Transfer Flow Gas Tank
When I bought my 2004 Ford, Sportsmobile said that the 46 gallon tank would only fill to 35 gallons — something about a new California "CARB" regulation. I was told that I could still fill those last 10 or so gallons, if I let the gas trickle in slowly at the pump, and that I might be able to improve things by inserting an air hose into the gas filler tube. With the tank completely full, I should have an extra 10 reserve gallons of gas after the gas gauge needle showed Empty.
This did not turn out to be completely as described. I could fill the tank all the way to 46 gallons, but those last 10 gallons took forever to fill — or, at least more than a minute per gallon! Stuffing an air hose down the gas filler tube didn't seem to help. And I found that when the gas gauge showed Empty, it really was empty! It turned out that for my van, those extra 10 gallons were burnt first, before the needle started to drop below Full.
I had read on various internet sites that other people having trouble filling the last 10 gallons on their Transfer Flow tanks had fixed the problem by trimming or piercing the vent tube that drops down into the tank.
So, I had my mechanic drop the tank. But, it turned out that my tank (perhaps because it's gas not diesel) wasn't the way the internet sites described — my tank couldn't be modified by trimming the vent tube. The tank was reinstalled unmodified.
Sometime later I talked with Heber at Sportsmobile, and he was kind enough to give me details on his idea of adding a second vent tube — from the top of the tank, and T-ed into the original tube. Here are the notes he sent me:
So, I brought the van into my mechanic, and had him drop the tank again! But it turned out that there was no tank-top plug where Heber's notes said one would be. So, the mechanic drilled a hole in the top of the tank and was able to add a new vent tube.
This photo is of the top-center area of the Transfer Flow tank as it is currently on my van. You're looking towards the driver side, so left is rear, right is front. I believe this photo shows the new custom vent tube added by my mechanic:
My mechanic warned me that he wasn't responsible for any potential issues — the Ford computer might throw a Check Engine code — but it's worked beautifully. Filling all the way is now a breeze, and I haven't had any problems.
UPDATE, a year later at 30,000 miles: This setup is still working fine. I recently learned that the Transfer Flow gas tank uses the stock Ford fuel pump — when that fuel pump died, so the gas tank had to be dropped... again!
More info about our Sportsmobile is here...