Thinking Of Plumbing My Two Tanks Together

[Poorman]

Thanks for the input! The issue of overflowing fuel while on a steep slope is something I hadn't considered. So I leveled the truck and took a bunch of measurements and then made those into a drawing on CAD where I could rotate the image and see what would happen on different slopes, I got a lot of info, I'll try to simplify it so that it's mostly relevant.
If the tanks were tied together as I propose: With 100% completely filled tanks (not the fill necks) the truck would have to be going up at an angle of 4 Degrees or a grade of 8.4% (think road sign warning of steep slope) before the fuel would reach the bottom of the rear fuel cap, or have the front tires raised 11.85 inches.
If the fuel level was down 1" or about 9.5% of the fuel gone then the truck would have to be going up at an angle of 6.5 degrees or a grade of 13.67% or have the front wheels 19.5 inches off the ground before the the fuel level reached the bottom of the rear fuel cap.
Interesting is that with the bone stock setup and no mods with the truck going up a slope of 8.57 degrees or 18.4% grade the fuel will reach the bottom of the FRONT fuel cap, this is with a full tank.
So I guess it could be a problem in an extreme scenario with full tanks, the problem does dissipate quickly as the fuel level goes down. The fuel caps appear to only let air in and seal fuel from coming out, is there another vent I can't find??, so a scenario like coming up a short steep embankment wouldn't be a problem, I wouldn't want to park on a steep slope for a long time. An easy solution would be to install a valve into the cross over line so you could have an option of stopping any fuel transfer if needed, or just clamp it.
It's good info to have but doesn't really concern me to much because I always have a camper on which means I'm always trying to stay level. I know the degree/ slope differences will be confusing to some, think of it this way, 360degrees in a circle, level is 0 and 90 is vertical. Grade is percentage of slope, fall vs Run, this is what you see on road signs, if you travel 100 feet and you drop 10 feet in elevation then you just traveled on a 10% grade (or 5.71 degrees)!
Also there isn't really any problem with going downhill it's only uphill because the front tank is long and the filler is towards the back.

 

[ISPKI]

What size are your two tanks? I believe there are a ton of variations in tank sizes, filler neck lengths, etc over the years. In my truck in my sig, the rear tank is ~22 gallons, I can fit almost 24 gallons in it filling up the neck somewhat. Also has long necks to reach the sides of a wide flatbed. My front tank is only 16 gallons however, plus a shorter neck. I think I have fit 17 gallons in it. Not sure if you took tank differential sizes into consideration. If the tanks are linked and of similar height, they should normalize their volumes. That may not matter in your application, but with mine, my front tank would over flow sitting on flat ground as the rear tank would transfer ~2 gallons into it - more than it can hold even filling up the neck. All subjective to the application though, my tanks might sit differently than those in another truck, especially a different model, plus changes in holding straps - mine are not factory and my rear tank sits higher than OEM as I made a pocket under the flatbed for the extra tank height to exist.

I think the more standard tanks were 16 and 19? Those were the sizes on my 94 F250 anyways. If you filled both tanks, the forward tank might receive ~1 gallon from the rear tank, depending on alot of variables. Just something to make sure you take into consideration in your calculations.

 

[Poorman]

Thanks ISPKI, This may sound almost stupid on my part but I really don't have any idea how big my rear tank is, when I bought the truck from my cousin 35 years ago he told me that it was really small, and after 50 miles tops it has the needle buried below the E mark, so I've just always treated it like it was a 5 gallon tank, it's completely hidden behind the spare so I've really never even seen it other than the edges. I'm thinking the sending unit may be adjusted all wrong and I just never questioned it. From what I'm reading it could be three times the size I've always thought, so I'm excited to get it out and have a look. I'm headed out to the shop right now to jack it up on blocks and start pulling both tanks. And also it appears the tops of the tanks are the same height. A lot of people are skeptical about this mod but I like it. Have you seen those aftermarket sumps? watched a video where they drilled a 3 inch hole in the bottom of the tank and attached the sump! Two screws! It's a real pretty unit but?? I might be a bit of a hacker but I would never do that!

 

[Old Goat]

In the center of the rear tank is the roll over valve, and a small rubber hose
attached onto a plastic nip ple that is attached high above, and with a plastic
thing in the end to keep dirt etc... out


Front tank is similar.

Goat

 

[Rdnck84_03]

For the standard F-series trucks 80 - 96 ( non C&C ) the rear tanks were 19 gal and long bed front is 19 gal, short bed front is 16.

I am unsure if ford ever offered any optional larger tanks for these, if they did I have never seen them ( for those that may not have been around for the "personal salvage yard" thread ) I have and have had a lot of these bodystyle trucks.

James

 

[franklin2]

For the standard F-series trucks 80 - 96 ( non C&C ) the rear tanks were 19 gal and long bed front is 19 gal, short bed front is 16.

I am unsure if ford ever offered any optional larger tanks for these, if they did I have never seen them ( for those that may not have been around for the "personal salvage yard" thread ) I have and have had a lot of these bodystyle trucks.

James

I believe you are correct on those sizes for pickups, and the cab and chassis tanks are different weird sizes and capacities.

 

[franklin2]

I think you might want the equalizer line larger also. Say you are filling the back tank first. As you are filling the back tank, it's going to "leak" to the front tank through the 5/16 line. Once the rear tank is full, you stop and move to the front tank, all the while the back tank is still leaking to the front tank. So how full would the rear tank actually be? I guess you could fill the front tank and then go back and top off the rear tank?

If you put a 1 inch line in there, could you just fill from the rear and the 1 inch line would be fast enough to fill both tanks through the rear filler?

 

[u2slow]

Bronco has a ~30gal rear tank. Also no room for spare tire.

I have a plastic rear C&C tank. Don't think it's more than 20gal. It's not pancake shaped. More like a squared-off keg.

I have wondered what it would take to squeeze a plastic dodge tank into the ford. They sit in the same spot, similar width frame.... but more than a 10gal improvement. Maybe have to move the driver side shock. I guess the 99+ superduty is similar size.

 

[Poorman]

Well I just finished dropping the rear tank, I started by taking off the trailer hitch framework because as others have said it hangs to the inside of the frame and the tank flange won't slip past easily. I also used a hole saw and put a 3" hole directly through the bed over the hose clamp that holds the filler neck to the tank, it would be impossible to access without the hole. Then used a floor jack , 3 pallets and some 2x6 to lower it down. I just cut the two fuel lines and the elect. wires because I don't plan on re-using them and there is no way in heck to disconnect the lines working from the bottom. So anyone that is dropping the tank to replace the sending unit take my advise and remove the bed and access from the top! I'm supper excited to find out that the tank is much larger than I ever thought! The gauge showed about a third left in the tank, I put a siphon hose in and drained 14 gallons out! and there's probably a gallon still sloshing around on the bottom. So it probably is 19 gallon, I'll have it completely dry so I'll measure accurately when Putting fuel back in so I'll know exactly. I'm thinking that because the tank is shaped like a pancake, it's not very deep so probably hard to adjust the float to be accurate. I'll try to get some pics if anyone is interested.

 

[franklin2]

All fuel tanks are setup with a reserve. If you use 16 gallons out of that 19 gallon tank, you are considered empty.

 

[Poorman]

n the center of the rear tank is the roll over valve, and a small rubber hose
attached onto a plastic nip ple that is attached high above, and with a plastic
thing in the end to keep dirt etc... out

You're right Goat, got the front tank out and there is one there also, except unlike the rear, the rubber tube lays across the top and then runs down to the bottom of the frame where it attaches. On the rear it attaches at the top of the frame, it was aged out, the end was split and the plastic valve thingy just fell out, it was so mudded up that it couldn't be functioning. Question, can someone tell me why or what the vent tube is supposed to do? the plastic end on the hose looks like it was designed to be a plug that leaks. My guess is that it is supposed to let air in as the fuel goes down but doesn't the cap do that also??Mine does, I can put a pipe up against the rubber gasket on the bottom of the cap and then I can suck through it but can't blow through it. Seems like the rubber tube vent would easily leak fuel out if over filled, seams redundant, I'm thinking I may remove it, tell me why not??

 

[franklin2]

All fuel tanks need a vent. When the weather warms up during the day the fuel will expand as it warms up and needs a place to go. You are correct, the fuel cap lets air in, but it will not let air out. That is what the small vent hose is for. The little plastic thing on the end of the hose is to keep mud daubers and other insects from plugging the end of the line.

The EPA hadn't cracked down yet on the diesels, so they just vented the tank to the atmosphere. The gas trucks have a network of pipes on these vent lines and they go up to the charcoal canister and are not vented to the atmosphere.

The white plastic thing in the top of the tank that the small vent plugs into is called a "roll-over valve". It has a metal ball in it. The metal ball normally sits on the bottom of this fitting, and the air goes in and out as it pleases. But if you are in a accident and the truck turns over, or I suppose if you overfill the tank, the fluid will push the ball up and it seals the vent off to keep fuel from leaking out the vent system. Not sure about the over filling part, but it would definitely keep the fluid from coming out if the truck was upside down.

The best modification you can make to your truck is to take the roll-over valve assembly out, and put a plumbing fitting in the rubber grommet and put a large vent hose on it. It really helps filling at the fuel station pump.

 

[Poorman]

The best modification you can make to your truck is to take the roll-over valve assembly out, and put a plumbing fitting in the rubber grommet and put a large vent hose on it. It really helps filling at the fuel station pump.

Thanks for clarifying how that valve works, The valve looks old and I really doubt it even works, they used cheap rubber hose that was split on the end, so I like the idea of just taking it out and replacing with a good hose, I think I'll try to run it up along the filler neck and punch it through so it's visible next to the filler cap, I want it up high and protected from mud, stock it was neither. Yes it will flow fuel when I'm upside down but I doubt I'll notice.

 

[Mulochico]

I did this over 10 years ago and have had very few issues.
1) I put shutoff valves on both tanks to be able to isolate the tanks if there is a problem.
2) I initially pulled off the front and returned to the rear. This can be an issue if you need to disconnect one of the tanks. I ended up pull and return to the front as it is lower at the bottom of the tank.
3) I plugged the supply and return on the rear tank near a brass f/f connection for supply and return so If I need to remove the front tank all I need to do is swap over to the lines for the rear. (a couple of hose clamps)
4) I combined the breathers ( I did the filler mod a long time ago) so that they evacuate the air evenly when fueling.
5) The front tank is usually around 1/2-3/4 when the rear shows empty (I left the gauge switch connected for S. and G)
6) The crossover line is as big of a diesel rated line as I could get (Forgot the exact size) This allows for enough transfer and I have not had any problems. I fuel just like stock as the tanks fill faster than gravity can transfer.
7) I put the line on the rear tank on the bottom of the drivers side front corner to allow as much room and drain as possible. The front tank feeds into the side of the tank a little lower than the drain on the rear tank.
7) I ended up using all brass fittings cleaning the tanks as well as I could of fuel residue and JB welding (don't hate, it hasn't leaked in over 10 years) the fittings to the tank while screwing them together (the fittings were a screw together clamping the side of the fuel tank between with the JB weld as a seal).
8) Again, no leaks except where the fitting on the front tank loosened up (the valve screws into the the threaded clamp fitting on the tank then the nipple threads into the valve. I don't have any pictures and it might be a few days, but if wanted I could get some.

This has been the cheapest and , for me, the best option. No buying a new tank, setting up senders etc. I too am a 4 decade long trucker and this was my inspiration. It does work, no issues if you take the time to clean the connections at the tank for the nipples, etc.

 

[Poorman]

Thanks for that post MULOCHICO, it gives me good confidence that this is a viable way to eliminate the FSV and 1 sensing unit (I'm tossing my rear), and a hand full of connections. The OEM FSV was built super good but after 39 years I know it's getting close to death and I'm pretty sure a new one won't last 39 years! Funny that you had a few problems with the fittings used to tap into the tank, I wasted lots of time running around town looking at every hardware and hydraulic store I could trying to find the right fitting but I was not happy with any of them so I spent the afternoon making my own from scratch. I decided to forego any shutoff valves (more to fail) and also am going with a very small line between the tanks, actually the line will be 1/2 inch but the ports in the fittings is only 1/4 inch, in my opinion smaller is just easier to seal and there's plenty of time for the fuel to transfer between tanks. I'm trying to fashion a bulbulus screen around the intake to prevent any debris from clogging it, that was my only concern with the smaller size.