See Figure 1
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 part of General Information & Maintenance , a screw-in type compression gauge is more accurate that 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.
- Make sure the battery is fully charged.
- Warm up the engine to normal operating temperature, then shut the engine OFF .
- Disable the ignition system by disconnecting the primary ignition wiring from the coil. On early models equipped with an external ignition coil it is also possible to disconnect the secondary wiring (coil-to-distributor lead).
- Remove all spark plugs.
- Block open the throttle linkage in the fully open position.
- 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, as the spark plug threads are easily ruined.
- Set the compression gauge to zero, then use the ignition switch to crank the engine through 4 compression strokes (four "puffs'' on the compression gauge).
- Record the highest reading, clear the compression gauge and repeat to be sure of your results. Record the highest obtained reading from both tests, then remove the compression gauge and repeat at each of the cylinders.
- Compare the highest reading of each cylinder to the readings of the other cylinders. No cylinder should be less than 70 percent of the highest reading. For example, if the highest reading was 150 psi (1035 kPa), then the lowest cylinder should not be below 105 psi (725 kPa).
A cylinder's compression pressure should not be below 100 psi (689 kPa) and the lowest cylinder should NOT be any lower than 70 percent of the highest cylinders reading.
- 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 rises after adding the oil, it is likely 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 would be 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 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.
Different engine conditions should yield appropriate compression test results:NORMAL
- Compression builds up quickly and evenly to the specified compression on each cylinder.PISTON RINGS
- Compression low on the first stroke, then tends to build up on the following strokes, but does not reach normal. This reading should be tested with the addition of a few shots of engine oil into the cylinder. If the compression increases considerably, the rings are leaking compression.VALVES
- Low on the first stroke, does not tend to build up on following strokes. This reading will stay around the same with a few shots of engine oil in the cylinder.HEAD GASKET
- The compression reading is low between two adjacent cylinders. The head gasket between the two cylinders may be blown. If there is signs of white smoke coming from the exhaust while the engine is running may indicate water leaking into the cylinder and being converted into steam. Check around the cylinder head-to-cylinder block area for signs of coolant and oil leakage, indicating a leaking head gasket.