84 300D Pressure in fuel tank
 

So, this car is still new to me and I'm working things out slowly. I had something very odd happen to me the other day.

I was driving down the road (this car has a 2 tank veggie system) and pulled into the gas station. The night before, I had filled up the diesel tank to the brim. So, I walk inside the station, come back out and see something leaking all over the ground - it's diesel fuel. It's pouring out like crazy! I go around and pull the cap off the fuel tank and it just shoots diesel fuel from out of the tank all over me! Extremely under pressure and I don't know why. Has this happened to anyone else before or know what causes this?

In all but the coldest times of the year it is best not to fill the tank to the 'brim', as the diesel is stored underground where it is usually cooler than inside the car's tank. Normal temperature difference expansion can cause this kind of pressure, IF the tank vent is plugged. This vent is located, if I remember correctly, forward of the rear wheel on the drivers side. It is a rubber hose ending in a small bell shaped device.

Chances are your tank vent is plugged. Check and clean the vent line. It is by the rivers side rear wheel. Normal looking black rubber hose with a bell shaped tip.

you return valve maybe not working,and you are sending veggie to diesel tank.Check fuel for smell,or feel to fuel with fingers to see if its slicker than diesel.

As mentioned, the vent may be clogged, run a stiff wire up through the end.

Fuel level monitoring
 

Are you monitoring the fuel level in both tanks?
I agree with Oldsinner... check to see if your return fuel line did not switch to the correct tank.
WVO contamination in the OEM diesel tank will eventually polymerize and create lots of problems, including plugging the vent system.

Thank you for the advice everyone - great information here. I'll take a look into it and I definitely learned my lesson - I will not overfill the tank!

How much diesel did the engine consume since filling the tank to the brim and you noticed it leaking? With a proper operating tank vent, that should not have happened. If your 2 tank VO system is not operating properly, it could be returning VO to the wrong tank (diesel tank) causing the leak you described. You may want to check it out. One easy way I can think of to do that is to fill the diesel tank almost to the brim (where you can see the fuel in the filler neck), then idle it in veg while watching the diesel tank filler neck level. It should not rise. If it does, you are returning VO to the diesel tank (purging) when it is not supposed to.