Compression test

1 When engine performance is poor, or if misfiring occurs which cannot be attributed to the ignition or fuel system, a compression test can provide diagnostic clues. If the test is performed regularly it can give warning of trouble before any other symptoms become apparent.

2 The engine must be at operating temperature, the battery must be fully charged and the spark plugs must be removed. The services of an assistant will also be required.

3 Disable the ignition system by disconnecting the coil LT feed. Fit the compression tester to No 1 spark plug hole.

(The type of tester which screws into the spark plug hole is to be preferred.) 4 Have the assistant hold the throttle wide open and crank the engine on the starter.

Record the highest reading obtained on the compression tester.

5 Repeat the test on the remaining cylinders, recording the pressure developed in each.

6 Desired pressures are given in the Specifications. If the pressure in any cylinder is low, introduce a teaspoonful of clean engine oil into the spark plug hole and repeat the test.

7 If the addition of oil temporarily improves the compression pressure, this indicates that bore, piston or piston ring wear was responsible for the pressure loss. No improvement suggests that leaking or burnt valves, or a blown head gasket, may be to blame.

8 A low reading from the two adjacent cylinders is almost certainly due to the head gasket between them having blown.

9 On completion of the test, refit the spark plugs and reconnect the coil LT feed.

    See also:

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