Testing for Compression Loss

For an engine to run, it must have three things:
• Sufficient compression
• Fuel (in a flammable ratio to air)
• Ignition (timed at the appropriate instant)
A quick way to check for uneven compression between cylinders is to disable the ignition system and crank the engine. An uneven cranking rhythm indicates unequal compression.


Compression Test
One of the most common and least expensive pieces of test equipment is the compression tester. A compression tester is simply a pressure gauge that is inserted into a spark plug hole. There are two styles of compression testers. One is held in place while cranking the engine (Figure 3.19a). It is handy on in-line engines because it is fast and easy. The other type is the screw-in tester. Spark plugs use one of two thread sizes. The tester

a

b
FIGURE 3.19 Compression testers. (a) This compression tester
can be used when there is easy access to the spark plug holes.
(b) A screw-in compression tester with adapters. The adapter shown
on top is for larger-diameter spark plug threads. The two adapters
beneath it are for different lengths (reaches) of the smaller spark plug
thread size.

shown in Figure 3.19b has adapters to accommodate both thread sizes and different thread lengths. It has

a Schrader valve, very similar to a tire valve, for saving the pressure in the gauge so that the technician

can read it.

The advantage of the screw-in tester is that it can be threaded into the plug hole, leaving the technician free to crank the engine.