1999 ML320 Substantial Fuel Leak
 

Hello ,

On a recent trip my 1999 ML320 developed a major fuel leak in the rear of the car. The leak is only apparent when the car is running and shortly thereafter. It completely stops, apparently after fuel pressure is bled down.
I took out the drivers side wheel well, to expose the fuel filter and the fuel lines. I also took out the rear seat and lifted the carper to expose the top of the fuel tank and the hose connections. All appeared dry and not leaking. I started the car and there was again, a massive fuel leak. It was hard to trace the origin and appeared to be running down the tank from the area of the filler neck inlet. I question whether it is possible for the filler neck to only leak with the car running and the system under pressure. Additionally, I can’t find the end of the small fuel line that runs from the fuel filter to somewhere on top of the tank in the area of the spare tire carrier. I see there are two large lines that run from the filter to the tank, as well as one large line that appears to run to the front of the car. Then there is a small, low pressure mystery line that comes off the filter to an unknown point over the fuel tank.
I replaced the fuel filter with an updated version and lines about 5 years ago without incident. Vehicle has been running fine to this point.
Any ideas? Does the mystery line go to a vent or canister that ‘s clogged? Could the fuel filler neck be defective?

Thanks for your kind attention.

Problem Solved; Maybe?
I removed the spare tire to get a better look at things on the top of the tank. Upon closer inspection, I found that the small mystery line from the fuel filter, AKA, fuel filter vent hose, is supposed to connect to the fuel tank just near the filler neck. Mine was not connected, as the short rubber hose connecting it was torn in half. Interestingly enough, in my further research, I stumbled on a post from 2010 on another forum , titled; “Fuel leaking driver side rear tire” where a fellow with the same year ML, 1999, had the EXACT same problem; a tear in this small hose at the rear top of the tank. I also learned from this post that this vent hose should be dry; well mine is filled with fuel. Soooooooooo, I’m assuming I have two issues, the torn line that needs repair, as well as a bad fuel pressure regulator in the filter that needs replacing. I will try to get the parts tomorrow from the dealer and report results for reference by future fuel leakers.
Thanks,
Out

As a follow up to anyone referencing this post in the future; problem solved. It looks like the vent hose mentioned above cracked off in the past, giving me a check engine evap code. Of course, I ignored it. Eventually, the diaphragm in the fuel pressure regulator failed, allowing fuel to spill out from this vent pipe. I reconnected the vent hose, and installed a new fuel filter/fuel pressure regulator, and it is a solid fix. Heck engine is also now not lit. As I have seen another post on the same problem ( as stated above) this small 1 inch vent hose may be a problem for others. Good luck.

Great info and follow up! This will really help folks in the future I bet.
Don't guess you snapped any pics? :D