New discovery on fuel issue

[Ron Sebastian]

I am posting in case someone else has this issue.
1. Truck runs when switching tanks left and right. Gauge reads past full but not pegged at 3 O'clock. (unknown amount of fuel in either tank).
2. When the right tank electrical connector is disconnected at the fuel pump (ground maintained), fuel gauge pegs past 3 O'clock and will not start.
3. When the tank is switched to the left tank, truck runs but the gauge is still pegged past 3 O'clock.
4. Reconnect the right tank electrical and the truck starts and runs as normal. Gauge goes back to just over full.
5. Disconnect the left tank electrical connection, (maintaining ground). Gauge stays at the just past full mark and the truck runs when switched right to left tank as normal.
6. At this point I have no idea the amount of fuel in either tank (over 10 gal each), nor do I know which tank fuel is being drawn from, but the truck runs smooth like velvet. Don't know where to turn, at wits end. Getting ready to throw in the towel and bring to a shop and pray.
7. I have replaced, the fuel sending unit on the left twice, brand new units, fuel switching valve new x 3, new relay and new switch in the cab.

 

[Jcoronado09]

Did you ever get this fixed??? I have a similar issue and was wondering if your fix could fix mine. Or I'm just gonna have to replace left sending unit.

My truck will start with Rh tank. I would flip to Lh and the truck would die. Turn the truck off and put the switch back to Rh I can hear the selector valve clunk. Also I can hear the Rh sending unit priming the fuel line. Then the truck would start. So I turned the truck off and switched to Lh. Heard the clunk from valve but no sending unit priming the fuel line. Truck would try to start but just cranks.

My gas gauge also reads full. I have disassembled my cluster cleaned my circuit connections, tested the gauge. That all seems fine besides light bulbs needs replaced. I also have unplugged the Lh tank. Turned the key forward and the gas gauge would go past full to 3 o clock position. Turn the truck off, plugged Lh back in, turned the key. The gauge goes back to full. If I unplug Rh the gauge stays at full.

I have checked my grounds from the tanks. They look solid. I have found in a different post to clean and tighten the head light grounds. Looked over the bus grounds under the dash. They looked good. Ground on on the E brake looks good.

Pretty much stuck from there. Only thing I can assume is that my Lh sending unit is no good and it's throwing everything els out of wack... one thing I don't understand is if I disconnect the Lh and the fuel gauge is supposed to read the Rh right? But it goes to 3 position.

 

[Ron Sebastian]

My fuel pump/sending unit was going bad, replaced all that. The pump was a high pressure model that blew out my selector valve and flooded my charcoal canister. That to some time to figure out Also believe it or not the ground going to the selector switch in the cab was bad, ran a whole new ground wire to the back of the switch. After all the removing and replacing, I accidently flipped the switch so it was reading left when it was sucking from the right. That and the bad switching valve was creating a very difficult to diagnose issue. All good now. New switch, valve and pump/sending unit (the correct ones).

Check to see that the connector/wires from the left tanking sending unit are good going to the valve.
Check the connector/wire on the rail from the selector valve to the gauge are good.
Check the ground on your selector switch in the cab.
Check all connector and all the wires going into the selector valve.
If all that is good, you might have a bad valve.

 

[JohnRod]

my gas gauge problem...filled the tank [goes to 3:00-as normal], drove 110 miles. next time I drove, the gauge was less than 1/4, which is way unnormal. I drove around corners, weaved, hit bumps. nothing would change. no massive leaks and locking cap, so...reading here, tank switch showed up.
Ah, he said...I just sweep the carpet...was it possible I hit my tank switch [custom drive side tank is empty]. yep, turned the key 'on', flipped the tank switch from L to R and wamo, gas back to 3/4. just about right.
so not to make you guys with real problems feel bad. but you did solve my problem.
thanks. Rod in AZ