GM Full-Size Trucks 1970-1979 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.

As mentioned in the Tools and Equipment section of Section 1, 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



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.
  2.  
  3. Remove all the spark plugs.
  4.  
  5. Disconnect the high tension lead from the ignition coil.
  6.  
  7. On fully open the throttle either by operating the carburetor throttle linkage by hand or by having an assistant floor the accelerator pedal.
  8.  
  9. Screw the compression gauge into the no.1 spark plug hole until the fitting is snug.
  10.  


WARNING
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 vehicles. 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.
  2.