Short answer
Every diesel has some blow-by: combustion gas that slips past the piston rings into the crankcase. Excess blow-by can come from worn rings or cylinders, damaged pistons, or turbo seal problems. A plugged crankcase ventilation (CCV) filter can also raise crankcase pressure. Shops measure crankcase pressure under set conditions instead of judging a puff from the oil cap.
Start with the easy checks
- Check the CCV filter service interval if your engine has one.
- Look for oil mist and leaks around seals.
- Track oil use.
Likely causes, most common first
- Worn rings or cylinders
- Common with high hours.
- Clogged CCV filter or breather
- Raises crankcase pressure.
- Turbo seals
- Can add pressure and oil carryover.
These are common causes, not a diagnosis. The same symptom can have a different cause on your engine; codes, tests and your service information decide.
When to stop and see a diesel tech
- A measured crankcase pressure test.
- Compression or leak-down testing.
Questions
Is the oil cap test a good blow-by check?
It is a rough sign at best. Shops measure crankcase pressure at set conditions for a real answer.
Related symptoms
How much oil should my diesel use, and when is it too much?
Troubleshoot →SmokeWhat does blue or gray-blue smoke from a diesel mean?
Troubleshoot →Guides & checklistsWhat should I check before buying a used diesel truck?
Troubleshoot →Where this comes from
- Engine Professional: Blow-by measurement and detection
- Nissens: Engine crankcase ventilation
- Manufacturer procedure filed with NHTSA: Check for excessive oil consumption (PDF)
Last reviewed 2026-10-19. General information only: confirm with your service information and a qualified diesel technician.