Wet vs Dry Compression Tests
Performing dry and wet compression tests helps diagnose engine issues by pinpointing the source of compression loss.
To do a dry compression test on a diesel engine, first warm up the engine, disable fuel and ignition, and fully depress the accelerator while cranking. Connect a compression gauge to each cylinder's injector hole and crank the engine through several compression strokes, recording the highest reading per cylinder. This shows overall compression health.
A wet compression test involves adding about one teaspoon of engine oil into the cylinder before cranking again. The oil temporarily seals the piston rings. If compression rises significantly (often 40 PSI or more) during the wet test compared to the dry test, it indicates worn or damaged piston rings as the source of compression loss. If there’s little or no change in readings, the problem likely lies with the valves, valve seats, or cylinder head gasket.
It's important to note that wet compression tests are not advised for diesel engines due to the risk of hydraulic lock from their high compression ratios, which can cause engine damage. Hence, diesel engines typically undergo only dry compression testing. Consistent readings among cylinders within about 10-20% of each other indicate good engine condition. Large deviations or low readings suggest internal wear or damage, guiding necessary repairs or rebuilds. Always consult your tractor's manual for specific procedures and pressure specs.
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