Tank Vent BURP

1989 Mercuiser Thundercaft

I have problem with fuel burping out the vent hole while filling with gas. I never fails gas comes out vent port about 2 min. into pumping gas no matter how fuel that tank is.

Problem:
I have theory that the fuel vent line has a P-trap configeration and when I pump gas it purges the trapped fuel from vent line.

Solution: Reroute fuel line to remove P-trap is removed thus no more trapped gas allowing good vent.

Hardspot: Is P-trap nessessary to keep gas from venting out, which really doesn't make sense or Problem is else where.

Thanks for your comments

 

In theory, the typical vent hose from a gas tank should travel up as it leaves the tank, and continues to rise until it exits the hull at the vent fitting. In practice, if the tank is installed near the side of the boat, under a side deck, for example, the hull is often not tall enough to allow much elevation gain between the top of the tank and the location of the vent fitting. As a result, a boat builder may put one or more loops in the vent line on the way from the tank to the vent fitting. These loops are not intended as a P-trap, which is designed to have standing fluid in it, but more as an impediment to fuel that may find its way to the vent fitting quite easily without the loops.

As you have noticed, fuel often finds its way to the fitting anyway. I think your assessment is correct in the fact that you are driving fuel that is standing in the vent line out of the fitting while fueling up the tank. Fuel entering the tank creates enough positive pressure in the tank to drive fuel that is in the vent line out of the vent fitting.

My own boat has this problem. I plan to run a new vent line, locating it in a manner that takes it from the tank to the rear bulkhead of the cabin on my boat, up the face of the cabin bulkhead to as high as I can take it, then back down to the vent fitting in its current location under the side deck. I will make sure that there be no low spots to collect fuel in the line. I imagine that fuel is unlikely to make its way all the way to the top of the bulkhead of the cabin. Therefore, no fuel is likey to come out of the vent fitting after the re-route of the vent line.

I have only had the boat in use for a few months, and I have not yet tried my fix. If you try something like this on your boat, please post how it works out!

 

Are you filling the tank with the boat on a trailer? This frequently occurs with boats on the trailer because the boat is not at the same attitude as it would be in the water, but the tank was installed with the tank level when the boat is afloat. So when you put it on the trailer the end of the tank with the vent is at the low end. So when you try to fill it, fuel burps out of the vent before it's full.

Zewe's suggestion also occurs.

Boats are supposed to be built such that all the fuel system fittings can be inspected, so there should be a way for you to look at the vent line. Do this before you start changing things to find out what the problem is first.

Of course, since Thundercraft is long out of business, there are few people around who are familiar with those boats.

As for the p-trap. It is rarely done. Most tank vents just go straight up to the tank vent. On a boat where there is a danger that water may enter the tank through the vent, then a p trap is put in to keep the water out. I don't recall thundercraft doing that. Of course what Zewe said may be true They may have put in a couple of loops.