A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period are all indicators of internal engine wear. Worn piston rings, scored and worn cylinder bores, blown head gaskets, sticking or burnt valves and worn valve seats are all possible culprits here. A check of each cylinder's compression will help you locate the problems.
As mentioned in the "Tools and Equipment" section of , a screw-in compression gauge is more accurate than the type you simply hold against the spark plug hole, although it takes slightly longer to use (it's worth it). To check compression:
- Warm the engine up to operating temperature.
- Remove all of the spark plugs.
- Disconnect the high tension wire from the ignition coil.
- Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug. Be very careful not to crossthread the hole, as the head is aluminum.
- Fully open the throttle either by operating the carburetor throttle linkage by hand, or on fuel injected cars having an assistant "floor" the accelerator pedal.
- Ask the assistant to crank the engine a few times using the ignition switch.
- Record the highest reading on the gauge.
The variation between or uniformity among cylinders is the important thing to look for when testing engine compression, not necessarily the highest or lowest cylinder readings.
- Repeat the procedure for the remaining cylinders, recording each cylinder's compression. The difference between each cylinder should be no more than 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 that 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.