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.
pressure is lower than normal, pressure is leaking out of the combustion chamber. Low
engine compression can be caused by the following conditions:
HEAD GASKET (head gasket ruptured).
ENGINE DAMAGE (hole in piston, broken valve, etc.).
VALVED SEAT (cylinder head seat damaged by combustion).
VALVE (valve face damaged by combustion heat).
RINGS OR CYLINDERS (part wear that prevents a ring-to-cylinder seal).
TRAIN TROUBLES (valve adjusted with insufficient clearance. This keeps the valve from
fully closing. Also, broken valve spring, seal, or retainer).
TIMING CHAIN OR BELT (loose or worn chain or belt has jumped over teeth, upsetting
To perform a compression test on a gasoline engine, use the
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.
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
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
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.
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 specifications.
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:
all injectors or glow plugs. Refer to the manufacturers manual for instructions.
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.
the fuel shut-off solenoid to disable the fuel injection pump.
the engine and note the highest reading on the gauge.
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.
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.
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.
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.