fuel spill -leak - aggravation
 

Filled up the SD the other day when diesel fuel was at the cheapest price I'd ever seen it (1.61). I noticed that when I opened the fuel tank cap, it was pressurized slightly, as I actually felt air rush out of it. I said to myself "This is odd". It was less than a quarter full at that point. I filled up the tank completely, so that fuel was visible in the filler neck.

On the drive home, it was a nice day, so I had the windows and sunroof open and could faintly smell that distinctive diesel fuel scent emanating from somewhere on the car.

Arriving home, I lifted the hood and noticed that cylinder #1 injector was soaking wet and diesel fuel had dampened the return line. Line didn't seem to be fitting very well, so I put a cable tie around it thinking that was the source of the leak. Parked the car overnight. Upon walking out to the car this afternoon, there was a HUGE puddle of diesel fuel underneath it! Like, gallons worth! :eek: :mad: The car was sitting in the sun and it was about 88 degrees.

Looked under the hood and diesel fuel is steady stream trickling from the injector, down the side of the engine and cascading onto everything below it and finally hitting the ground. What the heck? How can this happen if the engine isnt even running? :confused: I then opened the fuel cap and again, air rushed out, suggesting it was pressurized. This greatly lessened the fuel leak but didn't stop it. My neighbor came over and found that the injector return line was actually split towards the end, so my zip tying method of temporary repair wasn't effective in the slightest. He cut the split part of the line off, shortening the line and re-installed and its holding now. I am parking the car and leaving the fuel cap loose until I get this problem straightened out. That seems to stop the pressurizing of the fuel system and consequential leaking.

Ok, so I am thinking the vent for the fuel tank must be clogged. I've heard of vents clogging and not allowing air into the tank, thereby imploding the tank, but i'd not previously heard of this scenario affecting my car before. Have any of you? I searched the peachparts catalog but found no listing for a Diesel W126 fuel tank vent valve or anything similar. What gives? Is it called something else? What do you recommend? I have other means of transport so this car doesn't "need" to be driven daily, but obviously I'd like to get it going again. Please advise. Many thanks!

The tank diagnosis sounds correct, heating up in the sun and pressurizing the fuel past the shut off valve.

There are one or two aspects of MB's which seem to fall into a 'Bermuda Triangle' when it comes to detailed coverage :)

1. Atmospheric pressure-dependent full-load stop (ADA)

2. Metric v imperial flares on brake lines

3. Fuel tank vents/filters.

I think one of the reasons is they vary from country to country and possibly state to state making direct comparison difficult.

The tank vent for my W210 is located inside the front nearside wing, again this may vary for each model/year ect.. best bet is to physically trace it from source.

It's on my to-do list for both cars when the weather improves here.

The vent on your W126 as well as your W123 is behind the L/R wheel, slightly forward it.
It is a bell shaped rubber piece pointing down.
It could be plugged up not letting the tank vent properly.

Here is a Google search with some pictures.

https://www.google.com/search?q=mercedes+w126+fuel+tank+vent&biw=1138&bih=523&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjO78uG2c nLAhUKk4MKHbPGBo0Q_AUIBygC#imgrc=_



Charlie

Looking at one of the images from the Google search above has me wondering if this is for petrol only versions?

Petrol is obviously far more combustible than diesel which may explain the charcoal filter and breather.

http://ww2.justanswer.com/uploads/eurotec/2009-10-12_120949_42003555.jpg

Hi Charlie, thanks for the helpful link and advice! It actually took me a while to find that thing, they really have it tucked away, out of sight and you really wouldnt notice it casually unless you are specifically looking for it. And even then, wow is it hidden! Anyway, how exactly do I clear this clog? From looking at the diagram, that bell shaped vent valve has a few springs and plates inside it, so its not a simple matter of poking a piece of wire up there and clearing off a bit of dried mud. Upon looking at the one in my car it appears clean and i dont see or feel any obstruction on the very end of it. Kind of unsure how to proceed from here. Compressed air? Or would that just force the clog further into the vent valve and line? What would YOU do?

If you have return lines leaking, it is best to just purchase about a meter in length and replace all of it. It's pretty common for them to deteriate and split. Keep in mind the plug in the final end.

The fuel vent has to hit a certain pressure before it will release. With the severity of the leak you have, it may have trouble reaching that pressure.

The return line issue and the fuel leak, has been solved and taken care of. The main problem is that the fuel tank vent is seemingly clogged and I'm trying to deal with that right now. You bring up a good point about the tank having trouble reaching a specific pressure before the vent valve opens, which certainly aggravated the fuel leak. It dumped at least 4 gallons on the ground in a matter of hours. I understand that pressure will always find its easiest way out, but the tank is clearly not venting outward at all, as evidenced by the rush of air every time the fuel cap is opened. I'm really hoping its something simple, but it has me perplexed at the moment. Hoping somebody with more experience than I, will chime in here.