Austrian Heavy Diesel Vehicle Develops Intermittent Temperature Spikes After Cooling-System Service as Coolant Deaeration Line and Expansion-Tank Vent Flow Are Investigated

The Overheating Began After Cooling-System Service

A heavy diesel vehicle operating in Austria developed intermittent coolant-temperature spikes after a cooling-system service.

During ordinary driving, temperature appeared normal. During longer mountain climbs, however, the temperature could suddenly rise and then recover.

The coolant pump, thermostat and main radiator circuit did not initially show an obvious continuous fault.

Because the complaint began after draining and refilling the cooling system, technicians focused on how trapped air was being removed from the circuit.

Some Cooling Systems Depend on Continuous Deaeration

Many heavy-duty cooling systems use small vent or deaeration passages to move air and vapor from high points toward the expansion tank.

The flow concept can be represented as:

Engine/cylinder-head high point → Small deaeration line → Expansion tank → Gas separation

If the small line becomes restricted, the main coolant circuit may still circulate.

However, air trapped in local high points may not leave efficiently.

Why the Temperature Spike Was Intermittent

A persistent radiator blockage often creates a more predictable thermal problem.

Trapped gas can behave differently.

During load:

1. coolant temperature and local boiling tendency increase;
2. trapped air or vapor expands;
3. local coolant contact can be reduced;
4. sensor readings or actual metal temperature may rise rapidly;
5. the gas pocket may later move, allowing temperature to recover.

This creates an intermittent pattern that is harder to reproduce at idle.

The Inspection Followed the Gas Path

Technicians reviewed:

• small-diameter deaeration hoses;
• expansion-tank connections;
• vent fittings;
• hose routing;
• kinks or internal restriction;
• coolant fill procedure;
• and evidence of continuous return flow where applicable.

The exact expected vent-flow behavior depends on the engine design.

A small hose should not be assumed to be unimportant simply because the main radiator hoses flow normally.

A Blocked Deaeration Line Can Survive Routine Checks

The main cooling circuit can show:

• normal heater operation;
• apparently normal thermostat opening;
• normal pump drive;
• and reasonable temperature during light use.

That can lead technicians to conclude that all circulation is correct.

But coolant circulation and air removal are separate functions.

Why Mountain Operation Exposed the Problem

Long climbs increase engine load and heat rejection.

That operating condition can expand trapped gas and make marginal deaeration more visible.

Austria’s mountain routes therefore entered directly into the diagnostic scene rather than serving only as a geographic label.

Technical Lesson

This case highlights a frequently overlooked cooling-system distinction:

moving coolant
is not the same as
removing trapped air and vapor.

After cooling-system service, intermittent temperature spikes should prompt technicians to verify both the main coolant path and any manufacturer-designed continuous deaeration circuit.

FAQ

Can a small blocked vent line cause overheating?

On cooling systems that rely on continuous deaeration, restriction can contribute to trapped air and local thermal instability.

Is this the same as low coolant level?

No. The system can contain enough coolant overall while still retaining air in local high points.