The brief
Mr Wee's E250 had a strong petrol odour in the cabin while he was driving, the kind you don't ignore. A fuel smell is a safety issue, not a comfort one, so it gets dealt with straight away. The fuel pump on this car sits inside the tank, in a module that also seals the top of the tank, and on a saloon that tank is right under the rear passenger seat. As the pump module ages, the seals harden and the connections can weep, so fuel and fuel vapour escape, and because the tank is right under the cabin, the smell comes straight up inside. A worn, leaking fuel pump module doesn't reseal, so the assembly gets replaced and the seals renewed.
The diagnosis
The check traced the petrol smell to the fuel pump in the tank, the module worn and leaking fuel from around its seals, which is exactly why the cabin smelled of petrol while driving. The fuel lines, the tank itself and the rest of the fuel system were sound otherwise. That's a fuel pump module replacement with fresh seals, rather than trying to patch a worn seal that would soon weep again, on something as serious as a fuel leak.
The work
Working safely with the fuel system depressurised, the rear seat came up, the tank's pump module was disconnected and lifted out, and a new genuine Mercedes-spec fuel pump assembly fitted with a fresh tank seal and the connections checked tight. The fuel filter was renewed while it was in there, the system pressurised again and checked for leaks, and the area aired out. A road test confirmed no petrol smell in the cabin, the engine starting and running cleanly, and the fuel system holding pressure.
The outcome
No petrol smell in the cabin, no fuel weeping at the tank, the engine starting cleanly, and the fuel system holding pressure. The E250 went home safe and smell-free. A fuel leak is the sort of fault you fix once and properly, so replacing the whole pump module and the seals took care of it for good.