Coolant reservoir cap for Top models
- Coolant reservoir cap BMW 3 Series
- VW GOLF
- BMW 5 Series
- AUDI A4
- Coolant reservoir cap FORD FOCUS
- MERCEDES-BENZ C-Class
- FORD FIESTA
- MERCEDES-BENZ E-Class
- Coolant reservoir cap BMW 1 Series
- VAUXHALL ASTRA
- AUDI A3
- AUDI A6
- Coolant reservoir cap VW POLO
- VW PASSAT
- VW TRANSPORTER
- FORD MONDEO
- Coolant reservoir cap HONDA CIVIC
- MINI Hatchback
- VAUXHALL CORSA
- NISSAN QASHQAI

Radiator thermostat
(10000 items)
Water pump
(10000 items)
Radiator (engine cooler)
(10000 items)
Expansion tank (сoolant recovery tank)
(4922 items)
Blower motor resistor
(3895 items)
Coolant flange
(4058 items)Your Expansion tank cap (coolant reservoir cap) worn out? » Read more about common malfunctions • Repair • Replacement manual
The coolant reservoir cap is an important component of the cooling cycle of a car. It opens the expansion tank, enabling the driver or mechanic to add coolant. A defect of the cap is rare, but occurs every now and then. The cooling cycle in a car The cooling cycle in a car serves to prevent the engine from overheating. Combustion engines function by burning fuel in small explosions and using the resulting pressure to generate kinetic energy. The engine has no use for the resulting exhaust heat which needs to be discharged. This is the task of the cooling system. Simple and small engines of lawn mowers or small motorcycles still manage by air cooling. The exhaust heat generated by the cylinders is lead away from the engine via cooling fins and dissipated by airstream or a blower. This once very popular way of engine cooling gradually became insufficient due to engines becoming more powerful. It was therefore replaced by water or liquid cooling. The liquid cooling is a cycle, constantly pumping cooling liquid through the engine. The cooling of the liquid, generally a mix of water and glycol, is provided by a radiator. In its turn the radiator is cooled by an airstream from the outside or generated by a connected blower. No system of water ducts is perpetually tight. A small loss of cooling liquid over time is therefore unavoidable. Especially in old cars, the coolant level should be regularly checked. The filling point for coolant is where you find the expansion tank cap. Expansion tank warning function In addition to its sealing function, the expansion tank cap has an important warning function for the driver. If pressure rises too strongly, the expansion tank cap opens the cooling system. This happens at a pressure of ca. 1.5 bar. Overheating occurs when:
- - there is too little cooling liquid in the system
- the engine is under excessive strain (e.g. driving uphill or overloading)
- the cylinder head gasket is defective
- the radiator is defective
- the thermostat is defective