Author Topic: Fuel Gauge on Dual Tanks Not Working or so I thought  (Read 308 times)

Offline Johnret

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Fuel Gauge on Dual Tanks Not Working or so I thought
« on: May 14, 2019, 05:40:14 AM »
I have a 1985 Chevy Scottsdale, dual tanks from factory (NL2 option). I have replaced the mechanical fuel pump with in tank electric fuel pumps for EFI conversion. Each fuel pump is being powered by it's separate relay, this provides +12v to each of the fuel pumps when the selector switch is used. ON he selector switch I have tapped the LT.Grn and Drk.Grn wires to provide the volts to energized the relays for the fuel pumps. This seems to be working correctly. The fuel gauge reads completely empty when the left tank is selected and over full when the right tank is selected. I have checked the grounds from the original fuel pickups to frame and the grounds check good. The selected switch is the constant on rocker type, not the depress fully momentary type. I have noticed that the engine stalls when switching tanks while running, but starts and runs on either tank if now switched after initial startup. Does anyone have a suggestion on the fuel gauge issue?   So, I looked in the left tank and it is empty, apparently the gauge was correct. I had originally put 10 gallons in each tank. The left tank is now empty, the gauge says the right tank is over full. I will try to look in right tank later. I am thinking that maybe the fuel from the left tank maybe returning to the right tank. :-[
« Last Edit: May 14, 2019, 08:19:14 AM by Johnret »

Online bd

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Re: Fuel Gauge on Dual Tanks Not Working or so I thought
« Reply #1 on: May 14, 2019, 08:18:34 PM »
I have a 1985 Chevy Scottsdale, dual tanks from factory (NL2 option). I have replaced the mechanical fuel pump with in tank electric fuel pumps for EFI conversion. Each fuel pump is being powered by it's separate relay, this provides +12v to each of the fuel pumps when the selector switch is used. ON the selector switch I have tapped the LT.Grn and Drk.Grn wires to provide the volts to energize the relays for the fuel pumps. This seems to be working correctly. The fuel gauge reads completely empty when the left tank is selected and overfull when the right tank is selected. I have checked the grounds from the original fuel pickups to frame and the grounds check good. The selector switch is the constant on rocker type, not the depress fully momentary type. I have noticed that the engine stalls when switching tanks while running, but starts and runs on either tank if not switched after initial startup. Does anyone have a suggestion on the fuel gauge issue?   So, I looked in the left tank and it is empty, apparently the gauge was correct. I had originally put 10 gallons in each tank. The left tank is now empty, the gauge says the right tank is overfull. I will try to look in right tank later. I am thinking that maybe the fuel from the left tank maybe returning to the right tank. :-[

Is the engine presently running EFI/TBI or carburetor?  Are you using a factory type fuel tank selector valve?  Three-port or six-port?  Did you incorporate a return line to the fuel tanks and does the return line switch between tanks synchronously with the fuel supply line?  How "overfull" does the instrument cluster gauge indicate... what is the clock position of the fuel gauge needle when the right tank is selected?

For the left tank to empty into the right tank when the left tank is selected, either the fuel lines are plumbed incorrectly or the tank selector valve is not effectively switching the fuel return line.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline 75gmck25

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Re: Fuel Gauge on Dual Tanks Not Working or so I thought
« Reply #2 on: May 15, 2019, 05:29:41 AM »
Check to see if they replaced a six port valve (very hard to find these days), with a 3 port valve (available from several sources), and then connected the return line to only one tank.  A six port valve switches both return (3 lines) and supply (3 lines), while a 3 port line only switches the supply line.

Bruce

Offline Johnret

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Re: Fuel Gauge on Dual Tanks Not Working or so I thought
« Reply #3 on: May 15, 2019, 10:19:32 AM »
I am running the Holley Sniper EFI system. I have the factory 6 port valve on the truck. The Valve is new. I did incorporate the supply and return lines for the system. I can I tell if the valve is correctly switching the return line? The gauge on the left tank indicates empty, the gauge on the right tank now indicates 3/4, but I have been running the engine. I have verified that both fuel pumps are working by removing the fuel pumps relay individually to ensure the correct pump was being selected. So I am presently running only on the right tank and when I get to half tank I am going to put half a tank in the left side and run it that way to see what happens. If their is a way to check the return line switching I would love to hear about it.. Thanks for the replies.

Online bd

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Re: Fuel Gauge on Dual Tanks Not Working or so I thought
« Reply #4 on: May 15, 2019, 03:29:35 PM »
Perform the following procedure with the vehicle parked outside of any structures.  Have a suitably rated fire extinguisher readily available and within easy reach.  Remove the return line connecting to each fuel tank.  Route each line into a dry, fuel safe container.  Select the right tank and cycle the ignition switch.  Fuel should exit only the right fuel return line into its container. 

Switch tanks and cycle the ignition switch again.  Fuel should exit only the left fuel return line into its container.  Repeat several times, switching tanks each time.  Fuel should exit only the return line for which the tank is selected.

The issue with the engine stumbling upon switching tanks may be due to fuel draining from the line that sits idle while the engine is supplied from the opposing tank.  Another possibility is time lag and momentary loss of fuel pressure while one pump shuts off and the other energizes.  You may need to install test gauges and monitor fuel pressures to determine the exact cause for an accurate diagnosis and solution.
Rich
It's difficult to know just how much you don't know, until you know it....
In other words... if people learn by making mistakes, by now I should know just about everything!!
87 R10 Silverado Fleetside 355 MPFI 700R4 3.42 Locker (aka Rusty, aka Mater)

Offline Johnret

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Re: Fuel Gauge on Dual Tanks Not Working or so I thought
« Reply #5 on: May 15, 2019, 07:01:01 PM »
Thanks Rick I will give it a shot tomorrow, hopefully I would blow nothing up...... :)

Offline 78BIG-TEN

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Re: Fuel Gauge on Dual Tanks Not Working or so I thought
« Reply #6 on: May 16, 2019, 06:19:16 PM »
I remember having trouble with those 6 port valves.It was always on the gauge side.
If memory serves those valves are motorized hence the depress fully function.Wiring diagrams
show the gauge part going thru the valve to the respective tank unit.