logdog (Forum Supporter)
logdog (Forum Supporter) GRM+ Memberand UberDork
1/22/24 4:55 p.m.

I feel like I must be overlooking something on this thought exercise of doing dual tanks on a diesel van.

Currently has a side mount tank.  If a rear tank is added, and  assuming sumps are installed on both tanks to help pressure to the fuel pump, is there a reason they could not feed into a y block feeding the pump?  No need for switching valves and what not? 

I see the need for some sort of check valve on the smaller tank to keep from sucking air. 

What obvious thing am I overlooking?

wspohn UltraDork
1/22/24 5:15 p.m.

Many older Jags had twin pumps and tanks with a selector switch so you could switch to the full one when the other went empty.  Just stick ao one way check valve in each line to prevent feed back.

jharry3 GRM+ Memberand Dork
1/22/24 5:57 p.m.

Without a 3-way valve I'm thinking both tanks, depending upon elevation, will basically be drained at the same rate and the first tank emptied will be allowing air to be sucked into the engine's fuel line through the Y block.   

Maybe a check valve in the lines could prevent reverse flow provided its rated for blocking the reverse flow of air.  Both tank outlet lines upstream of a Y-block would need check valves because the higher tank will probably slowly gravity feed the lower tank at low fuel consumption times.

My dad had his pickup truck set  up with two tanks and a three way valve.    It worked pretty well. The valve handle was in the floor just to the left of the driver's seat and I could operate while driving.


Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
1/22/24 7:36 p.m.

At the pressures you'll need, I'm not sure how you could get a check valve that would actually prevent sucking air, and diesels really don't like air.  When you say "sumps are installed on both tanks to help pressure to the fuel pump," do you mean adding an in-tank pump, or just a low point to source fuel?

Refresh my memory on what the van is.  Dmax or Powerstroke would be lots different from 6.5LTD or 6.9 IDI

I think if you're planning a second tank, set up a pump in the rear tank that feeds the front tank.  When you get low on the front tank, flip a switch to pump more fuel forward to it.  But the problem with a y-block (without its own pump in each tank) will effectively let the tanks drain into each other.  If you park facing downhill or drive up a long mountain road, all of the fuel will drain into the other tank potentially uncovering the other tank's sump and sucking air.  You really need a way to isolate the two tanks of liquid.

If you put a pump in each tank,  you're relying on exactly equal pressure and flow from both pumps or one will drain faster than the other, or worse, it could overheat the weaker pump.  Maybe not, just trying to predict.  Even then, once one tank dries up, you're done without a way to isolate the tanks.

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
1/22/24 7:41 p.m.
wspohn said:

Many older Jags had twin pumps and tanks with a selector switch so you could switch to the full one when the other went empty.  Just stick ao one way check valve in each line to prevent feed back.

Agreed.  If you have a check valve that requires fuel to only flow in one direction, then all you need is DPST switch.  One position sends power to the rear tank's pump, the other sends juice to the front tank pump.

Again, though, depends on the diesel.  CP4 pumps are incredibly sensitive to low fuel lift pressure.  Others like a P7100 aren't as sensitive.

03Panther PowerDork
1/22/24 7:46 p.m.

The part being missed is the return line. 
Between a few class 8 trucks, and one MD (FL 50) with saddle tanks, both tanks draw at the same time through a y. With the return line through a y to each tank as well. 
But ILO of a sump, each tank has a straw installed from the top. With this set up, only one tank has a fuel level sending unit in it, and both tanks empty at the same time. 
I used an aftermarket dual fuel switch (I'll look up part number later) to split the tanks on the FL50 to be able to switch between tanks. Quite inexpensive at the time, and easy to set up, with all new lines. 

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
1/22/24 7:47 p.m.

grover GRM+ Memberand Dork
1/22/24 10:23 p.m.

Steal the ford dual tank switch system off of an old obs? 

03Panther PowerDork
1/23/24 12:20 a.m.

In reply to grover :

I've replaced one with stock for stock. Poor setup. The aftermarket one (I'll look up a part number eventually) is more like the late '70s early '80s gm setup. Probably cheaper than junkyard, and better setup. and new. 

03Panther PowerDork
1/23/24 12:34 a.m.

In reply to Curtis73 (Forum Supporter) :

Same size tanks, just add the return line. Y'd of course. I see possible bad things in store, with the different size tanks. 

Pete. (l33t FS)
Pete. (l33t FS) GRM+ Memberand MegaDork
1/23/24 5:48 a.m.

The fuel pumps should have one way valves in them, so you give each tank a pump and switch which one is being powered.

All you need to switch on the liquid side is where return fuel, if any, goes.

WillG80 GRM+ Memberand HalfDork
1/23/24 8:41 a.m.

There will never be a 50/50 split from the tanks through the Y valve. More fuel will be pulled from the tank with less resistance in the line. No matter what you do you'll probably never get them equal. Small variables like going up or down a hill, pressure differences in the tanks etc. will all play a role. That's why valves will probably be needed to direct fuel.

The easiest way to overcome this is to have a fuel pump in each tank with a switch for each on the dash. 

What about adding a second tank and simply connecting it to the first tank with a small electric transfer pump? When you you start getting low on fuel, flip the switch to pump the diesel across to the primary tank. 

Ian F (Forum Supporter)
Ian F (Forum Supporter) MegaDork
1/23/24 9:04 a.m.

Years ago I had a 1990 E150 with middle and rear tanks. A switch under the dash switched between them which also switched the level sender feed to the gas gauge.  My understanding is the switch toggled power between each of the in-tank fuel pumps.  The engine was EFI, so I'm not sure how it dealt with the return line.   

After some Google-fu... it turns out the selector valve is readily available (reviews of this particular version are mixed) and it switches both the supply and return lines at the same time.  I would say there's no reason to reinvent the wheel here. Just Google the Ford piping and wiring diagrams and copy that. 03panther's idea would probably work as well, but Ford definitely did dual tank diesels, so the info is out there to duplicate. 

03Panther PowerDork
1/23/24 9:05 a.m.

Dual tank switch

quick search pulled that up at summit. 
The detail oriented (most all of us, here) will notice it has two pairs of nipples - a supply and return, for tank one, and a supply and return , for tank two. And on the outlet side, yep, you guessed it, a supply, and a return for the engine. 
The Y setups do work well from the factory on MD (and larger) trucks. But I WAY prefer the switched. 
As GRM, we all like to reinvent the wheel, but, in this case, totally un-necessary. I'm sure the ac delco part is available from china with a weird sounding name, from same ass. line, for less, to the advanced shopper

03Panther PowerDork
1/23/24 9:12 a.m.

The last time I bought a kit (for way less) circa 2013 or so, it had the plug, wire and switch for dash mount. If ya find the right one, the tiny electric motor switches the valve, and level sending units, to run only one gauge. I ran two gauges, so I did not have to switch, to view aux tank level. 

Be careful not to buy the "two inlet, one outlet type by mistake!

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
1/23/24 11:59 a.m.

If logdog is on the same quest as I was, the factory stuff sucks.  I had a 95 Powerstroke F250 with dual tanks.  Everything about that truck was bulletproof... except the tank switching stuff.  It's not a hard job, but I got tired after doing it three times in 100k miles.  Nothing more frustrating being in the middle of nowhere, draining the main tank, hit the switch, and you run out of fuel because the solenoid didn't solenoid itself.

I think the two best possible solutions are either pumps in each tank with check valves (although I'm a little afraid because check valves aren't perfect, nor are they foolproof) to a Y block, or pumps in both tanks with one tank simply re-filling the other.  That seems to be the least tech and the least failure point to get this happening.  If it were me, I would make the big tank the main, and refill from the smaller tank.  Fuel gauges are intentionally slow to react so they don't bounce with slosh.  If you drain 28 gallons out of a 30 gallon tank and try to blindly refill it on the fly from a 40 gallon, it would be hard to tell when it was full.  But if you take 38 gallons out of a 40 gallon tank and refill it from a 30 gallon tank, no worries.

logdog (Forum Supporter)
logdog (Forum Supporter) GRM+ Memberand UberDork
1/23/24 1:32 p.m.

Lots of good stuff so far!  The vehicle is a 7.3 Powerstroke E350

The thing that appeals to me about a setup without switching valves is the simplicity (in theory) that it would act sort of like a 70gallon tank with no input needed by the driver and no moving parts.  The system 03 Panther described on class 8 and the FL50 sounds like what I am imagining, with the exception of my tanks being different volume and all the associated problems that would cause.

I guess I am looking to reinvent the wheel a bit when its unnecessary.  Im sure one of the dual tank switches posted would be an easier solution in the long run.  Gonna have to go down that rabbit hole!

03Panther PowerDork
1/23/24 1:46 p.m.

In reply to Curtis73 (Forum Supporter) :

Replaced one on my 95 444 (7.3, in liters)  250. Not impressed. Although it never failed again in another 150k (of random fuel sources ) like you say, not a good part. 
The aftermarket (gm style) I've used have been better. Two different gm 1 ton truck chassis's had failed original ones in them as bought by me. Minor mods to put the aftermarket in (Almost the same) and no farther problems. And the mod to the FL 50 done me good, as well  


No Time
No Time UltraDork
1/23/24 2:19 p.m.

My experience with the GM dual tank switching was that it worked and I never had to do anything to it over almost 300k miles. 

One key benefit of the implementation of a switch and valve is that you can run one tank down to 1/4, switch to the second tank and if you have water in the fuel or something else goes bad on the second tank, you still have a 1/4 tank of good fuel To return to after draining the fuel filter. 

Just something to consider, especially if you expect to be traveling and getting fuel from stations you don't deal with regularly. 

MiniDave HalfDork
1/23/24 2:35 p.m.

A friend did this type of setup on his VW bus, his spare tank being quite a bit larger than the stock tank. He simply has a transfer pump in the big tank that he activates when the stock tank shows low, fills it back up from the big tank and carries on down the road. Sort of a portable gas station! 

Karacticus GRM+ Memberand SuperDork
1/23/24 2:58 p.m.

If going one tank to another, you could also get fancy and add a float switch to the main tank to turn the reserve tank pump on, though I'd recommend an Off/On/Auto switch.

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