What are the symptoms of a faulty expansion tank cap UK?

A faulty expansion tank cap in UK cars typically causes coolant to spray from the reservoir. This reduces coolant flow to the engine, lowers system pressure, and can lead to overheating - potentially causing 'boiling over' in severe cases.

In UK vehicles like the Vauxhall Corsa or Ford Focus, the cooling system uses a pressurised design with an expansion tank. The cap contains pressure valves that maintain optimal system pressure (usually around 1.1 bar). When these fail:

1. Coolant leaks become visible around the reservoir or underneath the car
2. Temperature gauges show higher than normal readings
3. Warning lights may illuminate (particularly in modern cars like the Nissan Qashqai)
4. Reduced heater performance in the cabin

UK cooling systems use a mixture of antifreeze and water (typically 50/50). The expansion tank has clear MIN/MAX markers - always maintain coolant between these levels. For popular UK models like the BMW 1 Series, replacement caps should meet OEM specifications to maintain correct pressure ratings.

Note that in UK MOT tests, visible coolant leaks from the expansion tank would constitute a failure. Regular checks of the cap's seal and pressure rating can prevent more serious engine damage.