder's piston rings or bore are damaged or worn. If the pressure remains low, the valves may not be seating properly (a valve job is needed) or the head gasket may be blown near that cylinder. If compression in any two adjacent cylinders is low and if the addition of oil doesn't help the compression, there is leakage past the head gasket. Oil and coolant water in the combustion chamber can result from this problem. There may be evidence of water droplets on the engine dipstick when a head gasket has blown.