87 300D turbo tank/vent question

[kg6mov]

I've been cleaning up my recently purchased 87 300D turbo daily driver. I noticed when I started driving it that once I get under 1/2 tank for fuel the performance drops off and MPG's go down. I also noticed that the area inside the fuel door is always moist, which I assumed was residual fuel/oil.

Well . . .

Today when I filled up I noticed that there were streaks of diesel running down my rear fender. Looks like diesel was coming out of the tank vent hole that is under the filler cap. There was also a release of pressure when I opened the filler cap.

So vent something or other? I'm not largely familiar with fuel systems, I've never had to deal with it on my other vehicles.

[jay_bob]

The hole under the fuel cap is a drain to the pavement.

That is different than the tank vent. The tank vent is under the car on the right side, it looks like a little white plastic trumpet.

Are you sure the whoosh when removing the cap is pushing or sucking in. The tank vent is designed to admit air as the lift pump sucks out the fuel. That little pump could "suck a golf ball through a garden hose". In extreme cases the tank can collapse from atmospheric pressure.

If you have fuel dampness in the filler area that says to me the gasket in the filler cap is bad and you're getting sloshing in turns. No way fuel is coming up the drain tube in the fuel filler area from the tank (unless somebody did something really weird with the hoses). Maybe the exit of that tube is clogged, spilled fuel backs up in the tube, and sloshes out when you turn.

[kg6mov]

Interesting, the pressure release in the fuel cap feels like vacuum, not sure. I assumed that since it seems to seal air in pretty well it wasn't just fuel sloshing in turns.

I don't drive it like a camry though, so sloshing is definitely a possibility. I'll have a look at the vent and see if I can clean up the filler area to get a better idea of what's going on.