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 under Tools and Equipment in Chapter 1 of this manual, a screw-in type 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 obtain a more accurate reading. Follow the procedures below.
See Figures 1 and 2
- Warm up the engine to normal operating temperature.
- Remove all the spark plugs.
- Disconnect the high tension lead from the ignition coil.
- Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug.
- Ask an assistant to depress 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 through 4-5 compression strokes (complete revolutions) until the highest compression is shown on the gauge.
- 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 on the other cylinders. Readings should be similar for 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. 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 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:
- A special compression gauge adaptor suitable for diesel engines (because these engines have much greater compression pressures) must be used.
- Remove the injector tubes and remove the injectors from each cylinder.
- When fitting the compression gauge adaptor to the cylinder head, make sure the bleeder of the gauge (if equipped) is closed.
- When reinstalling the injector assemblies, install new washers underneath each injector.