Install the compression gauge in the recommended opening. A heat shield must be used to seal the gauge when it is installed in place of the injector.

Disconnect the fuel shut-off solenoid to disable the fuel injection pump.

Crank the engine and note the highest reading on the gauge.

A wet compression test should be used when cylinder pressure reads below the manufacturer's specifications. It helps you to determine what engine parts are causing the problem. Pour approximately 1 tablespoon of 30-weight motor oil into the cylinder through the spark plug or injector opening, then retest the compression pressure.

If the compression reading GOES UP with oil in the cylinder, the piston rings and cylinders may be worn and leaking pressure. The oil will temporarily coat and seal bad compression rings to increase pressure; however, if the compression reading STAYS ABOUT THE SAME, then engine valves or head gaskets may be leaking. The engine oil seals the rings, but does NOT seal a burned valve or a blown head gasket. In this way, a wet compression test helps diagnose low-compression problems.

Do NOT put too much oil into the cylinder during a wet compression test or a false reading may result. With excessive oil in the cylinder, compression readings go up even if the compression rings and cylinders are in good condition.

NOTE

Some manufacturers warn against performing a wet compression test on diesel engines. If too much oil is squirted into the cylinder, hydraulic lock and part damage may result, because oil does NOT compress in the small cylinder volume.

Compression readings for a gasoline engine should 3-46 run around 125 to 175 psi. The compression should not vary over 15 to 20 psi from the highest to the lowest cylinder. Readings must be within 10 to 15 percent of each other. Diesel engine compression readings average approximately 275 to 400 psi, depending on the design and compression ratio. Compression levels must not vary more than about 10 to 15 percent (30 to 50 psi). Look for cylinder variation during an engine compression check. If some cylinders have normal pressure readings and one or two have low readings, engine performance is reduced. If two adjacent cylinders read low, it might point to a blown head gasket between the two cylinders. If the compression pressure of a cylinder is low for the first few piston strokes and then increases to near normal, a sticking valve is indicated. Indications of valve troubles by compression test may be confirmed by taking vacuum gauge readings.

VACUUM GAUGE TEST

When an engine has an abnormal compression reading, it is likely that the cylinder head must be removed to repair the trouble. Nevertheless, the mechanics should test the vacuum of the engine with a gauge. The vacuum gauge provides a means of testing intake manifold vacuum, cranking vacuum, fuel pump vacuum, and booster pump vacuum. The vacuum gauge does NOT replace other test equipment, but rather supplements it and diagnoses engine trouble more conclusively.

Vacuum gauge readings are taken with the engine running and must be accurate to be of any value; therefore, the connection between the gauge and the intake manifold must be leakproof. Also, before the connection is made, see that the openings to the gauge and the intake manifold are free of dirt or other restrictions.

When a test is made at an elevation of 1,000 feet or less, an engine in good condition, idling at a speed of about 550 rpm, should give a steady reading from 17 to 22 inches on the vacuum gauge. The average reading will drop approximately 1 inch of vacuum per 1,000 feet at altitudes of 1,000 feet or higher above sea level.

When the throttle is opened and closed suddenly, the vacuum reading should first drop about 2 inches with the throttle open, and then come back to a high of about 24 inches before settling back to a steady reading as the engine idles, as shown in figure 3-77. This is normal for an engine in good operating condition.

If the gauge reading drops to about 15 inches and remains there, it would indicate compression leaks between the cylinder walls and the piston rings or power loss caused by incorrect ignition timing. A vacuum gauge pointer indicating a steady 10 inches, for example, usually means that valve timing of the engine is incorrect. Below-normal readings that change slowly between two limits, such as 14 and 16 inches, could indicate a number of problems. Among them are improper carburetor idling adjustment, maladjusted or