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 or 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 General Information & Maintenance , a screw-in type compression gauge is more accurate than the type you simply hold against the spark plug hole. Although the screw-in type takes slightly longer to use, it's worth it to obtain a more accurate reading.
See Figures 1 and 2
- Before checking the engine compression, make sure that the battery is fully charged.
- Warm up the engine to its normal operating temperature.
- Shut off the engine, then remove all spark plugs.
- Disconnect the high tension lead from the ignition coil.
- 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. Use extra care on aluminum cylinder heads, as the threads in these heads are easily ruined.
- Ask an assistant to depress and hold the accelerator pedal fully (on both carbureted and fuel injected vehicles). 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 readings of the other cylinders. The readings should be similar throughout the engine.
A cylinder's compression pressure is usually acceptable if it is not less than 80% of the highest reading. For example, if the highest reading is 150 psi, the lowest should be no lower than 120 psi. Also, no cylinder should be less than 100 psi.
- 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 probably 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.
See Figure 3
Checking cylinder compression on diesel engines is basically the same procedure as on gasoline engines, except for the following:
Before checking the engine compression, make sure that the battery is fully charged.
- A special compression gauge adaptor suitable for diesel engines must be used (since these engines have much greater compression pressures).
- Remove the fuel lines and gaskets, and then remove the injectors from each cylinder. See Fuel System .
Don't forget to remove the washer and gasket underneath each injector; otherwise, it may get lost when the engine is cranked.
- When fitting the compression gauge adaptor to the cylinder head, make sure the gauge bleeder (if equipped) is closed.
- When reinstalling the injector assemblies, install new washers underneath each injector. See Fuel System .