Fuel Tank Pressurizing

[TimG]

I recently upgraded my fuel feed line from the stock 5/16" line to a 3/8". I also installed a bosch 044 pump and an inline fuel filter. From the in-tank 255 Walboro to the FPR is completely aftermarket. I am using the stock 5/16" fuel line as my fuel return.

The issue I'm having is that my fuel tank seems to be pressurizing and fuel is constantly trying to seep out of the sending unit between the gasket and the tank. (All 6 studs are functioning and tight). It never did this before so I'm confused. What I did notice is that with the fuel cap off, the seeping stops but as soon as you put the cap back on, it starts seeping again. This was performed on a 1/8 full tank and then I added some more gas and it's now at 1/2 tank. It did this before and after the fuel refill.

What would be causing the tank to build up so much pressure? My only thought would be the return line is returning more fuel than the pump is taking, but that doesn't make any logical sense.

Ideas?

 

[Defiant]

No, I doubt your fuel system is creating fuel and increasing your amount. However, it is getting heated during its trip up to the engine and back to the tank. The expansion rate for gasoline is around .0007 per Fahrenheit degree. There is also the effect of magic, which the petroleum producers try to explain away as "vapor pressure and temperature" and other such doublespeak.
However, the reason you're getting seep around the sender is that the O ring on the sender flange isn't sealing, from dirt, dents, or simple age.
Have you done things (such as elimination) with your charcoal canister and the vapor recovery system?

 

[TimG]

The charcoal canister has been eliminated and it was done sometime in 2004 or 2005. I bought a new daily driver 2 weeks ago so the car is literally going to be driven to the track and occasionally to work. The nipple vent for the canister is left open to the atmosphere on the tank right now and free from obstruction.

Maybe you're right and the seal isn't perfect (I've re-installed it a half dozen times in the last 2 days though). There definitely isn't any dirt or debree, it's been cleaned up very nicely. I do have a new gasket which should be in stock today and I'm going to check squarness of the sending unit when I go home after work today.

The odd part of the whole thing is that the sending unit is leaking like it's pressurized. My only theory is that there is so much fuel being returned to the tank, that it's causing the fuel to splash up to the sending unit and it's leaking out of a bad sealed unit. Take a look at this picture and you'll see where the fuel is returned too inside the tank. (btw - this isn't my car in the picture)

 

[Boosted98gsx]

did you loop the lines that run to the canister after you eliminated it? If you did, it can cause over pressurization. But if you dont use a check valve, you can get o2 in the tank, which is BAD. So, don't loop the lines, but add a check valve, and make sure the + vent is far enough away from anything hot.

 

[TimG]

I lifted the sending unit while the car was started to see where the fuel was coming from. It is definitely coming from a small port on the top of the fuel pump that is right beside the fuel feed nipple. Any ideas if that's normal or not?