GM Full Size Vans 1967-1986 Repair Guide

Checking Engine Compression

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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.

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 for gasoline and diesel engine trucks.

GASOLINE ENGINES



See Figure 1



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Fig. Fig. 1: A screw-in type compression gauge is more accurate and easier to use without an assistant

  1. Warm up the engine to normal operating temperature.
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  3. Remove all spark plugs.
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  5. Disconnect the high tension lead from the ignition coil.
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  7. On fully open the throttle either by operating the carburetor throttle linkage by hand or by having an assistant floor the accelerator pedal.
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  9. Screw the compression gauge into the no.1 spark plug hole until the fitting is snug.
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Be careful not to crossthread the plug hole. On aluminum cylinder heads use extra care, as the threads in these heads are easily ruined.

  1. Ask an assistant to depress the accelerator pedal fully on both carbureted and fuel injected trucks. 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.
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  3. 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 Electrical . The specs in this chart are maximum values.
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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.

  1. 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.
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DIESEL ENGINES



Checking cylinder compression on diesel engines is basically the same procedure as on gasoline engines except for the following:

  1. A special compression gauge adaptor suitable for diesel engines (because these engines have much greater compression pressures) must be used.
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  3. Remove the injector tubes and remove the injectors from each cylinder.
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  5. When fitting the compression gauge adaptor to the cylinder head, make sure the bleeder of the gauge (if equipped) is closed.
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  7. When reinstalling the injector assemblies, install new washers underneath each injector.
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