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 war. 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 information of , 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. Follow the procedures below.Gasoline Engines
- Warm up the engine to normal operating temperature.
- Remove all the spark plugs.
- Disconnect the high tension lead from the ignition coil or the distributor harness connection if so equipped.
- On carbureted cars fully open the throttle either by operating the carburetor throttle linkage by hand or by having an assistant floor the accelerator pedal. On fuel injected cars, disconnect the cold start valve and all injector connections.
- 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 ruined.
- 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 four 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 .
A cylinder's compression pressure is usually acceptable if it is not less than 80% of maximum. The difference between any two cylinders should be no more than 12-14 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.
Maximum cylinder compression for all Datsun/Nissan gasoline engines covered in this guide (except the L24 engine in the 1977-78 810) is 171 psi at 350 rpm. Minimum cylinder compression is 128 psi at 350 rpm. Maximum compression pressure for the 1977-78 810 L24 is 185 psi at 350 rpm. Maximum compression pressure for the V6 engine is 173 psi and the minimum is 128 psi; both are measured at 300 rpm. Compression pressures for the LD28 diesel are 455 psi maximum, 356 psi minimum at 200 rpm. When analyzing compression test results, look for uniformity among cylinders, rather than specific pressures.