Valve adjusting shims may also be used on OHC
engines for the cam-to-valve clearance. To determine
whether shims are required, measure the valve
clearance with a feeler gauge. Then, if needed remove
or change the shim thickness as necessary.
Other OHC engines have an Allen adjusting screw
in the cam followers. Turning the screw changes the
valve clearance. Always refer to the manufacturers
manual for detailed instructions.
A compression test is one of the most common
methods for determining the mechanical condition of an
engine. It should be done when symptoms (engine miss,
rough idle, puffing noise in induction or exhaust) point
to major engine problems. Measure compression
pressures of all cylinders with a compression gauge (fig.
3-76). Then compare them with each other and with the
manufacturer's specifications for a new engine. This
provides an accurate indication of engine condition.
When gauge pressure is lower than normal,
pressure is leaking out of the combustion chamber. Low
engine compression can be caused by the following
BLOWN HEAD GASKET (head gasket
PHYSICAL ENGINE DAMAGE (hole in
piston, broken valve, etc.).
BURNED VALVED SEAT (cylinder head seat
damaged by combustion).
BURNED VALVE (valve face damaged by
Figure 3-76.Cylinder compression tester.
WORN RINGS OR CYLINDERS (part wear
that prevents a ring-to-cylinder seal).
VALVE TRAIN TROUBLES (valve adjusted
with insufficient clearance. This keeps the valve
from fully closing. Also, broken valve spring,
seal, or retainer).
JUMPED TIMING CHAIN OR BELT (loose
or worn chain or belt has jumped over teeth,
upsetting valve timing).
To perform a compression test on a gasoline engine,
use the following procedures:
Remove all spark plugs so the engine can rotate
easily. Block open the carburetor or fuel
injection pump throttle plate. This prevents
restricted air flow into the engine.
Disable the ignition system to prevent sparks
from arcing out of the disconnected spark plug
wires. Usually, the feed wire going to the
ignition coil can be removed to disable the
If the engine is equipped with electronic fuel
injection, it should also be disabled to prevent
fuel from spraying into the engine. Check the
manufacturers manual for specific directions.
Screw the compression gauge into one of the
spark plug holes. Some gauges have a tapered
rubber-end plug and must be held by hand
securely in the spark plug opening until the
highest reading is obtained.
Crank the engine and let the engine rotate for
about four to six compression strokes
(compression gauge needle moves four to six
times). Write down the gauge readings for each
cylinder and compare them to the manufacturers
The compression test for a diesel engine is similar
to that of a gasoline engine; however, do not use the
compression gauge intended for a gasoline engine. It
can be damaged by the high-compression-stroke
pressure. A diesel gauge must be used that reads up to
approximately 600 psi.
To perform a diesel compression test, use the
Remove all injectors or glow plugs. Refer to the
manufacturers manual for instructions.