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.
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:
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 fuelinjection, 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 o 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.Ensure batteries are completely charged and the engine starter motor is in good operating condition.
2.Retighten the cylinder head bolts to specified torque. (120NM + Rotate to 900).
3.Warm up the Engine until the coolant temperature reaches 750 C to 850C.
4.Remove all high Pressure pipes/lines from the injectors.
5.Disconnect the fuel shut-off solenoid connection to disable the fuel injection pump (Rotary) or keep stop lever cable in pulled condition.
6.Remove the 1st injector and crank the engine for a while to purge the gases from the cylinder.
7.Install compression gauge adaptor on the injector mounting hole and connect the compression gauge measurement tool.
8.Crank the engine for 3 seconds and note down the reading of compression gauge after maintaining the 200 – 300 RPM
9.Repeat the step – 7 & 8 for other cylinders
If the reading is low than specified , go for wet compression test
Wet Test: Pour 3 – 5 drops around the piston rings by oil can through injector hole.
Repeat the step from 7 & 8, if pressure increases on wet test that indicating the leak in cylinders and piston rings.
If the compression pressure remains same as Dry test that indicating the valve leaks and head gasket leaks.
Do the above test for all cylinders and note down the readings to analyze the conditions of each cylinder.
Tips:
Look for cylinder variation during the test. 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.
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.
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