Why is this fuel tank leaking?

[greenlantern]

I have an idea why this thing on my diesel tank is slowly leaking, but I want to ask knowledgeable people before I embarrass myself.

I'm not even sure what this fitting is for. It's a little screw-on cap and I tightened it down with a pliers, but it's still getting wet within an hour.
Fuel tank is full


What is this thing, why is it leaking, and how can I stop it?

[Bill O]

Is your vent clogged? Check the vent screen in the hull for mud wasps, etc.


Possibly slightly over filled the tank. Next time fill w/less. Besides physically removing some fuel, could run the motor (from that tank) to lower the fuel level.

[greenlantern]

... So this was my wild guess... That the tank is very full and gravity pressure is forcing fuel out of this thing. The filler neck is higher than the tank and is topped up.

I sooo don't want to pump out fuel but I thinki need to try this. Unless someone else has any ideas.

[greenlantern]

But what is this fitting for anyway? Could it have a bad seal?

[tkeithlu]

It looks like a gummed up painted over pressure release. In any case, Bill's advice holds - by some means you are pressurizing a full tank, probably by simply over filling. If you filled until an automatic nozzle shut off, then you filled the fill hose as well as the tank, and there is gravity pressure on this fitting. That's the most likely scenario, because a clogged vent hose would release air/vapor rather than fuel if the tank was not over-filled.

[thinwater]

Identify where the hoses go. The pipe is probably engine supply. One on the hoses is the fill and one is likely the vent ... so where is the return?


You need to know what each does.

[DougR]

THe tank is probably leaking because the fill pipe is full of fuel, as others have noted.

The engine feed pipe is the one with the ball valve in line, the hose to the left is the return line from the engine, as it appears to have “return” written on the tank. The hose to the right is the vent, as it appears to have a “V” written beside it. The fill pipe is elsewhere, connected directly to the tank, not in the picture as it will be a 1 1/2” hose.

The fitting in question is likely for a dipstick, as the cap is made to be removed readily and the chain keeps it from being lost. Plus there is no fuel gage sender in sight. Or, perhaps it’s a stripping tube to strip water from the tank.

[team karst]

Understand that many, if not most, rubber compounds will degrade in the presence of petro fuels. Neoprene, which has good weather ability, breaks down with fuels and oil. Cant tell from here, but check that removable cap seal. Make it viton or bunaN. Even butyl, silicone rubber and epdm are very bad.

[bil56]

for hydrocarbon resistant seals, VITON and no more problems

[greenlantern]

Wow... This forum is amazing. I fixed it this morning after sorting out the fuel lines.

DougR correctly identified all the fuel lines and the guilty dipstick cap. I had it cranked down tightly, but was still leaking. Team Karst identified the reason. I was about to use chewing gum to reseal the cap, but at the last minute decided to use some marine goop (can't read the foreign label). Now it's dry. Problem solved for now.


I will try to find some VITON for the possible failure of the stuff I used

[thinwater]

Originally Posted by team karst

... Even butyl, silicone rubber and epdm are very bad.

I would take the work "even" out of this. Butyl and EPDM are atrociously bad with hydrocarbons. Good with water, but horrible if any oil is present.


Silicone depends on the compound.



Viton vs. buna (nitrile) is not open/shut. Viton is better at high temperatures but it is stiffer, does not wear as well, and is not as suitable at low temperatures. I spent years in refineries, and nitrile was typically the material of choice for gauging holes.


The important thing is to replace the seals when needed.