Advice Needed: Adding stock 19 gal rear fuel tank to single tank truck

[Tyler]

Please poke holes in my plan, Bullnosers!

Some other guys have done projects similar to this but without closure on the thread, so I don't have the full story.  And I'm looking to do it just a little different.

I found a new EFI model rear fuel tank in a donor truck at a local JY.  My truck only has the single 16 gal tank and was carbureted (see signature).

I plan to remove the crossmember, tank, straps, spare tire mount, etc. from the donor truck as well as the wiring harness and heater/blower/fuel tank selector module from the dash.  Then install into my truck.  I THINK I can use a relay setup to power the fuel pump in this rear tank, routed through a toggle switch that I will put on the dash somewhere.  Then, I will route the 19 gal tank fuel pump outlet directly into the 16 gal side tank sending unit (this is no longer used due to converting to Holley Sniper and retro-fit in tank fuel pump and the screen has been removed).  This allows me to fill the side tank from the rear tank using the switch on the dash and the stock in-tank fuel pump from the 19 gal rear EFI tank.

I intend on cutting a hole in the bed behind the wheel well and mounting a filler neck from underneath (by my eyeballing, it should line up pretty close to the rear tank fill location on dual tank trucks).

I THINK I can wire the existing 16 gal side tank sending unit level detection to the scavenged JY dash unit and then run a parallel set of wiring to the "new" rear tank sending unit.  This will allow me to see the level in the rear tank when needed so that I don't run the fuel pump dry.

Of course, I'll post up pics and lessons learned when I'm done.
I know I haven't considered or checked if the dash module from the donor truck will connect to my non-AC unit and probably a few other finer points as well.  Inertia switch? Overflow?

Prove me wrong.  Prove me right.  Advice?  Things I haven't considered?  Standing by...

[ArdWrknTrk]

I think you need to hear from Dave (Fuzzface2)
AFAIK he's the one here who has put dual tanks in a SWB truck... but he doesn't have an aftermarket Sniper setup.

[Rusty_S85]

I thought of trying to do a dual tank setup but I found them to be a pain on the sniper install I did on a 78 GMC with dual tanks, switching valve is rated for 50 PSI from a 80s pickup but the valve wouldnt switch with the engine running due to the 50 psi fuel pressure.  I theorize it would probably work better as a puller pump through the switching valve but that also means a noisier pump, a pump that wont last as long as it is more prone to running hot.

One Idea I had on my short wheelbase truck as I thought of getting the large 50 gallon I think it is rear tank from a place like LMC and then use a transfer pump to pump fuel from that tank into my saddle tank and just use my saddle tank as the supply tank for my Sniper.  Down side to this is you would need quite a large volume pump to be able to fill the saddle tank up in quick order and then you would need some kind of safety to shut the transfer pump off in case you dont catch it and switch it off manually.

Personally Ive been looking at having the 16 gallon saddle tank lengthened on the front end.  Measurements I took shows I should be able to add 3 inches to the front of the saddle tank which if my calculations are correct should turn the 16 gallon saddle tank into a 21 gallon saddle tank.  Only draw back in my mind on this is all that extra weight if the straps could handle that and if that extra 5 gallons would cause the truck to sit with a lean to the driver side.  In any case I know the saddle tank on mine will have to be cut open regardless to weld in a baffle as 85-86 trucks didnt have baffled fuel tanks, they used a lift pump to a pump reservoir that the high pressure fuel injection pump on the frame sucked from and I wont be doing this on my truck.

But in the end that is one of the reasons I decided to scrap my idea of adding a second aft tank to my truck.  Ever since I did that 78 GMC with that 85/86 GM switching valve and found it would not switch on the fly I just have no idea how I personally could add a dual tank to my truck and have a functional switching valve without putting the pressure pump on the frame rail.

If you do it the way I was thinking of on mine and use a transfer pump to fill your saddle tank from the aft tank what you can do is get a AN bulkhead fitting and drill a hole in the top of your saddle tank and use one of those bulkhead fittings from the inside to the outside to allow an attachment point for the transfer line from the rear tank.  Great thing about doing the AN bulkhead connector which some come with sealing washers for this kind of thing you can use another AN hose end on the inside with a hose long enough to keep it near the bottom of the tank to prevent aeriation.

[FuzzFace2]

When I get on my pc I will reread this and post what I think.
Dave ----

[Tyler]

Rusty, my plan was to use the in tank pump for the rear tank as a transfer pump.  I would manually control it from a toggle switch that I'll put on the dash (probably on the center portion above the radio).

The in-tank pump would push fuel directly into the saddle tank through the saddle tank sending unit tube, which is now retired and capped off.

The stock dash switch on the heater/blower control unit would only be used for fuel level indication so I could see level in both tanks when needed.

[myrl883]

This seems like a good plan. Much simpler than making all of the otherwise necessary plumbing connections for a normal dual tank setup.

You'll just have to be careful of the overflow potential since the rear tank is larger than the front. This is (also) a good thing because you shouldn't ever run the rear tank dry and kill a fuel pump.

[FuzzFace2]

Ok on PC now
What do they do on the FI trucks with dual tanks from the factory?
I think the tank valve is switched by the fuel PSI IIRC and this also changes the gauge.
I just don't know what that PSI is but why not do that set up on the truck?

So you want to get everything for the rear tank like I think you said you got.
Tank, straps, the cross member just in front of the tank the straps go to.

As for the wiring. On my truck (81 F100) the dual tank wiring is all part of the main & frame harness.
My parts truck was also a 81 F100 so it was just swap harnesses and I was good to go.

Now I think you are using a newer dual tank harness in your truck so I don't know what you will run in to or have to make work.
I think to go through all the work to have a level reading on the rear tank and have that pump transfer gas to the from is a lot of work.

If me and I could not get both tanks to work like the factory did I would set up the rear 19 gal. as the main and the 16 gal. as the back up and transfer it to the rear.

On the fill tube you are going to fill through the wheel opening?
How much road dirt is going to be kicked up on it?
Did you get all that from the parts truck?
I like the clean factory look and why I molded the rear filler door into the fender.
If you have a 2nd door assy. you could have a shop or you mount it in your bed side.

I know not a lot of help here sorry.
If there is something I might be able to shed more light on ask and hope I have the answer.
Dave ----

[Tyler]

I appreciate all the thought and feedback, Dave.

I postponed the JY trip until I did more research.  No parts in hand just yet.

I might scrap the idea all together and just deal with having less gas and more stops...  I'm only going to drive the truck across country again once, LoL.

When I get back home to WA, it will be rare to take trips over 100 miles and the truck will not be driven much. It wouldn't be great to have all that fuel sitting there for prolonged periods of time.

I still like the idea of doing the "upgrade" though.

On Tue, Jan 5, 2021, 6:41 PM FuzzFace2 [via Bullnose Enthusiasts Forum] <[hidden email]> wrote:

Ok on PC now
What do they do on the FI trucks with dual tanks from the factory?
I think the tank valve is switched by the fuel PSI IIRC and this also changes the gauge.
I just don't know what that PSI is but why not do that set up on the truck?

So you want to get everything for the rear tank like I think you said you got.
Tank, straps, the cross member just in front of the tank the straps go to.

As for the wiring. On my truck (81 F100) the dual tank wiring is all part of the main & frame harness.
My parts truck was also a 81 F100 so it was just swap harnesses and I was good to go.

Now I think you are using a newer dual tank harness in your truck so I don't know what you will run in to or have to make work.
I think to go through all the work to have a level reading on the rear tank and have that pump transfer gas to the from is a lot of work.

If me and I could not get both tanks to work like the factory did I would set up the rear 19 gal. as the main and the 16 gal. as the back up and transfer it to the rear.

On the fill tube you are going to fill through the wheel opening?
How much road dirt is going to be kicked up on it?
Did you get all that from the parts truck?
I like the clean factory look and why I molded the rear filler door into the fender.
If you have a 2nd door assy. you could have a shop or you mount it in your bed side.

I know not a lot of help here sorry.
If there is something I might be able to shed more light on ask and hope I have the answer.
Dave ----

Dave G.
81 F100 flare side 300 six / AA OD / NP435 / 2.75 gear
http://cars.grantskingdom1.com/index.php/1980-Ford-F100?page=1
81 F100 style side 300 six/SROD parts truck -RIP
http://cars.grantskingdom1.com/index.php/1981-Ford-F100

---

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NAML

[Gary Lewis]

What about just going with a Bronco rear tank?  They came in 28 - 33 gallon versions, but there are lots of 38 gallon tanks available.  In fact, I have one that's new that I'm not going to use.  I'd sell it for half price but the shipping might make the whole thing more expensive than buying one from a vendor that has good shipping rates.

A Bronco tank will fit our trucks, but you lose your spare tire spot.

[Tyler]

Perhaps, I should focus on getting the truck running again before I undertake this adventure.  I'll keep the  Bronco tank in mind.  The smaller of those is the most attractive to me.

[Rusty_S85]

FuzzFace2 wrote

Ok on PC now
What do they do on the FI trucks with dual tanks from the factory?
I think the tank valve is switched by the fuel PSI IIRC and this also changes the gauge.
I just don't know what that PSI is but why not do that set up on the truck?

So you want to get everything for the rear tank like I think you said you got.
Tank, straps, the cross member just in front of the tank the straps go to.

As for the wiring. On my truck (81 F100) the dual tank wiring is all part of the main & frame harness.
My parts truck was also a 81 F100 so it was just swap harnesses and I was good to go.

Now I think you are using a newer dual tank harness in your truck so I don't know what you will run in to or have to make work.
I think to go through all the work to have a level reading on the rear tank and have that pump transfer gas to the from is a lot of work.

If me and I could not get both tanks to work like the factory did I would set up the rear 19 gal. as the main and the 16 gal. as the back up and transfer it to the rear.

On the fill tube you are going to fill through the wheel opening?
How much road dirt is going to be kicked up on it?
Did you get all that from the parts truck?
I like the clean factory look and why I molded the rear filler door into the fender.
If you have a 2nd door assy. you could have a shop or you mount it in your bed side.

I know not a lot of help here sorry.
If there is something I might be able to shed more light on ask and hope I have the answer.
Dave ----

That was a one of the many reasons why I decided to not attempt a rear tank on my truck.  I didnt like the idea of mounting another door on the fiberglass fender it would be too hard to revert back to OE if for some reason I decided to restore back to factory new condition some day.  I also didnt like the idea of having a filler through the wood in the bed.  I figured my choices were to stick with the OE 16 gallon saddle tank and mount a jerry can carrier on a roll bar if I install one or while the tank is at the local truck shop to have the top cut open so a baffle could be welded in take and inquire with them about chopping the forward nose of the tank off and adding about 3" worth of metal to length the tank forward.  This can be done on mine as Im not 4x4 and 3" of length added based off the heigth should be good for a easy 5 gallons of fuel.  In theory I should see a 21 gallon saddle tank that would retain the short wheelbase mounting pattern.  I just dont know if this would stress the mounting straps doing this like wise I dont know if the extra 5 gallons would be enough to offset the truck and induce a lean in the truck with a full tank.

So for me personally I am still unsure what I will do till I am at that point of fuel injection install.

As far as the OE setup goes, you have a lift pump in each tank with your high pressure pump on the frame rail.  The way I understand it is the switching valve switches feed and return from each low pressure lift pump for saddle tank or rear tank which feeds a reservior canister that acts as a fuel bowl on a carb allowing the high pressure fuel pump on the frame to always have fuel even when cornering.  This is why installing a high pressure pump in the tank of a 85-86 truck like I will be doing will not be perfect as there is no baffle to prevent fuel slosh from resulting in your pump picking up air when you get around 1/4 tank.  This is why I personally will be having my new tank cut open to have a baffle welded in to prevent this from happening.  I could run the same reservior as the OE systems used but I dont like the thought of mounting a fuel pump on the frame rail.  They are noisy, they get hot, and they dont last long.  Only benefit of them is they are easy to replace when they fail vs the in tank pump that requires the tank to be dropped.

[Tyler]

I definitely understand the baffle concern when cornering.  I ended up putting a piece of Holley Hydramat in my tank, attached to the retro-fit in-tank pump.  The matting I purchased was not nearly as large as I wanted but there was a $450 price difference between the piece I bought and the one that "should" have gone in my tank to prevent fuel starvation down to the last drop.  Also, I would have needed to create my own adapter at the pump suction connection to make the larger piece work and I didn't feel like I could do that given the way the suction of the Holley pump is designed.  The tech rep also couldn't help me much, and these guys have the best customer service that I've experienced on the phone.  It's like they care about what you are trying to do. Seriously.
The optimistic side of having the hydramat in there is that the fuel return line dumps right on top of it and as long as that material has some contact with fuel, it won't let your pump lose suction.  Apparently...we'll see, I guess.

If you end up going with a HP fuel pump on the rail and keep your original tank, or use a non Holley drop in fuel pump with a threaded fitting on the suction, I would seriously look into the Hydramat if you'll be doing any cornering or steep angles at less than 1/4 tank.  As for now, I'm going to continue to carry extra fuel with me and save this project for the future.  I do like the idea of getting rid of the saddle tank all together and just running a single, slightly larger rear tank for balance of weight purposes but that could just be the 22 years of submarines, LOL.

[FuzzFace2]

Rusty, another factory stock fender in storage would take care of the rear door if you wanted to go back to stock.
I also think once you have the larger rear tank with the smaller front one you will never go back to just 1 small tank.

Also if I under stand you right on the factory EFI dual tanks the tanks have a low PSI pump and that is what changes the what tanks is used and the gauge as it is done in that assy.

So why cant that be used and a high psi pump on the frame, again like factory, to run the after market EFI? Dose the factory high PSI not put out enough to run the after market EFI?
Dose the after market EFI have a frame pump to use then you could use the low PSI pumps.

You know what they say ... If there's a will there's a way


I would go for either the factory 19 gal rear tank or any of the larger and not use the 16 gal side tank.
If you have a flare side and need a rear tank door it has to be from a flare side as the style side door is different size and shape and will not fit the fender right.
I would just move the front door to the rear and the part you cut from the rear just glass it in to the hole up front.

If you have a style side it would be easy to come up with one from the junk yard and weld it in place on your bed.

Just my .02
Dave ----

[Rusty_S85]

Tyler wrote

I definitely understand the baffle concern when cornering.  I ended up putting a piece of Holley Hydramat in my tank, attached to the retro-fit in-tank pump.  The matting I purchased was not nearly as large as I wanted but there was a $450 price difference between the piece I bought and the one that "should" have gone in my tank to prevent fuel starvation down to the last drop.  Also, I would have needed to create my own adapter at the pump suction connection to make the larger piece work and I didn't feel like I could do that given the way the suction of the Holley pump is designed.  The tech rep also couldn't help me much, and these guys have the best customer service that I've experienced on the phone.  It's like they care about what you are trying to do. Seriously.
The optimistic side of having the hydramat in there is that the fuel return line dumps right on top of it and as long as that material has some contact with fuel, it won't let your pump lose suction.  Apparently...we'll see, I guess.

If you end up going with a HP fuel pump on the rail and keep your original tank, or use a non Holley drop in fuel pump with a threaded fitting on the suction, I would seriously look into the Hydramat if you'll be doing any cornering or steep angles at less than 1/4 tank.  As for now, I'm going to continue to carry extra fuel with me and save this project for the future.  I do like the idea of getting rid of the saddle tank all together and just running a single, slightly larger rear tank for balance of weight purposes but that could just be the 22 years of submarines, LOL.

On mine I will be using the 16-111 Hydramat which is a 15" x 3" mat with a 11mm fitting with a 6 pin locking position and I am looking at a GSS342BX Walbro Electric pump that is 67 GPH/255LPH which is recommended volume by Holley and it has a 11mm inlet for $90.

So my idea is to use the Hydramat in conjunction with a baffle to get the most and help prevent aeriation of fuel.

I actually have a parts list of everything I am looking at doing and for the EFI itself the following is what I either already have bought and got put up or is to be bought.

x1 : 5/16" to -6AN quick disconnect
x3 : 3/8" to -6AN quick disconnect
x1 : 3/8" to -6AN male quick disconnect
x6 : 3/8" to -6AN Vaporguard Hosebarb
x1 : 00-04 Corvette fuel filter with built in 50psi regulator
x1 : Holley throttle lever extentsion
x1 : '85-'86 16.5 gallon saddle tank
x1 : '85-'86 saddle tank sending unit
x1 : '85-'86 fuel pump connector
x1 : Walbro Electric fuel pump 67GPH/255LPH with 11mm inlet
x1 : 3"x15" Hydramat with 11mm outlet

One of the things I am seriously contemplating about changing is picking up a stainless steel OE fuel supply line from Inline Tube its for a long wheel base and its in 3/8" but could cut it down to fit my application then can AN flare the hardline at the engine to use an AN hose flex joint between engine and frame.  Then I can flare the hardline into a male quick disconnect to fit the fuel filter/regulator then run rubber lines from the filter to the sending unit.  This would change my parts list some and it would change the amount of work Id have to do but I feel would be worth it as I feel the hardline would not just look better but it would provide a longer period of time with no issues.

[Rusty_S85]

FuzzFace2 wrote

Rusty, another factory stock fender in storage would take care of the rear door if you wanted to go back to stock.
I also think once you have the larger rear tank with the smaller front one you will never go back to just 1 small tank.

Also if I under stand you right on the factory EFI dual tanks the tanks have a low PSI pump and that is what changes the what tanks is used and the gauge as it is done in that assy.

So why cant that be used and a high psi pump on the frame, again like factory, to run the after market EFI? Dose the factory high PSI not put out enough to run the after market EFI?
Dose the after market EFI have a frame pump to use then you could use the low PSI pumps.

You know what they say ... If there's a will there's a way


I would go for either the factory 19 gal rear tank or any of the larger and not use the 16 gal side tank.
If you have a flare side and need a rear tank door it has to be from a flare side as the style side door is different size and shape and will not fit the fender right.
I would just move the front door to the rear and the part you cut from the rear just glass it in to the hole up front.

If you have a style side it would be easy to come up with one from the junk yard and weld it in place on your bed.

Just my .02
Dave ----

If I could source a spare fender I would consider doing it that is for sure.

As far as swapping the in tank pumps from low pressure to high pressure pumps it can be done but I dont know about Ford but I know the GMC I did this on the switching valve was marked 50 PSI max which is the regulated pressure of the sniper system.  That GMC wired up properly would not switch tanks while the engine was running.  You have to cycle the key to get the valve to switch, i theorize it has to do with the fuel pressure on the switching valve that was locking it in place.  I feel this would be the same issue one would have using a Ford dual tank setup and simply swapping the in tank lift pumps out for high pressure pumps.  Holley does offer a frame mounted electric pump one could use with a OE factory lift pump dual tank setup but those pumps dont seem to last long, Ive seen lots of people complain about how noisy their holley pumps are even their in tank holley retrofit pumps then they complain about the pumps failing not long after.  Why I am avoiding using one of holleys pumps and looking at a walbro pump that I can get for less money.

If I could find another fender to fabricate a second door for a rear tank what I probably will do is put a large pump in the rear tank and have the outlet of that pump connect to the saddle tank and use my saddle tank as my feed tank for my sniper and use the rear as a bed mounted auxiliary tank, those use gravity to feed fuel into your main tank as you need it but in this case it would be a electric transfer pump you switch on when you need more fuel.

Mine is a flareside and I can find the fuel tank doors and pockets and all that, ive came across them before on ebay.  Just havent seen the fiberglass fenders before online nor locally.