The cap should be a tight fit. If it is at all loose then the most likley scenario is you've not fitted it straight/correctly. Chances are you've only hooked one side of it in place. It's a bit fiddly to get on right particularly if the tank is still in place. You need to push it down firmly and evenly before your turn it to lock it in place.

There have been cases of Teneres with dodgy caps, but its mostly R/X models with the problem.

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