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 Tools and Equipment 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 it takes slightly longer to use. It's worth it to obtain a more accurate reading.
See Figure 1
- Warm up the engine to normal operating temperature. Turn the engine OFF .
- Remove all spark plugs.
- Disable the ignition system by disconnecting the battery feed to the coil.
- Fully open the throttle either by operating the carburetor throttle linkage by hand or by having an assistant floor the accelerator pedal.
- Apply engine oil to the gauge fitting and 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 crossthreaded.
- 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.
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 does not help the compression, there is leakage past the head gasket. Oil and coolant in the combustion chamber can result from this problem. There may be evidence of coolant droplets on the engine dipstick when a head gasket has blown. Another sign of a blown head gasket is white smoke coming from the exhaust pipe when the engine is at normal operating temperature.
See Figure 2
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.
Do not forget to remove the washer 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 bleeder of the gauge (if equipped) is closed.
- When reinstalling the injector assemblies, install new washers underneath each injector.