Croatian Legacy Mechanical Diesel Engine Shows Rising Oil Level Despite Normal Injector and Lift-Pump Checks as Injection-Pump Drive-Shaft Seal Leakage Is Investigated

Fuel Was Entering the Crankcase Through an Unexpected Route

A legacy mechanical diesel engine used in Croatia showed a steadily rising engine-oil level.

The lubricant also developed signs consistent with fuel dilution.

Injector leakage was investigated, and the mechanical lift-pump diaphragm did not show the expected evidence of fuel passing into the engine.

The diagnostic question then became:

What other component contains diesel fuel and also has a mechanical interface with the engine?

Attention shifted to the injection-pump drive-shaft seal.

Mechanical Injection Pumps Can Sit Across Two Fluid Environments

Depending on engine and pump design, the injection pump may be driven directly from:

• timing gears;
• an intermediate drive;
• or another engine-side mechanism.

The pump contains diesel fuel internally, while the drive shaft connects toward an engine timing or lubricated area.

A seal maintains separation between these environments.

The possible leak path is:

Fuel inside injection pump → Drive-shaft seal → Engine/timing side → Crankcase oil

Why the Oil Level Rose Gradually

A shaft-seal leak may not create an immediate external fuel leak.

Instead, small amounts of diesel can migrate into the engine over many hours of operation.

The operator may notice:

• oil level increasing;
• oil becoming thinner;
• reduced lubricant viscosity;
• or a fuel odor.

This gradual trend makes maintenance records especially useful.

Other Fuel-Dilution Routes Were Excluded First

The workshop considered common alternatives, including:

• injector nozzle leakage;
• incomplete combustion;
• lift-pump diaphragm failure;
• and repeated short-run operation.

The injection-pump shaft seal became more relevant only after the evidence did not support those paths.

Pump Condition and Crankcase Condition Were Reviewed Together

Inspection included:

• drive-shaft play;
• seal condition;
• pump housing leakage;
• pump mounting interface;
• timing-area contamination;
• and engine-oil condition.

Excessive shaft movement can also damage a seal, so replacing the seal alone may not correct the underlying cause.

Why This Matters Beyond Oil Level

Fuel dilution can reduce oil viscosity and therefore affect:

• bearing protection;
• oil pressure;
• turbocharger lubrication;
• and wear control.

For that reason, a rising oil level should not be treated as harmless.

Technical Lesson

This Croatian case highlights the importance of cross-system leak paths.

When engine oil is gaining volume, technicians should identify where another fluid can physically enter the lubrication system rather than assuming the injectors are always responsible.

FAQ

Can an injection-pump shaft seal allow diesel into engine oil?

On pump designs where the fuel side and engine drive side are separated by a shaft seal, seal failure can create such a path.

Is a rising oil level always caused by injector leakage?

No. Multiple fuel-dilution paths are possible and should be separated diagnostically.