I suggested that R.A, contact Nicholas Froome of www.w124.co.uk who seems to know more about the model than anyone else in the UK, and this is what Nicholas came up with: “The fact that it leaks after startup implies the leak is post-pump - otherwise it'd leak when parked. Assuming a 220 is like a 300 and 320, there is a pump / pumps / damping chamber assembly under the car. That feeds the fuel rail via a long steel pipe. If the leak were by the pump it should start leaking immediately the ignition is turned on The fuel rail return comes back to the tank via a return line. There will be some pressure in this line but logically not as much as in the feed. Also I'd expect pressure in the return to take a couple of seconds to build up. So I'd suspect the leak, if there is one, to be in the return pipework. Three more things: Leaks in the tanks of estates are common. The tank leaks where the filler neck is welded to the tank body. Filling the tank above the level of the neck provokes the leak. It could be that the tank has a leak that is provoked by the fuel return being pressurised. You'd have to check a diagram showing where the return is located to see if this might be a factor. There is a breather from the top of the tank to the filler neck to allow air to escape from the tank when displaced by incoming fuel. If the tank has ever been removed - or if the original fitter was having a bad day - the link pipe might be missing or misrouted. I have seen a diesel with the pipe completely missing. There is a charcoal canister on the inner wing that is used to purge fuel vapour. The canister should be replaced regularly but is frequently overlooked. There is a mechanism to cyclically purge this canister. How this might affect the system I don't know but, assuming you haven't done it already, I think I would replace it as a second step. My first step would be to replace the fuel filler cap with a new one from Mercedes. Finally, just because the pump and filter have been replaced does not mean they cannot be at fault. I'd look very hard at the pipework into and out of them and replace any flexible pipes and all pipe clips with the correct original MB parts. I use a specialist called Alan Robertson in Hove (01273 770838 / ajrobertson@daisybroadband.co.uk). He is a proper old-school engineer and I am sure he could get to the bottom of this problem.”