Mystery fuel leak 2003 MKII Plastic tank

[Stucker]

Mystery fuel leak 2003 MKII Plastic tank

So I had a fuel leak this past weekend.    I was making a 150 mile trip between Kingston and Toronto Ontario.   Before leaving I filled the tank so full that it came out of overflow valve on transom (not intentional).   I started my trip with two hours of motoring before sailing on a port tack with some waves and 15 knots of wind.   After several hours of sailing I noticed that some diesel ha been leaking out of tank.   Because of the healing angle some of the diesel ended up traversing corner of the aft wall to underneath aft cabin and along the side of the prop shaft and ended up coming out the side of engine compartment onto aft cabin floor.   I inspected all hoses and everything looks intact and tight.  My best guess is that the diesel might be coming out of gasket under fuel valve at top of tank?   After cleaning and inspecting I did not have any more leaks once I burned off another 30 litres of fuel (7 gal?).  seems like a full tank issue.   I have attached pics.   Red indicates the area the diesel came out from behind the wood access covers.

Any tips on trying to trouble shoot and find the leak?  My thought was to try and fill again to max levels and recreate the problem by going for a sail on a port tack, this time with no bedding and cushions to soak it up.  Then I can be down there and see it happen.  Is this a bad idea?  It's a plastic tank.  Would love to know what you would do. 

There is always something 

[KWKloeber]

Scott,

Of course the caveat is that this is lacking any "hands on" your particular situation (or about the tank on the C34,) but the valve fitting looks to me like it was tightend down WAY too much and has deformed the gasket.  Or fuel has attacked and deformed it. 

See my caveat.

It also looks discolored around there (fuel?)

In general though, IIWMB (C-30) I would
   - close the valve
   - plug (or properly pinch off) the tank vent hose.
   - use the fuel return hose to put 1-2 psi of air into the tank.   With just a hand pump, nothing to bulge the tank, just to locate a air leak.  Alternately you can use one of my favorite tools, a Horror Fright brake system bleeder pump.
https://www.harborfreight.com/brake-bleeder-and-vacuum-pump-kit-63391.html  But that will reveal a leak, not its location.
   - brush or spray the suspect area(s) with a little dishsoap in water or better kids' bubble maker (it lets bubbles grow larger and easier to spot.)  There's recipes for it on the 'net.

I don't know which engine you have but the return would be either on the front (M-25, from injector #1) or the rear (injector #3, later engines.)

See my caveat.

[Stu Jackson]

Just playing detective here:

Fueled enough to fill the vent.  There is a possibility that the top of the hose connecting the vent to the transom fitting may be loose.  That is the end of the tank where your red lines shows the leak appeared visually.

Of course, all caveats about liquids applies. 

[Jim Hardesty]

Scott,
During my cruise this summer many of the fuel docks had rules that only employees pumped.  Was overfilled a number of times, ie the fuel splashed out the fill and vent.  I hate it when that happens.  Afterwards the only fuel leaked from the vent.  Nothing in the cabin.
I think Ken's trouble shooting is spot on.  Don't need to overfill again.
Also, I keep a log of engine hours and fuel.  FWIW I also have a white board at the nav station that has the last fill engine hours.  Simple division gives me a gallon per hour usage.  For Shamrock it's .6 to .7 Gal/Hour more when motoring hard.  Simple multiplication using the hours since last top-off will give the gallons to pump.  That's if the meter is visible from the nozzle.  Doesn't work if you tell them 11 gallons and they pump 13   
Hope you get that diesel smell out of the boat.  I hear fabric softener helps.
Jim

[Stu Jackson]

................
................
Also, I keep a log of engine hours and fuel.  FWIW I also have a white board at the nav station that has the last fill engine hours.  Simple division gives me a gallon per hour usage. 
...................
...................
Hope you get that diesel smell out of the boat.  I hear fabric softener helps.
Jim

Fuel consumption is the key.  KNOW yours and tell them how much to pump.

Pure Ayre is what you're looking for, great stuff.

[ErikN]

We had the same problem when we bought our boat. We finally discovered that there were two leaks. One was through the fuel level sender--the gasket at the base (that sits on the tank) and the seal around the sender in the middle had fine cracks. The other was in the fuel supply for our Espar heater. The fuel standpipe is connected to the fuel line with a 2" piece of flexible hose, and that flexible hose was cracked. We found one, congratulated ourselves on fixing it...then discovered the other. A surprising amount of fuel leaked through when the tank was overfilled, but they were relatively easy to replace.