GM Firebird 1967-1981 Repair Guide

Checking Engine Compression


See Figures 1, 2 and 3

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Fig. Fig. 1: A compression gauge can be used to diagnose many engine problems

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Fig. Fig. 2: Screw the compression fitting into the spark plug hole ...

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Fig. Fig. 3: ... then, while an assistant cranks the engine, read the compression gauge

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 section of General Information And Maintenance , a screw-in type compression gauge is more accurate then 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.

  1. Warm up the engine to normal operating temperature.
  3. Remove all the spark plugs.
  5. Disconnect the high tension lead from the ignition coil.
  7. Fully open the throttle, either by operating the carburetor throttle linkage by hand or by having an assistant hold the accelerator pedal to the floor.
  9. Screw the compression gauge into the No. 1 spark plug hold 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 damaged.

  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 4 to 6 revolutions using the ignition switch. Repeat the test two or three times until a consistent reading is obtained.
  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. The lowest reading recorded should not be less than 70% of the highest reading and no cylinder should be less than 100 psi.

For example, if the highest reading obtained was 150 psi. then the lowest acceptable cylinder reading would be 105 psi. (150 - 105). The difference between any two cylinders 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.