auxiliary fuel tank fabrication/plumbing questions.

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Hello all ! been quite a while. Anyways I conjured up a new project for the truck that I figured I would get done before camping season comes along. Being that were 78 feet long with the fifth wheel and boat ive always had to pull in for fueling of course, which isn't the end of the world but isn't FUN at times, also being that theres no diesel at the lake I felt it just made sense to fabricate myself an aluminium auxiliary fuel tank.Here is my plan! Iam going to essentially make an "invisible" tank, i have an existing tool box for the truck 64" x 24" x 21" high, i am going to cut out 8" from the toolbox height and weld in a new floor to compensate for the height of the auxiliary tank which will sit in the exact foot print of the tool box and will be 8" deep. Roughly 50 gallons of extra fuel, great! I like a nice clean look, so i don't want ugly hoses/filler necks/greasy diesel residue all over the place so! when the new floor is welded into the tool box i will be cutting a slot on the driver hand side so that all of my fittings and equipment for the auxiliary tank will be neatly tucked away AND locked up, the tool box will have a partitioned and sealed off area to house all of this equipment. hard to explain all of this without pictures however i will have some to post on Monday.Here are my questions! and plans. If anyone has any input as to a better way i am all ears!- I will be tying into the existing filler neck on the truck, i will be removing the filler neck and welding on a fitting to accept the fuel hose from the auxiliary tank. I seen the rubber hose splice kit for sale... don't like that idea! more places to leak in my opinion.- Now! I was going to go simple and go gravity feed with a ball valve, however i scrapped that idea, i don't fill other equipment so i have no use for a nozzle/pump setup, so i decided to go with a 12V electric fuel transfer pump with a simple IN and OUT preferably an 8-10 GPM... i really don't want to take 30min to fill the existing truck tank. However i have yet to choose a pump to go with, but by the looks of things 3/8" seems to be the most common ? any recommendations? I will be wiring a switch for the pump in the cab of the truck, and will be monitoring the fuel transfer via the stock fuel guage in the trucks cluster. - Here is what i am thinking for the plumbing situation nothing set in stone of course. SO! for your own visualization the tank will be sitting flat on the truck... 8" high 64" long, 24" wide with the toolbox sitting on top.  on the driver side of the truck going through the slot of the tool box into the tank i will have a filler neck, a SUCTION line that protrudes through the tank that sits maybe 1/2" from the bottom of the tank? and lastly i think i will weld in another bung with nipple beside those two. The reason for extra nipple is.... i was thinking when i had the trucks filler neck out that i would ALSO weld a fitting on to the trucks vent line... and run another hose from the extra nipple ontop of the auxiliary tank to the trucks vent in order to vent the auxiliary tank, what are your guys thoughts on this? The only negative i see is if when filling the aux tank, that i end up filling the vent line if i over fill the tank. i know i could also get a venting cap to alleviate this problem, however i thought if i could successfully vent the aux tank this way i could get a tight sealing filler cap for the aux tank and prevent some fumes in the tool box.SO my plan is of course to hook up my hose to the suction line and then that would lead to a ball valve which is bolted to the box wall and then that would lead to the IN port on the fuel pump which will be bolted to the inside of the box, directly beside the pump on the OUT port i will be pumping into a fuel filter/water separator, and then a hose coming from that filter will make its way down to the trucks fuel filler neck via a hole in the box w/ grommet.I guess my biggest questions are, do any of you see an issue with this setup? will i have issues with suction having my suction tube through the tank from the top sitting 1/2" from the bottom of the tank? And your opinions on my venting solution? i am not apposed to just using a vented diesel fill cap, but i thought maybe tying into the fuel tanks vent system may be a cleaner option.any recommendations for a pump?Construction of the tank will be 1/4" aluminium... overkill for sure but my tool box will be sitting on top, and i had a 4x8 sheet in stock. Baffles, and i will be making my own aluminium bungs on the lathe.
Reply:I have a 115 gallon L tank for the back of the pickup. One issue I have is that it's hard to get the tank 100% empty. There's a lot of fuel in the 60" x 30" bottom when the pump no longer can pick up fuel and I have the pick up cut so it will suck down to about 1/2"- 3/4". To empty the tank, I'll drive around the corner and park the truck diagonally down hill in the back parking lot of the shopping center. This slants the whole tank diagonally so the fuel pickup is now lower than most of the tank and I can get maybe another 10-15 gallons out of the tank that way. I keep the 12' hose coiled right next to the tank between the tank and the body and you'd hardly know it's there. To pump, I simply open the side window of the cap, pull out on coil of hose with the nozzle and fill just like at the pump. Pump/hose assembly is rigged in the drivers side since that's where the fuel fill for the truck is.I have mine set up to one of the GPI 12v pumps you can get at Northern or Tractor supply. Mine has the regular fuel nozzle and 12' hose. I run their water block fuel filters that absorb moisture, then shut down when full. This prevents me from pumping any water from say condensation into my main vehicle tank. I will say a vent is VERY important. I got reminded of that the other day. Guy who helps me occasionally was going to pump 130 gallon from the one tank in the truck into the 500 gallon tank at the shop. Anyways he hooks everything up, but after a short bit, the fuel just isn't flowing, so he pulls off the nozzle and things get better for a bit, but then slows way down again. Turns out he forgot to open up the fill on the tank, and he's creating a vacuum in the tank since it doesn't have a direct vent. The original vent would allow fuel to slosh out if it was too full, and since that tank is only used to move bulk fuel to the shop and fuel from the shop to the house, it was removed temporarily until I can get the chance to put the new self sealing vent in place.I've seen some neat rigs where they equipped both the pump and the hose with flat faced hydraulic QD fittings on utility bodies. They keep the pump on the tank, but have the plumbing run to where it's most convenient. The hose etc is stored in a side box. To fuel, you simply plug the hose into the QD fitting and away you go. The shop tank and the bulk transfer tank have cheap alum bayonet fittings installed. This allows me to easily add more hose if needed and allows me to install the one 12v pump on a drum if need be without spinning pump and hose together. I have a blank cap to seal things if the hose assembly isn't installed.If I was going to rig direct into the trucks fuel system, I'd look at the 2 tank aux systems like come factory. That way you simply hit a switch in the cab and then draw direct from the aux tank. They have systems designed to be used with waste veggie oil/ bio diesel where you have an aux tank added in the back..No government ever voluntarily reduces itself in size. Government programs, once launched, never disappear. Actually, a government bureau is the nearest thing to eternal life we'll ever see on this earth! Ronald Reagan
Reply:I think on problem you might have is overfilling your trucks tank. If you fill the tank and go by the gauge on your instrument cluster it will be easy to do. Most vehicles fuel gauge are dampened so that they do not change as the fuel moves around in the tank. Leave your ignition on one time while you fuel your truck at the station and watch your gauge it probably will not show as full when the pump shuts off.Mike
Reply:thanks for the updates fellas much appreciated! quite looking forward to the setup. went shopping today and purchased some items picked up a 10GPM 3/4" diesel pump, and a water block fuel filter setup. Just going to cut the clips off the pump wiring and splice in a section of wiring to run my switch up to the cab. Decided I am going to hard pipe in the pump and fuel filter to make it more ridged and of course put some pipe clamps inside the box to support the whole setup.Now one question I do have! tank venting.... So my goal is to make this setup as tidy and fume free as possible, dealt with diesel all my life and I know how bad and unsightly  fuel tanks/filler necks can get. SO! on this tank, of course I will have the safety rollover vent valve, this item has a check ball in it that allows to vent the tank when in the upright position, when you overfill the tank it forces the ball up and seals so you don't leak any fluid. In the case of a rollover situation the ball will also seal and not allow any fluid to leak out. In the operating position its supposed to vent the tank.My question is will this safety vent valve on its own be adequate enough to vent my auxiliary fuel tank? I have two options of sizes, 1/2" valve with a 5/16" hose barb, OR 3/4" valve with 1/2" hose barb. Or will I also need a vented fuel cap. iam quite on the fence about this option. Id prefer to run JUST the safety vent valve to hopefully provide a more "fumeless" setup as compared to what the vented fuel cap might provide.My plan for the safety vent valve, would be to connect a line to the hose barb, and run that to a barbed fitting on the inside of my tool box.... that lead to a simple 90* fitting on the outside of my toolbox. Attached Images
Reply:This all sounds really expensive and overthought. Somewhere I have seen a kit to gravity fill the OEM tank from the auxiliary tank in the bed of the truck. No pumps, no hassle. Just drive and enjoy.
Reply:Originally Posted by 12V71This all sounds really expensive and overthought. Somewhere I have seen a kit to gravity fill the OEM tank from the auxiliary tank in the bed of the truck. No pumps, no hassle. Just drive and enjoy.
Reply:Headspace-allow for at least 6% tank volume for this with diesel fuel. 10% for gasoline.This allows for heat expansion of the fuel (and it does expand). IOW-this is normally created viathe filler neck protruding down into the tank top a calculated amount to form an air pocket abovethe max. fuel level. Open tank top vent can protrude the same to help this pocket formation.Blackbird
Reply:I have a gravity feed from my aux tank to my main tank, ball valve is on drivers side of tank. Honestly, I think the only way it would leak was just a shoddy install. I've owned and installed 2, my present truck and my last truck with no issues. As far as your setup goes, I think the roll over vent would suffice, maybe throw in a vented cap if you're concernedSent from my Nexus 6 using Tapatalk
Reply:Gravity feed is illegal if its directly to the engine/carb. Sent from my Nexus 6 using Tapatalk
Reply:It sounds like it might be possible with your setup for fuel to siphon from the aux tank into the main tank after the pump is shut off.  Even if it siphons slowly, you could get a big mess after the truck sits overnight.I'd want some way to empty the last little bit out of the tank to periodically remove water that condenses in the tank.  Like a drain in one corner.Lincoln AC 225Miller Auto Arc 130 (Mig)Thermal Arc 186 AC/DC Inverter (Tig/Stick) - the "no excuses" machine
Reply:Originally Posted by ThobIt sounds like it might be possible with your setup for fuel to siphon from the aux tank into the main tank after the pump is shut off.  Even if it siphons slowly, you could get a big mess after the truck sits overnight.I'd want some way to empty the last little bit out of the tank to periodically remove water that condenses in the tank.  Like a drain in one corner.