Diesel Engines and Compression

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Diesel engines are internal combustion engines where diesel fuel is burned inside of the engine chamber by a combustion process where the ignition is brought about by high temperature compressed air and pressure. A small amount of diesel fuel is injected (sprayed) at the end of the compression stroke cycle by a fuel injector into the cylinder, and the diesel “mist” is sprayed into the hot air inside the chamber which ignites the fuel, providing a diesel engine’s ignition. The burning, expanding gases push the piston, translating that energy into forward movement. The ability to confine this explosion in the cylinder is measured and called compression. Any leak out of the cylinder is defined as compression loss, which can severely lower the power output of the engine and damge engine components..

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The 7 Steps of How to do a Compression Test on a Tractor Engine

 

1. Warm up the engine. Warm up the engine to normal operating temperature.​

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2. Stop the engine. Stop the engine and let it cool to 75 degrees or less.

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​3. Remove the fuel lines. Remove the high-pressure fuel injection lines as an assembly from the engine. Always remove or install the high-pressure fuel injection lines as an assembly whenever possible. Note: Disassembling the high-pressure fuel injection lines from the retainers or bending any of the fuel lines will make it difficult to reinstall the fuel lines. To prevent "rounding" the fuel line nuts use a "line" or "flare nut" wrench.  Close any fuel valves in the fuel supply line and clean the area to keep any contaminants from entering the fuel system. Place a drain pan under the fuel injection pump to catch any spillage. Loosen the fuel line nuts at the fuel injection pump. Next, loosen the fuel line nuts at the fuel injectors. Use one wrench to hold the fuel return line nut and fuel return line from rotating. Use a second wrench to loosen the fuel line nut Repeat with the remaining fuel injectors. Finish loosening all the fuel line nuts and remove the high-pressure fuel lines as an assembly being careful not to bend any of the fuel lines. Be sure to protect the fuel system from contamination by plugging or covering all open connections. Plug or cap all openings.

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4. Remove one fuel injector.  Remove one fuel injector from the test cylinder. Turn off the fuel supply valve in the fuel supply line. Disconnect the fuel injection stop solenoid at the connector. Crank the engine for a few seconds with the stop solenoid disconnected no injection state before installing the compression gauge adapter which will expel any residual fuel from the cylinder. 

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5. Install one injector gasket. Install one injector gasket at the tip end of the compression gauge adapter. Install the compression gauge and the compression gauge adapter at the cylinder to be measured.

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6. Conduct the test and record the reading. Next, crank the engine until the compression gauge reading is stabilized. Record the reading and take an average of the high and low readings. This is the cylinders average compression.

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7. Reassemble. After performing the compression check and reading, remove the compression gauge and compression gauge adapter from the cylinder. Reinstall the fuel injector, high-pressure fuel injection lines and reconnect the stop solenoid. Turn on the fuel supply valve and reconnect the injection pump stop solenoid. Prime the fuel system. Check for leaks.

 

Consult your tractor service manual or technical manual for the proper compression settings for your engine to determine if the test meets specifications. If your tractor is meeting the previously described symptoms, and the compression test results are lower than manufacturer specifications, One major cause of a loss of compression in a tractor engine is a blown head gasket. If you suspect that your machine may have a bad head gasket, see our article "How to Diagnose and Repair a Tractor Engine Head Gasket."