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 manufacturer's manual for detailed instructions.

COMPRESSION TEST

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 conditions:

BURNED VALVE (valve face damaged by combustion heat).

BURNED VALVED SEAT (cylinder head seat damaged by combustion).

PHYSICAL ENGINE DAMAGE (hole in piston, broken valve, etc.).

BLOWN HEAD GASKET (head gasket ruptured).

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 system.

If the engine is equipped with electronic fuel injection, it should also be disabled to prevent fuel from spraying into the engine. Check the manufacturer's 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 manufacturer's specifications.

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 following procedures:

Remove all injectors or glow plugs. Refer to the manufacturer's manual for instructions.