Blow-by is combustion pressure that leaks past the piston rings into the crankcase. Every diesel has some, which the crankcase ventilation system handles. Excessive blow-by, seen as heavy vapor or oil being pushed from the breather or blow-by tube, oil consumption and oil leaks from crankcase pressure, usually means worn rings or cylinders, and sometimes a problem that only mimics it, such as a leaking turbo or a blocked breather.
Diesel blow-by symptoms
- Vapor or smoke pouring from the blow-by tube or breather, especially under load
- Oil residue around the breather and engine bay
- Rising oil consumption
- Oil leaks from seals pushed out by crankcase pressure
- Loss of power and harder starting as compression drops
Common causes
| Cause | Notes |
|---|---|
| Worn or stuck piston rings | The classic cause on high-mileage or dusted engines |
| Cylinder wear or scoring | From dirt ingestion through a leaking intake or poor filtration |
| Damaged piston | From a failed injector or overheating |
| Turbo oil seals | Can add oil and vapor that looks like blow-by |
| Blocked crankcase ventilation | Normal blow-by with nowhere to go raises crankcase pressure |
How to test for blow-by
A blow-by tester or a calibrated orifice on the blow-by tube measures the flow while the engine runs, ideally under load, and compares it with a specification or with known-good engines. A simpler check is to watch the tube with the engine warm and under load: heavy, pulsing vapor is a warning sign. A compression or cylinder leakage test tells you which cylinders are worn; see compression and leak-down tests.
Blow-by fixes
- Ventilation: clean or replace a blocked breather or filter first.
- Catch cans: collect oil from crankcase vapor but do not fix the cause.
- Additives: may free stuck rings in mild cases but cannot repair worn parts.
- Rings and cylinders: a rebuild, or a replacement short block or long block, is the real fix for worn engines.
Cummins blow-by
Cummins engines vent crankcase vapor through a blow-by tube or a filtered breather depending on the year. A big increase in vapor on an engine that tows hard is the classic sign of ring wear, often after dust got past a leaking intake; see piston rings.
Prevent excessive blow-by
Keep the air filter and intake tract sealed, change oil on schedule, avoid long idling, and fix injector problems quickly. When rings and cylinders are worn, our D&J 5.9 Cummins and 6.7 Cummins short and long blocks are listed by year; see short block vs long block.
More guides: diesel knocking · diesel black smoke.
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