To control blow-by in a diesel, measure crankcase pressure using the procedure and limit for that exact engine, then determine whether the cause is restricted ventilation or internal wear. Smoke or vapor at the oil filler can prompt an inspection, but it cannot establish that an engine has excessive blow-by or identify its cause.
What blow-by is—and why high crankcase pressure is not a diagnosis
Blow-by is combustion gas that passes the piston-and-ring sealing path and enters the crankcase. Some gas entering the crankcase is part of engine operation; the concern is a measurement or symptoms that exceed the manufacturer’s criteria.
High crankcase pressure can reflect excessive gas entering the crankcase, but it can also occur when the ventilation route is restricted. Caterpillar’s C13/C15/C18 troubleshooting procedure considers both plugged crankcase ventilation and damaged or worn internal components. Treat pressure as a finding to investigate, not proof of worn rings.
Excess pressure can contribute to oil leakage at gaskets and seals. Caterpillar’s procedure also notes that high blow-by can restrict the ventilation system more quickly, so a ventilation fault and internal leakage may compound one another.
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Measure blow-by using the exact engine procedure
A visual check at the oil filler is not a substitute for a crankcase-pressure or blow-by test. Cummins service literature for ISM/ISMe/QSM11 describes measurement with a water manometer, pressure gauge, or transducer connected using the applicable blow-by measurement service tool. Use the procedure for your engine family; neither the test connection nor a pressure value should be transferred to another engine.
Test conditions matter. Caterpillar’s “Measuring Engine Blow-by and Diluting Crankcase Emissions” document says, for the procedure it describes, “All measurements are to be taken with the engine running at rated speed, load, and temperature.” An idle-only observation or improvised setup may not produce a result that can be compared with the engine’s specification.
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There is no single acceptable blow-by figure established for all diesel engines. Find the exact engine model and configuration, consult its current OEM service information, and compare a measurement taken under the stated conditions with that source’s limit. Cummins provides a service-literature lookup by engine model and category: Cummins QuickServe Online.
Check for Excessive Engine Crankcase Pressure (Blow-By)
The following ordered checks reflect a Caterpillar C13/C15/C18 troubleshooting example, not a universal checklist for every diesel. Follow the diagnostic procedure for your own engine before removing parts or drawing conclusions.
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- Confirm oil level. Check it as directed in the engine’s manual; do not assume pressure alone identifies the fault.
- Inspect the crankcase ventilation route. Look for a plugged or restricted open ventilation system and service it according to the applicable instructions.
- Check the related components. In the Caterpillar example, inspect the check valve and oil-disposal tube.
- Assess sensor plausibility. Verify the sensor reading and condition using the model-specific diagnostic procedure.
- Measure blow-by if ring leakage is suspected. Use the specified tool, connection, operating conditions, and pass/fail criteria before considering internal engine problems.
Caterpillar’s procedure identifies damaged pistons or rings, worn valve guides, and failed turbocharger seals as possible internal causes. It states, “Damaged pistons or rings can cause too much pressure in the crankcase.” These are possibilities to investigate—not conclusions that can be made from filler-cap vapor alone.
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For a measurement tool, check compatibility with the engine maker’s service adapter, a suitable pressure range and resolution, and the test conditions required by the correct manual. Cummins literature names water manometers, pressure gauges, and transducers; Caterpillar’s separate measurement document describes its Blow-by/Air Flow Indicator, part number 8T2700. These examples do not establish a universally compatible test kit.
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If inspection finds a restricted replaceable ventilation filter, use the replacement specified for the exact engine and follow its Operation and Maintenance Manual. A filter may be serviceable, but replacing it does not repair worn pistons, rings, valve guides, or a failed turbocharger seal.
Repair the designed ventilation system as specified. Do not treat removing, blocking, or rerouting crankcase ventilation as a generic fix; the procedures cited here do not establish a safe universal rerouting method.
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Use model-specific service information before deciding what is excessive
The cited procedures cover selected engine families, not every diesel configuration. For Caterpillar C13/C15/C18, the high-pressure troubleshooting example is reproduced by a third-party maintenance site; the Caterpillar measurement document is hosted on an official Caterpillar domain. The Cummins ISM/ISMe/QSM11 measurement procedure is reproduced by a third-party site, while the service-literature lookup is official. Confirm the current manual and revision for your engine before applying a test setup, interpreting a symptom, or ordering a part.
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