Norwegian Marine Diesel Engine Shows Rising Crankcase Pressure and Intake Oil Mist Despite an Open Breather Path as Centrifugal Oil-Separator Rotor Operation Is Investigated

The Breather Was Open but the System Still Did Not Work Correctly

A marine diesel engine operating in Norway developed increasing crankcase pressure and visible oil mist in the intake during extended running.

Initial inspection showed that the main crankcase breather passage was not obviously blocked.

Because airflow through the breather existed, the system was first considered functional.

Further diagnosis showed that flow through the system and effective oil separation were two different questions.

Attention shifted to the centrifugal oil separator.

A Separator Must Do More Than Pass Gas

Some closed crankcase ventilation systems use a centrifugal separator to remove oil droplets from blow-by gases.

The process can be simplified as:

Crankcase gases → Separator rotor → Centrifugal separation → Oil returned → Cleaner gas sent onward

If the rotor does not rotate correctly, the gas path may remain open while separation efficiency falls.

This can create a misleading situation where the system is not blocked, yet it is still malfunctioning.

Rotor Function Became the Key Variable

Possible faults may include:

• rotor sticking;
• contamination;
• bearing wear;
• drive mechanism failure;
• restricted oil return;
• or internal damage.

Technicians therefore reviewed whether the separator actually performed its intended dynamic function.

A stationary but unobstructed separator is not equivalent to a healthy separator.

Why Oil Mist Increased

If oil droplets are not separated effectively, more lubricant can travel with crankcase gases toward the intake.

Possible results include:

• oily intake piping;
• higher oil consumption;
• deposit formation;
• smoke changes;
• and contamination of downstream air-system components.

At the same time, poor separator operation may alter crankcase ventilation characteristics and contribute to pressure changes.

Norway’s Marine Duty Made Long Operating Time Important

Marine diesel engines can operate continuously for extended periods.

This allowed oil carryover and crankcase-pressure changes to accumulate gradually.

A short harbor test could show an open breather while failing to reproduce the oil-separation problem that developed over time.

Other Causes Still Required Exclusion

Technicians also considered:

• excessive engine blow-by;
• blocked oil return;
• crankcase regulating valve condition;
• intake suction effects;
• and internal engine wear.

The separator rotor was not treated as the only possible cause.

Why Injector Diagnosis Was Not the Main Direction

Oil mist entering the intake can influence smoke and combustion appearance.

Those symptoms may be mistaken for over-fueling.

However, diesel injectors cannot correct an excessive lubricating-oil carryover problem originating in crankcase ventilation.

Technical Lesson

This Norwegian marine case highlights an important system-diagnosis principle:

a passage can be open while the component inside that passage is functionally ineffective.

Crankcase ventilation diagnosis should therefore evaluate both gas flow and oil-separation performance.

FAQ

Can a centrifugal crankcase separator fail even if the breather is not blocked?

Yes. Rotor or separation-function faults can exist while gas still passes through the system.

Can poor oil separation increase intake oil contamination?

Yes. More oil mist can remain entrained in crankcase gases if separation efficiency falls.