1973-1991 Dual fuel tank systems theory of operation

[Dleslie212]

Originally Posted by hatzie

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

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

[Dleslie212]

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.

[Hart_Rod]

Originally Posted by Dleslie212

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. That seems like the only way you could get fuel transfer from one tank to the other.

I have no idea wtf is happening.

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…

[Dleslie212]

Originally Posted by Hart_Rod

Look at post #146. It sure seems like your return lines are crossed.

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