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