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:

• BLOWN HEAD GASKET (head gasket ruptured).
• PHYSICAL ENGINE DAMAGE (hole in piston, broken valve, etc.).
• BURNED VALVED SEAT (cylinder head seat damaged by combustion).
• BURNED VALVE (valve face damaged by combustion heat).
• 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:

1.      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.

2. 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.
3. 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.
4. 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.
5. 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:

1. Remove all injectors or glow plugs. Refer to the manufacturer’s manual for instructions.
2. 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.
3. Disconnect the fuel shut-off solenoid to disable the fuel injection pump.
4. 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.