Fuel system diagram? Setting up EFI return

[JoeliusZ28]

Anyone know where i can find a fuel system diagram/schematic of a 73-86 dual tank truck?

More specifically, I'm trying to plum the return line from an aftermarket EFI system. Wondering if its possible to route it into the return from the stock mechanical pump and have it toggle with the feed with the OEM dash switch.

Thanks!

 

[6872xtc]

J don't have any specifications on what size the return line should be, but I would try it.

I am assuming that you have an '81 and up, because the '79 that I had, (K30) didn't have a valve capable of controlling a return line. But I don't know if the 1/2 tons had return lines.

 

[imiceman44]

J don't have any specifications on what size the return line should be, but I would try it.

I am assuming that you have an '81 and up, because the '79 that I had, (K30) didn't have a valve capable of controlling a return line. But I don't know if the 1/2 tons had return lines.

I believe he has an 84 so yes he has return line on the valve

 

[JoeliusZ28]

Ok this is good news. Should i tee into that line or plug the mechanical return? I’m running at 40psi so there will probably be quite a bit more volume than that setup was designed for. I’ll have to look at the diameter of the hose.

 

[imiceman44]

Ok this is good news. Should i tee into that line or plug the mechanical return? I’m running at 40psi so there will probably be quite a bit more volume than that setup was designed for. I’ll have to look at the diameter of the hose.

The consensus has been that the valve can not handle that pressure so no one is using it, that I know of.
I probably would try it in a controlled environment and if it looks like it can take it then you are good.

 

[JoeliusZ28]

The consensus has been that the valve can not handle that pressure so no one is using it, that I know of.
I probably would try it in a controlled environment and if it looks like it can take it then you are good.

Well, i probably didnt state that right. The 40psi is kind of irrelevant since thats what the feed is regulated to but the return is unregulated. I just suspect there will be a larger volume of fuel going through it than what the stock mechanical pump would be generating, but maybe not since it is feeding the electric pump. The selector valve would not have 40psi on on either side since the feed only pressurizes after the mechanical pump.

The fluid dynamics of this is admittedly a little over my head, but my gut feel says this will work.

 

[imiceman44]

Well, i probably didnt state that right. The 40psi is kind of irrelevant since thats what the feed is regulated to but the return is unregulated. I just suspect there will be a larger volume of fuel going through it than what the stock mechanical pump would be generating, but maybe not since it is feeding the electric pump. The selector valve would not have 40psi on on either side since the feed only pressurizes after the mechanical pump.

The fluid dynamics of this is admittedly a little over my head, but my gut feel says this will work.

If you only use it for return you will be good

 

[Wes Harden]

Not sure I understand, You have an electric pump with system delivery pressure of 40 psi. This pressure is run through a mechanical pump also ? and you hope to use the return from the mechanical pump ?

 

[Blue85]

He has a factory dual tank setup, which has the selector valve and extra lines compared to a single-tank setup. Rather than start over with different lines and selector valve, he wants to re-use the stuff that's already there. The selector valve was not designed for EFI pressures, but the high pressure is only between the pump and the regulator (regulator is a restriction designed to create pressure). Pressure returning to the tank is lower pressure because the only restriction is the lines and valve.

Not sure where the planned location of the pump is, but it sounds like it will be upstream of the valve, between the valve and the engine. Pumps doing suction don't work as well as in-tank pumps, but I understand the hesitancy to put in two in-tank pumps. Will it be mounted lower than the tanks?

 

[JoeliusZ28]

I am using the Fitech FCC 2.0 setup, and have been for over a year now as an unfinished install. The stock mechanical pump feeds the FCC 2.0 which pressurizes the fuel to 40 psi to the injection unit where it dead-heads. Ideally id run my return from here but i think if i did that the mechanical pump would not keep up with the electric one. The fuel feed line between the FCC and the injection unit is the only line of the fuel system that is high pressure. I currently have a return line running from the FCC 2.0 back to the filler neck vents (manually capped to only go to the passenger side tank right now). I did that because it didn't even dawn on me that my mechanical pump had a return, all other mechanical pump vehicles ive worked on did not so i didn’t even look.

So what i am intending to do here is remove the return line i installed a year ago. Then, connect the FCC’s return line to the OEM return line. I’ll either have to tee into that line, or plug the mechanical return. Thats the only thing I’m undecided on.

i do know that the even though FCC return is not pressurized, it IS returning a substantial amount of fuel. At one point i had my return line plumbed to both tanks simultaneously. I was hoping that with the way the tanks were vented the fuel might find the correct tank on its own, or at least stop filling a tank once it was full. That design didnt work out. It would transfer the fuel in one tank to other in less than 10 minutes, and if one filled, it wouldnt stop and go to the other tank. Instead it would backfeed up the vent to the engine bay where im surprised it didn't start a fire on me the first time it happened. so yeah that experiment didn’t quite work out

 

[Blue85]

Is it set up like this?

http://forum.73-87chevytrucks.com/smforum/index.php?topic=9072.0

 

[JoeliusZ28]

@Blue85 the top diagram is the “factree” configuration i believe, yes.