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nunzio 06-23-2008 12:57 PM

Huh? - fuel tank issue
In the last two weeks I motored for 20 hours. Fuel tank gauge is at 1/4 for aq 35 gallon tank. Went to fill up at the fuel dock and it would only take two gallons and began to overlow from the overflow vent and from the main fill. Checked tank and there are three hoses. The main fill, one to the vent, and one to the engine.

So the fuel should only exit the vent if the tank was filled. It is a Catalina 36 with a Universal 25 Diesel.

Any ideas what is happening? :confused:

BarryL 06-23-2008 01:16 PM

The diesel fairy must have visited your boat the night before.

It seems like you must have a blockage somewhere. I think you need to remove the fill hose and see what is going on.

Good luck,

JohnRPollard 06-23-2008 01:19 PM

Is the boat new to you? Or is this a change from previous behaviour?

If the boat is new to you and this is your first fill-up, you may just have a slow-filling tank. This is sometimes caused by a low-point in the fill line before it reaches the tank, in which case patience will be necessary while filling (and a fuel-dock that isn't very busy).

Another possibility is that you used the high-volume pump at the fuel dock. These are intended for filling motorboats that take on hundreds of gallons of fuel. They pump so fast that they generate a lot of foam, and they fill the tank so quickly that that little air vent cannot vent fast enough -- resulting in back pressure and more foam. Try to find a fuel-dock with a standard-flow pump.

Of course, there's also the possibility your fuel gauge is malfunctioning. I would expect that engine to use about 1/3 gallon per hour. So 20 hours of motoring should have used about 7 gallons of fuel. Maybe there's more in there than indicated?

nunzio 06-23-2008 01:57 PM

New boat and my first fill up. Good point about the fast fill although we were trying to fill slowly. Question: Where is the vent line connected. Is it the fill ine or the furl tank? I think my first note could be wrong that the thiord line on the tank id a "return" line and not the vent which means the vent line may be tapped off the fill line.

Rockter 06-23-2008 02:05 PM

If the tank has an inspection hatch, loosen it a little and watch for fuel squirting past the seal. If it does not squirt past the seal, take it off and have a look inside. If it squirts past the seal, the tank is full right into the filler hose.

If it's not full, have a friend fill the tank from the filler line and have a wee look in to see if the fuel is entering the tank OK.

I fitted a quick-release inspection hatch to the tank top, and I fill from there, in the cabin. Personally, filling right to the deck fitting can give an old tank a hard time as it raises the average pressure in the tank by about 1.5 psi (4ft head diesel), and 1.5 psi over the top area of the tank... maybe 4 x 4 ft, that's (2304 square inch).... will be a fair old load on there, close to 1.5 tons, on the roof of the tank, and probably the floor too.

Very few boat owners would park a 1.5 ton automobile on the roof of the tank, but plenty of them use 1.5 tons to try to lift the roof off the tank, or blow the floor out of it. All they have to do is fill their 4x4 ft tank right to the deck filler entry, 4 ft above the tank top.

Not for me that one.


nunzio 06-23-2008 02:23 PM

The tank is definitely not full. I'm going down there today to track the vent hose.


JohnRPollard 06-23-2008 02:24 PM


Originally Posted by nunzio (Post 332472)
New boat and my first fill up. Good point about the fast fill although we were trying to fill slowly. Question: Where is the vent line connected. Is it the fill ine or the furl tank? I think my first note could be wrong that the thiord line on the tank id a "return" line and not the vent which means the vent line may be tapped off the fill line.

Usually the vent line is tapped right into or near the top of the tank, then leads from there up to a high point in the hull or side of the coachroof.

The return line is for excess fuel that the fuel injection system does not use, so it gets pumped back to the tank from the injectors. You should be able to trace that line fairly easily. There should also be a fourth line, for fuel delivery coming from the tank to the fuel filter/pump. It will look much like the return line.

Now that I know this boat is new to you, and assuming you weren't using a high-speed pump, my hunch is that you have a low-point in the fuel fill line. Go down to the tank and see if the fuel fill line dips down a bit below the fitting where it attaches to the tank. If so, that is the culprit and you can try to fix it or live with it by pumping slowly. Sometimes there's no way to route that fill line without it taking a little dip before it gets to the tank.

camaraderie 06-23-2008 05:39 PM

My guess is a blocked vent line...either with an insect next (wasps seem to love them) or with fuel collecting in a low point in the vent line and blocking the air. Disconnect the vent line at the tank and try to blow it out...If that is it...trace the line to eliminate low loops and future problems. If the tank air can't escape as you are filling it creates surges backup the fill line and gives the impression that the tank is overflowing.

nunzio 06-23-2008 06:43 PM

You both are correct. The fuel line dips a bit below the tank and then routes to the top. Apparently I just need to fill it VERY slowly as the fuel will just spew out the vent.

Pub911 06-23-2008 11:06 PM

Just to add some confusion to the thread....why are there only 3 hoses? Doesn't the engine have a return hose back to the tank for the diesel overflow? If you're boat is equipped with a 'lift pump', meaning it's not feeding fuel to the engine via gravity, but rather, it's assisted by a low pressure electric pump plumbed in line between the tank and the engine (typically between the primary and secondary filters) ...well if it has one of those try this...When the key is on and the engine is not running...listen carefully to the you hear fuel dribling back into it? If you do, it's coming from a 4th hose to the tank.

Now this may have nothing to do with the problem on the surface, but it may suggest you need to take a much closer look at the plumbing to get a deeper hopes of improving your diagnosis.

Good luck.

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