Tank cap diesel leak
 

I didn't turn the tank cap (I can't call it gas cap if I pump diesel, now, can I?) on all the way earlier today. The tank wasn't more than 3/4 full, but when I got home there was quite a bit of diesel on the rear fender. I don't drive very agressively, so I don't think the diesel sloshed around enough to spill over the filler.
I also noted earlier that there there is quite a 'hiss' when I open the cap to load up with Bio-diesel (You all do pump bio-diesel, right?), as if there is a lot of positive or negative pressure in the tank. Now that I am thinking of it, there is always some diesel marks around the filler door, no matter how often I clean the area around the flap.

Do I have tank pressure issues or what?

Does this car have a similar setup as gas powered cars with a carbon canister and a negative pressure (partial vacuum) on the fuel system at all times?

RE: Tank cap diesel leak
 

Don't know what model you are talking about, but a bit of vacuum/pressure (depending on ambient temperature) is normal. If pressure is excessive than you may have a blocked fuel tank vent pipe.

Depending on your car, the vent pipe is usually visible, and accessible from underneath. Most of the time it exits somewhere near the rear axle. It could be cleaned out with a coat hanger, or something similar.

Phil

RE: Tank cap diesel leak
 

Ah, yes, that would help wouldn't it. Thanks.

The car is an '87 300D.

The pressure difference is really quite pronounced, so I'll check the vent line.

Two other questions:

1) If the tank is vented, why would there be any pressure difference, however small? Is there a valve in between the vent and the tank?
2) I noticed an excess dump hole under the cap, presumably to flush away spilled diesel while pumping without staining the bodywork. Does that have a line as well?

It still does not explain why the area around the flap is always stained. I would think diesel would have to be forced out the cap to have that happen. I am quite sure I keep everything clean when I pump diesel.

Anyway, thanks for the info.

Bart

RE: Tank cap diesel leak
 

Bart,

1) At a fuel pressure of 100-300mbr the fuel gases can escape through the cap. This is only the case if the vent line is blocked. If the system is operating properly a pressure of up-to 50mbr can be present in the tank.

2) The "dump hole" that you are refering to, is meant to direct any spilled fuel away from the body and through a hose down to the ground. Make sure that the hose is not blocked. Clean it with a coat hanger, or high pressure air/water if necessary.

Hope that helps


Phil

RE: Tank cap diesel leak
 

hi phil. i am learning something on diesel from you now ;)