See Figures 1 through 5
A noticeable lack of power, excessive oil consumption and/or poor fuel mileage measured over an extended period are indicators of internal engine wear. Worn piston rings, scored/worn cylinder bores, blown head gaskets, sticking or burnt valves and worn valve seats are all possible culprits. A check of each cylinder's compression will help you locate problems.
- Always use a fully charged battery and warm the engine to operating temperature.
- Turn the ignition switch OFF .
- Remove all spark plugs.
- Disconnect the high tension lead from the ignition coil.
- Properly relieve the fuel system pressure.
- Screw the compression gauge into the No. 1 spark plug hole until the fitting is snug. On VG30E engines, use a compression tester that has a rubber tip on the end with a diameter of less than 0.79 in. (20mm), or else it may snag on the cylinder head during removal.
Be careful not to crossthread the plug hole. On aluminum cylinder heads use extra care, as the threads in these heads are easily ruined.
- 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 to the compression pressure specification in the Tune-Up Specifications chart in Engine Performance and Tune-up . The specifications in this chart are maximum values.
Compression pressure specifications are included in Chapter 2, but the main thing to look for is uniformity; a cylinder's compression is usually acceptable if it is not less than 80% of the highest cylinders's 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 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 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.