43DS - Main Diesel Tank Filling Issue

[wheelsucker]

I'm wondering if anybody else has come across the following issue:

Background:

Previous owner installed a Blue Seas tank monitoring system for both the factory installed main diesel tank and aftermarket auxiliary tank installed in the V-berth in place of the 3rd water tank.  Since we carry 95 gallons, we've only filled the main tank three times in the two years that we've owned the boat since we transfer from the aux tank as fuel is consumed from the main tank.  Filling the main has occurred without issue and the diesel bubbles out of the main tank vent port as expected each time when tank is full.

At the end of last season we ended up with 38 gallons indicated in the main tank and 0 in the aux tank.  We intended to go to the fuel dock at the end of fall to fill both tanks but life got in the way. 

Issue:

Fast forward to this June when we were finally able to get back on the boat and actually do some work.  One of the first things was going to the fuel dock and filling the tanks.  Main tank still read 38 gallons and aux read 0.  We started filling the main tank and diesel immediately came out of the vent port.  Wondering if somehow the gauge was off (17 gallons is a lot!), we stopped on the main and moved on to the aux - filled it with the 40 gallons with no issues.  Today I was finally able to get the inspection port door off the main tank and saw that the diesel level was 3 inches below the bottom of the inspection port base, so it looks like the  Blue Seas gauge is still calibrated correctly.

Question:

Per Papawads thread on Diesel Hoses, it does look like the vent line drops before it heads aft to the vent port.  Is it possible enough diesel somehow got trapped at the low spot of the vent line during the last fill in 2021 and got blown out the vent port once enough air pressure had built up in the tank to make it appear the main tank was full?  Has anybody else experienced this?

TIA

BTW - I had no idea the main tank was so large, extending underneath part of the galley cupboards.  We haven't had to replace the water muffler yet am not looking forward to getting that main tank out.  I know people have done it but hard to believe that the tank will actually make it out through the floor opening.

[wheelsucker]

Answering my own question - transferred from auxiliary tank to main tank on during trip in San Juan Islands.  My wife said she could smell the diesel and feel the displaced air from the vent, but no diesel came out of the vent.  So my only conclusion is that some small amount of diesel somehow got trapped at the low point of the vent line during the trip last year - weird.

[papawads]

Once we shorted the vent line to make it run only uphill, we’ve not had a repeat of the problem. Beforehand, Even leaning the boat (sufficiently) to port on a starboard tack, if the tank was full-ish, would put diesel in that line ‘reservoir’.
Presume the same happens whenever the tank is filled up to the max, as the fuel pump won’t stop until there is resistance…. ergo a ‘head’ of diesel in the feed pipe.