u/Brilliant-Paint-2381

Hi all,

I’ve got a Mercedes W221 S350L (diesel) and I’ve recently discovered a pretty significant fuel leak. I’m trying to figure out whether this is definitely a fuel tank issue or if it could be something else before I commit to an expensive repair.

Here’s everything I know:

I first noticed a strong diesel smell and then saw puddles forming on the driveway

The leak seems to be around the middle to rear of the car, driver’s side

I placed a bowl under the car and measured the leak overnight,

It’s losing ~300 ml over ~15.5 hours while parked

So roughly ~0.5L per day just sitting, not to mention the amount that’s collected first in the undertray before overflowing out into the bowl.

The leak appears more to one side, but I suspect that might be because fuel is collecting in the undertray and then draining out from one side (path of least resistance)

I took it to a Mercedes specialist who I’ve gone to before and he said the issue is the fuel tank, and that mud or water got trapped in the undertray which sat against the tank and caused corrosion from the outside.

Quoted me ~£3,500 to replace the tank

I’m not 100% convinced it’s definitely the tank, I’ve looked online and everywhere I’ve looked said that the fuel tank for my car is made of plastic.

Could this instead be:

Fuel lines near the tank?

Sender unit / seals on top of the tank?

Does this sound like a typical tank failure on a W221?

Are these tanks even metal or plastic on this model? (I’ve heard conflicting things)

Has anyone seen similar symptoms caused by lines or seals instead of the tank?

Is there a way to confirm it’s the tank without just replacing it?

Any advice would be massively appreciated — just trying to avoid throwing £3.5k at the wrong problem.

Thanks!