Fuel cap is stuck, leaks or will not lock

The fuel cap is hard to open, the key will not turn, the cap will not close or lock, or fuel or water gets past it. The cap has three jobs: seal the filler, let air into the tank through its vent, and lock. Dust and sand in the lock, a dry hinge, a hardened or broken seal, or a bent cap after a fall are the usual causes.

Is it safe to ride?

With care, as long as the cap closes and seals. If the cap will not close, or fuel splashes out of it, do not ride: fuel can spill onto the hot engine.

Quick checks first

• Is the key the correct one, clean and not bent or badly worn?
• Is there sand or dirt in the keyway or around the hinge?
• Is the rubber seal under the cap whole and seated?
• Has the bike been dropped on the tank side?
• Do you hear a hiss when the cap opens?
• A cap may have a small vent hole, a one-way valve, or be sealed with a separate vent tube. A blocked vent stops fuel flowing.
• Keep plastic tanks and caps out of strong sun where possible. Plastic parts become brittle after years of sunlight.

Diagnose it step by step

1Will the key go in and turn?

• No: blow dirt out of the keyway and puff in a little dry graphite or PTFE lock lubricant. Work the key gently several times. Do not force it with pliers or a screwdriver. Clean and lubricate the fuel cap.
• Still stuck: the lock is broken or the key is worn. A spare key may work; otherwise replace the cap set or ask a locksmith or dealer. Never drill a cap on a tank that holds fuel.
• Yes: go to step 2.

2Does the key turn but the cap will not open or close?

• The latch or hinge is dirty, dry or bent. Clean and lubricate it. If the bike has been dropped, check the cap and tank neck for bending: see The bike has been dropped or crashed. A bent cap is replaced.
• Opens and closes: go to step 3.

3Is there a fuel smell, a wet ring round the cap, or water in the fuel?

• Look at the seal: cracked, flat, swollen or out of its groove. Replace the seal or the cap. Do not overfill the tank. See Water or dirt in the fuel if water got in, and drain it.
• No leak: go to step 4.

4When you open the cap after a ride, is there a hiss of air rushing in?

• Yes: the vent in the cap is blocked. Clean it: Check the fuel tank vent. See Engine stops after a few kilometres and the cap hisses.
• No: the cap is fine.

Causes, most likely first