Lithuanian Heavy Diesel Vehicle Develops a Collapsed Coolant Hose After Overnight Cool-Down and Later Thermal Problems as Pressure-Cap Vacuum-Valve Operation Is Investigated

The Problem Appeared After the Engine Cooled

A heavy diesel vehicle in Lithuania operated normally when hot but repeatedly showed a partially collapsed upper cooling hose after being parked overnight.

On the following day, the vehicle could also develop unusual coolant-flow behavior during extended operation.

Because the engine did not show a classic hot overpressure problem, the workshop investigated what happened while the cooling system cooled down.

Attention moved to the vacuum-relief function inside the pressure cap.

Cooling Systems Experience Both Positive and Negative Pressure

When the engine is hot, coolant expands and system pressure rises.

When the engine is switched off and cools:

Coolant temperature falls → Coolant volume contracts → System pressure decreases

The cooling system must accommodate that volume reduction.

Depending on design, the pressure cap may contain a vacuum valve that permits coolant return from an expansion/recovery reservoir or otherwise prevents excessive negative pressure.

If the valve cannot operate correctly, vacuum can develop inside the cooling circuit.

Hose Collapse Was a Physical Clue

A flexible hose can deform inward if pressure inside becomes sufficiently lower than atmospheric pressure outside.

The sequence can be:

Engine hot → System cools → Coolant contracts → Vacuum valve fails to compensate → Hose collapses

This is different from a hose that collapses only at high RPM because of pump suction.

The timing of the deformation—after overnight cooling—was a key diagnostic clue.

Areas Reviewed

Technicians inspected:

• pressure-cap vacuum valve;
• cap sealing surfaces;
• recovery-hose condition;
• expansion tank level;
• return passage;
• hose structural integrity;
• and correct cap application.

A pressure cap that fits physically may still have an incorrect pressure or vacuum characteristic for the system.

Why the Next-Day Thermal Complaint Could Follow

If a hose remains deformed or the recovery process fails, coolant distribution may be affected after restart.

The system can also begin the next operating cycle with an abnormal coolant level or trapped-air condition.

That can contribute to temperature instability under heavy load.

The cap should not automatically be blamed for every collapsed hose, but the shutdown-and-cooldown sequence strongly supported inspection of the vacuum side of the system.

Lithuanian Cold Nights Made the Transition More Visible

Large temperature changes between operating temperature and overnight ambient conditions can create a significant cooling contraction cycle.

That made the pressure-recovery phase especially relevant in this application.

Technical Lesson

Cooling-system diagnosis should consider both:

how pressure is controlled while the engine heats

and

how vacuum is controlled while the engine cools.

A cap is therefore not only an overpressure safety component.

FAQ

Can a faulty pressure cap cause a radiator hose to collapse after shutdown?

Yes. If the vacuum-relief function does not operate correctly, excessive negative pressure can develop.

Is this the same as a blocked thermostat?

No. The fault occurs during system pressure recovery as coolant contracts.