How to tell if your car expansion tank cap is faulty UK

A faulty expansion tank cap (also called a coolant reservoir cap) can cause coolant to leak or spray from the system, leading to engine overheating. In UK cars like the Vauxhall Corsa or Ford Focus, common signs include coolant loss, overheating warnings, and visible leaks around the reservoir. The expansion tank cap maintains proper pressure (typically 1.1-1.4 bar for most UK models) in the cooling system. When faulty, it fails to regulate pressure, causing coolant to boil at lower temperatures ('kettling'). Modern UK-spec vehicles like the Nissan Qashqai use pressurised systems where the cap's pressure relief valve is crucial. Check for: 1) Coolant spraying from the overflow pipe, 2) Collapsed radiator hoses when cold, 3) Overheating despite adequate coolant levels. The UK MOT test doesn't specifically check the cap, but coolant leaks from a faulty cap could cause a fail. For accurate diagnosis, use a pressure tester (available at Halfords) to verify if the cap holds the manufacturer's specified pressure.