Re: 1973-1991 Dual fuel tank systems theory of operation
 

That's what I was thinking. I can easily just cross the return lines, but it bugs me not knowing why that's happening

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I went ahead and replaced the six port valve today, just to make sure it wasn't a problem with the valve itself. As I was connecting the six lines, I double checked the paperwork/diagram and the routing of the lines to make sure everything was in the correct spot.

Right now, both my tanks are fairly full. I filled them a few weeks ago, drove maybe 20 miles, and since then has been just idling in the driveway a total of maybe 20 minutes - so both the tanks are still pretty full. I don't have a fuel gauge hooked up just yet.

When running on one tank, if given long enough, the opposite tank starts spewing gas, as if the returns were backwards but they're not. If I switch tanks when the spewing begins, it will stop spewing after a second, and after awhile start spewing from the opposite tank

The strange part is that, before I swapped the valve today, the old valve and lines had been in place for almost three years with no problems, even with both tanks completely full. Nothing ha changed between now and then except for a complete rewire of the truck, selector switch included. Tanks, senders, straps, fuel lines and regulator were all replaced at the same time about three years ago.

Anyone have any ideas?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Do have a charcoal canister and is it connected to the vent line?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

No charcoal canister. The vents on the two tanks are teed together by the selector valve, with a single line running up to a breather under the hood. But no canister

Re: 1973-1991 Dual fuel tank systems theory of operation
 

The charcoal canister isn't a big deal to plumb and it isn't a performance hit.
It's only three connections.

1. a vacuum line from the valve to a ported vacuum source,
2. a tee to the PCV valve for the purge port on the canister
3. a connection to the vents on the tanks

Re: 1973-1991 Dual fuel tank systems theory of operation
 

How would a charcoal canister keep gas from spewing out of the tanks?

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I spent a bit of time after work today troubleshooting. I started the truck up running on the left tank, and removed the gas cap. After a few minutes, I can see the gas level rising up through the fill port, until eventually it starts spilling out the fill port on the left tank.

Once that happened, I removed the power wire from the selector switch that feeds the left tank, so only the right tank was hooked to the switch, and started up again. Same result

I disconnected the two vents where they tee together and plugged them off to rule out fuel somehow crossing from one tank to the other via the breather hose, and same result.

Finally, just for ****s and giggles, I swapped the return lines at the fuel selector valve, and same result.

I have no idea wtf is happening.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Look at post #146. It sure seems like your return lines are crossed. That seems like the only way you could get fuel transfer from one tank to the other…

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I swapped them last night just to try it out, with thr exact same results

Re: 1973-1991 Dual fuel tank systems theory of operation
 

I would completely disconnect one of the tanks feed and return line and then power up the other side. This should help you narrow it down.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Great info in here! It is nice to know I can ditch the earlier problematic switch with the simple 2-position switch from the newer trucks. I'm hoping to get my second tank operational sometime this year. I have a feeling I should pick up the switch and a new valve at minimum, then go from there. The entire 10 years I have owned the truck I am not even sure what is in the left tank if anything lol.

Re: 1973-1991 Dual fuel tank systems theory of operation
 

Did you try this?