Ford Mustang/Capri 1979-1988 Repair Guide

Checking Engine Compression


See Figure 1

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 problem(s).

As mentioned in the Tools and Equipment portion of General Information & Maintenance , a screw-in type compression gauge is more accurate than the type you simply hold against the spark plug hole, even though it takes slightly longer to use. The little bit of additional time is worth it, however, to obtain a more accurate reading. To check engine compression, 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 floor the accelerator pedal.
  9. 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.

Click image to see an enlarged view

Fig. Fig. 1: The screw-in type compression gauge is more accurate

  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.
  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 for each cylinder to those of the other cylinders. Generally, a cylinder's compression pressure is acceptable if it is at least 80% of the highest reading of all other cylinders. For example, if the maximum reading of any cylinder in a given engine is 150 psi, then none of the highest readings of the other cylinders should fall below 120 psi.
  5. 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 does not 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.