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Section 4 Engine Service
Testing Compression
A cylinder compression test can be a fi rst step
toward determining the condition of the upper major
mechanical parts of the engine. This test is especially
valuable if an engine lacks power, runs poorly, and
shows little or no improvement after fuel system and
ignition adjustments. To perform a compression test:
1. Run the engine until it is warm.
2. Disconnect all drives to the engine.
3. Open the choke and throttle valves wide.
4. Remove the air cleaner.
5. Remove the spark plug and insert the
compression gauge. See Figure 13-28.
6. Crank the engine as fast as possible and read
the gauge. Repeat the test to ensure accuracy.
Engines equipped with compression release
camshafts may have to be cranked in reverse rotation
to obtain an accurate reading.
Note
Engines E En n s s equipped q p d with compression m release
camshafts ca a s s a may a h ave t o be cranked in reverse rotation
t to o obtain b b n n an n accurate a u reading. i
An engine producing a compression less than
the minimum specifi ed by the manufacturer usu-
ally has one or more of the following problems:
• Leaking cylinder head gasket.
• Warped cylinder head.
• Worn piston rings.
• Worn cylinder bore.
• Damaged piston.
• Burned or warped valves.
• Improper valve clearance.
• Broken or weak valve springs.
To determine whether the valves or rings are
at fault, pour a tablespoonful of SAE 30 oil into the
spark plug hole. Crank the engine several times to
spread the oil and repeat the compression test. The
heavy oil will temporarily seal leakage at the rings.
If the compression does not improve, the leakage
is due to problems with the valves, cylinder head,
or piston. This does not disprove a problem with
the rings; the engine may have worn rings in addi-
tion to its other problems. On the other hand, if the
compression is much higher than the original test,
the leakage is due solely to defective piston rings.
Figure 13-27.
This optical tachometer can measure rpm without
contacting the engine by aiming it at a rotating part that
has a refl ective tape attached. (Monarch Instrument)
Figure 13-28.
A compression test can indicate the condition of various
mechanical components of an engine.