Fuel Tank Leak Testing, Round 2 and a Very Unpleasant Surprise!

After the completion of the second service bulletin on the fuel tank, I allowed the Proseal to set up and cure for a few weeks.  Today, with spring-like temperatures in the 70's, it was time to check the leak status of my service bulletin modifications as well as the optional fuel tank vent installation.  Following picture shows the setup, much like the Round 1 version performed in May 2013.

Notice the upgrade from green colored water to blue!  In this shot above, the tank is pressured up to almost 27 inches water column, resulting in 1 psi pressure to the tank.  There is a leak!

Turns out it is one of the two screws that Van's insisted needed a lock washer underneath, I believe, to insure proper grounding of the electronic fuel sender unit.  You will see bubbles around the screw in the 10 o'clock position below.

There were the other usual culprits causing leaks during the pressurization, like the fuel tank filler neck, where it adjoins the tank with the large diameter radiator hose.

 Another source of an imperfect seal was the tank vent's 45 degree bulkhead fitting, which had been firmly potted in a nice glob of Proseal.  The leak is better seen in the pictures below, with bubbles forming at the base of the cap - not the Proseal!

Those leaks aside, the 10'oclock screw on the sender unit was the only legitimate leak that was observed.  Note the tightened screw and the distinct lack of bubbles while under the second pressure test of the day.  I would say we are good to go!  But stay tuned.....the plot is about to thicken.

Not 30 minutes after I left the garage and put everything away, basking in the sweet glow of victory of a tight, nonleaking fuel tank, I got a phone call from Dick Gossen, another RV-12 Viking builder from the great state of Texas.

The conversation went something like this:  "Tom, I've think I've found a problem with our fuel tanks and the way they might not function properly with the stock fuel system as constructed for the Rotax engine...".  It was too funny to get upset about.

The long and short of it is that the fuel return line is probably in the wrong location for fuel tanks that operate with a Viking engine.  The fuel injected Viking is of course, a pressurized system that utilizes the stock return line, originally designed to minimize vapor lock with a Rotax engine.  The sump, where the fuel outlet is located is at the front of the tank.  The fuel return line terminates at the rear of the tank, pointing upward into the center of the filler neck.  This location of the return line is the best way to cool warmed fuel in order to prevent vapor lock.  Unfortunately this characteristic works completely against the Viking's requirement for returning fuel to the sump as quickly as possible.

The Viking engine with its automotive fuel pumps delivers fuel at the rate of about 30 gallons per hour, with a return rate of about 25 gallons per hour, netting a nominal consumption of about 5 gallons per hour.  Under this assumption, once the fuel level reaches the 5-6 gallons remaining in the tank, fuel is mostly contained in the baffled, sump area.  Fuel goes in and out of this smaller (approximately 1 quart) area via a series of 4-5 small holes at the base of the baffles, where they contact the floor of the tank.  Perhaps you can see the potential problem.  Fuel returned to the tank at the opposite end cannot "trickle" its way into the sump area fast enough to overcome the rapidly dwindling fuel supply...and a fuel starvation situation could exist.  Combine this with the possibility of a go around, where what little fuel remains is flowing out of the sump area and you could have a major problem on your hands.

If Dick's assumption is correct, and I have no reason to believe that it is not, it will mean opening the tank one more time to greatly shorten, relocate and secure the termination of the return line.  It only makes sense with our high pressure fuel injected engine to return unused fuel as close to the sump/fuel pickup area as practical, without creating other problems.

Dick has initially suggested shortening the return line such that as it enters the tank, adjacent to the fuel pickup fitting and travels vertically toward the top of the tank, that it stop before exceeding the height of the baffle wall and immediately take a 90 degree turn and point directly to one of the four baffle walls.  This configuration would keep returned fuel in the immediate area of the sump and hopefully minimize any flow disturbances as fuel leaves the outlet.

Onward through the fog!