Fuel tank cap

Fuel tank caps seal the filler opening, keeping rainwater and dust out of the fuel and preventing spills. Depending on the tank design, pressure equalisation takes place through the cap or through a separate tank vent. An incorrect cap type can therefore not only cause leaks but also disrupt the fuel supply. On MAN and Mercedes trucks, buses and vans, the seal, locking mechanism and venting function must therefore be assessed as a single technical system.

Always identify parts using the vehicle’s chassis number. Visual comparison or selection based solely on the model name is never sufficient. Tank designs and filler necks can vary within ranges such as MAN TGS, TGX and TGE or Mercedes Actros, Atego and Sprinter. The installed tank configuration is also decisive on buses such as Lion’s City and Citaro. The same applies to trucks such as MAN TGL and TGM or Mercedes Arocs and Econic, coaches such as MAN Lion’s Coach and Mercedes Tourismo and Travego, and vans such as Mercedes Vito. Other bus models on which the tank design must be checked include MAN Lion’s Intercity and Lion’s Regio and Mercedes Conecto and Intouro. The engine variant alone does not determine the correct cap; where tanks have been replaced, also check the part number and technical specifications of the installed tank.

There are screw-on and bayonet caps, locking and non-locking versions, and caps with or without integrated venting. Selection parameters include the filler neck diameter, thread pitch or bayonet geometry, installation depth, seal profile and any opening pressure specified for pressure equalisation valves. The fit and axial preload must provide uniform seal compression without deforming the filler neck.

Housings may be made of fuel-resistant plastics or metals. Seal materials include NBR (nitrile butadiene rubber) and FKM (fluoroelastomer), depending on the specification. Check suitability for the fuel used, the biodiesel content and the temperature range; the material name alone does not guarantee compatibility.

Traces of diesel around the filler neck may indicate hardened sealing rings, damaged sealing surfaces or insufficient closing force. Stiff lock cylinders and worn bayonet lugs can sometimes prevent full locking. Water ingress can occur when a cracked seal or deformed filler neck allows rainwater through. A blocked vent can cause a vacuum to build up, resulting in loss of power or fuel starvation under load; these symptoms alone, however, do not prove that the tank cap is faulty.

1. Check the initial conditions: park the vehicle safely, switch off the engine and remove dirt around the filler opening before opening the cap. Keep ignition sources away from the work area and collect any spilled fuel.
2. Inspect the seal and filler neck: look for cracks, swelling, corrosion and deformation. Measure the relevant diameters and closure geometry with a vernier caliper; for threaded closures, use a thread pitch gauge if necessary. Compare the results with the part specification.
3. Assess the closure: check that the thread or bayonet engages fully and that the seal makes contact all the way around. Also inspect the lock cylinder, retaining mechanism and tether chain, where fitted.
4. Test pressure equalisation: if a vacuum is suspected, measure it with a suitable low-range pressure gauge using the specified test adapter. Test vented caps separately with a pressure/vacuum tester with limiting controls, using the manufacturer’s specified values. Do not apply uncontrolled compressed air to the tank.

Replace damaged caps or separately available seals only in accordance with the parts and repair instructions. A new cap will not compensate for a bent filler neck or a blocked external vent line. Never fit a non-vented version in place of a specified vented cap, and do not drill a vent hole as a makeshift solution.

Clean the sealing surface without allowing dirt into the tank, and close the cap to the specified stop position. Then check for leaks and correct locking. If water contamination recurs, the area around the filler neck, the tank vent and the water separator must also be inspected. Braem can provide technical support with selection based on the chassis number and tank design.