TBI Conversion fuel pressure

[skyhigh4by]

My truck is an 82 with dual tanks originally carbed but now has an 87 tbi with holley TB and walbro pump.

I can't get my fuel pressure below 15 PSI. I figured out today that the problem is the return line. I took it off and ran return straight in to a jerry can with 3/8" hose and I was able to adjust pressure low enough that engine wouldn't even run (2-4psi)

So I crawled underneath and started to look at the steel return line and the selector valve(which doesn't work) and realized that the stock steel line is TINY (1/4"???) Then I decided to look up the part #s for the selector valves and from 82-87 the aftermarket uses the same # but GM part # differs. Is there a difference between the 82 valve and the 87 valve? Do I need one for an 87? Also is that tiny return line whats causing my problems? Too small to return the fuel my pump is putting out. The pump is 43 psi 155 L/hr and is mounted on the inner wheel well.

I want to change out my selector valve anyways because my fuel gauge isn't working and it would be nice to have use of both tanks.

 

[dyeager535]

Really can't tell you what GM did on carbed returns, but pretty sure all the early FI stuff was 3/8 feed and 5/16" return. The 1/4" might work on the TBI pumps that put out 13PSI on a good day, the volume being returned from the 43PSI pump is a very likely issue. What happens if you run a hose from the tank side return on the valve into a bucket? Can you adjust it down then?

Those valves are an utter mess. You might try searching on here, I think the differences have been explained before, but with all the EVAP and return lines they tend to get complicated.

 

[skyhigh4by]

Really can't tell you what GM did on carbed returns, but pretty sure all the early FI stuff was 3/8 feed and 5/16" return. The 1/4" might work on the TBI pumps that put out 13PSI on a good day, the volume being returned from the 43PSI pump is a very likely issue. What happens if you run a hose from the tank side return on the valve into a bucket? Can you adjust it down then?

Those valves are an utter mess. You might try searching on here, I think the differences have been explained before, but with all the EVAP and return lines they tend to get complicated.

Lots of volume returning back with the pump Im using. It doesn't take long to fill a 5 gallon jerry can.

I didn't try checking the return after the valve but maybe I will tomorrow just to eliminate the possibility.

Im actually thinking now that since one of my tanks need to be replaced anyways that Im just gonna bypass the selector valve altogether and run new lines.

The only thing I don't know how to figure out is what wires to splice to get the fuel gauge hooked up. I want to use the driver side tank. What wires to I need to splice together at the selector valve?

 

[dyeager535]

Just need to figure out which is the sending unit wire (it's only one) and which is the pump 12V wire.

Wish I had a bunch of money to experiment, but my theory is that if you ran a non-vented cap on the primary tank, a vented cap on the secondary tank, and ran a line from the secondary tank pickup (fuel outlet) line to a fitting on the primary tank that didn't have a tube on it (dumped at the "top" of the tank) you could eliminate the switching valve and the secondary tank fuel pump. You'd be able to forget about the fuel gauge switching either, the tanks would draw down at the same rate so the fuel gauge would be accurate.

It sounds complex, but in reality it's just a siphon from one tank to the other that never needs to be started manually, and the line from one tank to the other is the only fabricated piece needing made. Perhaps other than figuring out how to dump that line into the top of the tank.

Removing the valve any other way starts to cause issues with fuel return.

Just my idea. I hate that dual tank setup, especially with EFI.

 

[skyhigh4by]

Well since its an 82 it never had a pump inside the tanks I should only need to find the one wire for the sending unit. What about ground though, will I need to find the ground for that tank for the sending unit to work?

I like that siphon idea but I think that off camber situations and sloshing could likely disrupt the siphon effect??

I have dual 20G tanks so Im sure one will be enough for anywhere Im going.

I think Ill just take the rotted tank right out and everything that goes with it all the way up to and including the selector valve.

 

[dyeager535]

If going with one tank, yep, no reason to keep it.

Ground for the sending unit/tank is just a wire from the sending unit to the frame or structural member.