Finnish Heavy Diesel Engine Maintains Normal Cold Oil Pressure but Loses Real Pressure When Hot as Oil-Pickup Tube O-Ring and Suction-Side Sealing Are Investigated

The Low Oil Pressure Was Real

A heavy diesel engine in Finland showed healthy oil pressure after cold startup but progressively lower pressure once the engine reached operating temperature.

Unlike a previous sensor-related scenario, independent measurement confirmed that the pressure drop was real.

The oil pump and bearings were initially considered, but another possibility became important: air entering the lubrication circuit before the pump.

An Oil Pump Must Receive a Solid Column of Oil

The supply path can be simplified as:

Oil sump → Pickup tube → Pickup seal/O-ring → Pump inlet → Oil pump → Galleries

The pump is designed to move lubricating oil.

If the pickup connection leaks air, the pump can receive a mixture of oil and gas rather than a fully liquid supply.

This can reduce effective pumping and contribute to unstable pressure.

Why Cold Operation Looked Normal

Cold oil has greater viscosity.

A marginal pickup seal may behave differently while:

• the oil is thicker;
• clearances are colder;
• and operating conditions are less thermally expanded.

As the oil warms, several changes occur simultaneously:

• viscosity decreases;
• seal material changes temperature;
• metal dimensions change;
• and pump inlet conditions become more sensitive.

A worn or hardened pickup-tube O-ring may then allow more air entry.

The Diagnostic Path Started Before the Pump

Instead of assuming the pump itself was worn, technicians reviewed:

• pickup tube attachment;
• O-ring condition;
• cracks in the pickup;
• sump oil level;
• suction-screen condition;
• pump inlet sealing;
• and evidence of aerated oil.

The key principle was:

A good pump cannot produce stable pressure if its inlet supply is compromised.

Pump Cavitation and Air Ingress Are Different From Sensor Error

An inaccurate oil-pressure sensor creates an electronic reporting problem.

Suction-side air entry creates a real hydraulic problem.

That distinction matters because both can display the same dashboard warning.

The Finnish application therefore used direct pressure verification before moving deeper into the lubrication system.

Why Hot Idle Was the Best Test Condition

At hot idle:

• oil viscosity is lower;
• pump speed is relatively low;
• and internal engine leakage is proportionally more significant.

A marginal suction-side problem may therefore become easier to identify in this state.

Exact pressure limits remain engine-specific.

Technical Lesson

This case demonstrates why lubrication diagnosis should begin at the oil source and move forward:

Sump → Pickup → Pump → Galleries → Bearings

When real oil pressure falls only after warm-up, technicians should not restrict the investigation to pump wear or bearing clearance.

The pump’s inlet sealing condition can be equally important.

FAQ

Can an oil-pickup O-ring cause hot low oil pressure?

If it allows air into the pump inlet, actual oil-pump performance can be affected.

Why can cold pressure remain normal?

Oil viscosity, seal behavior and pump conditions differ significantly between cold and hot operation.