The brief
The CLS350 had been getting sluggish, the throttle response a bit lazy, fuel economy on the owner's usual routes had dropped, and there was a faint smell of petrol in the cabin after a drive. That last one is the part you don't sit on, so he brought it in before the smell became a real hazard.
The fuel pump lives inside the tank and pushes petrol up to the engine at a set pressure, through a filter that keeps grit out of the injectors. When the pump weakens it can't hold that pressure under load, so the engine is starved when you ask for power. And if the seal where the module sits in the tank is weeping, that's where the fuel smell comes from.
The diagnosis
We put a gauge on the fuel system and found two problems, not one. The pump was no longer holding its rated pressure when the engine was working hard, which explains the flat acceleration and the economy drop. And the seal around the in-tank pump and filter module was weeping, which is the source of the petrol smell in the cabin.
With the pump tired and the module's seal gone, the right fix was to replace the pump and filter as a complete unit with fresh sealing rings, rather than chase one half of the problem and leave the other.
The work
We released the fuel pressure first, then lifted the rear seat to get at the access hatch over the tank. The old in-tank pump and filter module came out, and a new Mercedes-spec unit went in, sealed back to the tank with new rings.
Then the system was primed, the engine started, and the fuel pressure read on the gauge across idle and load to confirm it was holding spec.
A check around the access hatch confirmed there was no weep, and a road test confirmed the engine pulled cleanly with the power back.
The outcome
No more petrol smell in the cabin, the throttle response is back, and fuel economy has returned to where the owner expects it.
The CLS350 went home with the fuel system sealed and feeding clean, full pressure right across the rev range. Replacing the pump and filter as a set means the supply side is sorted properly, and the smell, which was the worrying part, is gone at the source.