two-Tank fuel return question - PeachParts Mercedes-Benz Forum

psaboic · #1 07-24-2013, 02:34 PM

Hi folks,

We are in the process of installing a two-tank system into my brother's 84 300D, and I have run across a question regarding fuel return. He wants to have the set up such that the fuel returns to the respective tank it came from with no looping. (i.e when running on Diesel the fuel returns to the diesel tank and when running on WVO it returns to the WVO tank)

We are using two 3-port fuel switching valves and have the fuel supply all hooked up and set to go, but the issue is with the fuel return. How can we hook it up so that it sends fuel back to the specific tank? The way I understand it, these valves basically take two different supply inputs and switch it to allow one or the other input to feed the output.

However with the fuel return, we need one input that is switched between two different outputs (the exact opposite of the fuel supply hookup). So, since we are taking the fuel return from out of the cigar hose we have the one input, but how do we hook it up to the valve so we can switch the returned fuel to the respective tank? Is it simply a matter of hooking the hoses up differently to the valve, or???

I know this must have been encountered by many others before, so HELP!!!

Thanks,

Glenn

Jimec3 · #2 07-24-2013, 04:02 PM

Think of the return valve as a reverse of your supply valve. It takes one input and sends it to two outputs...same valve just backwards. Or am I misunderstanding your question

psaboic · #3 07-24-2013, 04:40 PM

Jimec3,

Sounds like you understand my question. The thing I am wondering about is how I would hook up the valve on the return side. Normally (on the supply side), the valve is hooked up so one output is fed by two inputs (depending on position of the valve).

My question is, can a three port valve be hooked up so you have one input and two outputs (where the outputs are controlled by the position of the switch)? And if so, how do you hook it up.

Maybe I am over-thinking this as I am assuming the valves only allow fluid flow in one direction (fluid in through the input port and out through the output ports). If the valves allow flow in any direction, then hooking it up would just be the reverse of how the supply lines are hooked up to the valve.

Phillytwotank · #4 07-24-2013, 04:47 PM

you'll need a third valve

psaboic · #5 07-24-2013, 07:47 PM

How would that work/be hooked up using a third valve?

psaboic · #6 07-24-2013, 08:24 PM

Ok, let me start over a bit just to make sure I am asking this the right way. We are hooking up a two tank system and want the fuel returns to each go back to the respective tank (D2 return to D2 tank and WVO return to WVO tank)

To supply the lift pump/IP, we are using a three port valve hooked up so the two different fuel types come into the valve, then depending on which way the valve is switched determines which fuel goes into the lift pump/IP.

My question is can I hook up the other 3 port valve in such a way that instead of two fuel inputs going to a single output, I can have a single input (return fuel line from the cigar hose) go to two outputs (return fuel to WVO tank or Diesel tank depending on position of valve)?

I would think this could be done simply by hooking up the 3-port valve for the return fuel opposite from the way the supply 3-port valve is hooked up, unless the 3-port valves only allow fuel flow in one direction.

So, in a nutshell, can I get my desired results by hooking up the return 3-port valve opposite of the way I have the supply 3-port hooked up?

If not, why?

Thank you,

Glenn

Phillytwotank · #7 07-24-2013, 08:30 PM

Forget my answer from before. New answer... Sounds totally doable to me.

psaboic · #8 07-24-2013, 08:43 PM

Philly,

Thanks, I kind of figured it would work as I did not think the 3-port valves were designed to allow flow in only one direction.

BTW the answer to my question got to be your 500th post (I feel a bit honored) <grin>

Anyone else have any experiences or advice to add?

GregMN · #9 07-24-2013, 08:59 PM

When running on diesel: return the fuel to the diesel tank
If you loop the diesel return back to the input stream it will get hot and will not lubricate the IP as much

When running on VO: loop the fuel back to the input
If you return it to the tank, you are running it through the filter many, many, times and at a unnecessarily high flow rate. It can get oxygenated which can lead to polymerization. VO should make one pass from the tank to the engine compartment.

If you are using 3 port valves, you need a 3rd valve.

Valve #1: selects the fuel source
2 inputs from diesel filter or from VO filter
1 output to lift pump to IP

Valve #2: selects the return fuel destination
1 input from return fuel line from valve #3
2 outputs to diesel tank or VO tank

Valve #3: selects whether to return the fuel to a tank or to loop the fuel back to the fuel input stream
1 input from return fuel from IP and injectors
2 outputs to valve #2 or to "T" into the fuel stream before the lift pump

Read this, it might help: 1991 350 SDL Conversion to WVO

psaboic · #10 07-24-2013, 10:14 PM

Good read on the conversion

Since we do not live in MN and the coldest it usually gets in the winter around here is low 30's give or take, my brother doesn't want to loop the fuel return line. His friend tried it that way and ran into a ton of issues with trapped air in the system that he could not purge. Hence why he wants to just return the fuel to their respective tanks.

That said, it sounds like we could get away with just two 3-port valves.

Valve 1 is the supply valve taking 2 inputs: (from diesel tank or from WVO tank) and going to 1 output to lift pump/IP

Valve 2 is for the return fuel lines. It would have to be hooked up backwards from the way the supply valve is hooked up. One input (return fuel line from the cigar hose downstream of the spin on fuel filter) and 2 outputs to either the diesel tank or the VO tank.
One

Jimec3 · #11 07-25-2013, 10:15 AM

Maybe start without the WVO loop to ensure you have everything running but add the third valve later.

psaboic · #12 07-25-2013, 11:42 AM

Sounds like a good idea. I'll push for that when we finish the install...

moon161 · #13 07-25-2013, 12:06 PM

There's a banjo bolt on top of the fuel filter you want to replace with a plain bolt so you don't cross contaminate your diesel circuit and filter.

grease lightnig · #14 07-25-2013, 12:14 PM

Fuel Input: diesel goes to the Normally Open Position (NO) the Veg connects to the (NC) Normally Closed Position. The Output going to the IP goes to the Common Side

Fuel Return: The common side connects to the cigar hose that comes from the engine return. The (NO) Normally open side goes to the diesel tank. The (NC) Normally Closed side goes to the Veg Tank.

That is easy enough. But also you need to have two switches or one 3 position electric switch to flush the system so veg does not go into the diesel tank.

To do this with no power to the switch the diesel will go in and return goes to the diesel tank when you flip the switch on then the veg goes into the engine and the return goes to the veg tank. When you flush the Fuel in switch is off and diesel goes into the engine and the return switch is on and goes to the veg tank for a short period of time 15-30 seconds on average. Then both switches are turned off.

When you buy a switch get a 3 position with momentary hold. This means Middle Position is off. Up is on and Down has to be held down for it to work. That way it is really hard to flush all of the diesel into the veg tank. You are only flushing as long as you are holding the switch.

moon161 · #15 07-25-2013, 05:41 PM

Momentary hold is a good idea. Now and then I have pumped 1/2 tank of diesel into the grease.