Auxiliary tank gas-transfer puzzle 73 GMC

[MusicMan70]

Because of the known venting issues on the standard tank and slow-filling/burping at the gas pump, I figured I'd just use the Auxiliary until I remedy the problem on the standard tank(aux tank fills quite easliy as it should, unlike the factory unit).

The main/standard tank had about 1/4 tank of gas left in it per the gauge. I filled up the Aux tank and noted 20 gallons at the pump(I had never filled it up before). Flipped the toggle under the dash and it began to move and read full plus.

So I'm driving home from work next day and noticed my gas gauge needle had dropped to about an 1/8th...Hmnnn. I check my toggle to make sure I didn't flip it on accident; I hadn't. Once home, I flipped the toggle over to the Main/Standard tank and it read Full+..?! But how the....anyways

I shut off the truck and when I get out to inspect , I immediately smell gas. Look under to see gas dripping from the passenger-side area of truck. I go to remove the gas cap on the Pass/main tank side and it starts to spurt out. I remove the cap on the Aux/driver-side, thinking that may help relieve some pressure; nope still spurting main side.

I get a bucket and take the main tank cap off and it spurts and pours out gasoline. I siphoned out about 5 gallons and that cured my spurting-gas dripping issue.

1- I looked under the truck and the valve switch unit was not leaking
2- Aux gas cap says "VACUUM PRESSURE" Main cap says "Anti-surge" inside
and they both "whoosh" when I remove the caps(when they have some gas)


Q1-Is this Valve system designed to keep the main tank full or should they operate as separate fuel tank units?
Q2-Does this mean I need to replace the valve or caps or both?

Here are some pics and thanks in advance for any guidance

[lowesguy]

Wow strange ? never had this issue when I had 2 tanks,removed the aux tank 8 years ago along with all the piping and such,what a mess, I've never figured out why its such a pain to gas the factory installed tank,have to fill slow or it all comes out? must be some vent issue? emission crap or something?

[lowesguy]

Maybe that valve is bad,if its letting fuel flow from one tank to another,they should be separate

[hatzie]

Do you have a fuel return line?

[MusicMan70]

Thank you lowes and hatzie for your responses. Yes hatzie, I do have a return line from the pump going back to the main tank.

[chengny]

That (the return line) is the only way that fuel could be moved from one tank to the other.

The diaphram in the fuel pump is probably leaking. Cut and plug the return line and install a new gas pump when you get a chance. Get the kind with 2 ports instead of 3.

There is no real reason for that return line - or a 3 port pump. Don''t listen to the old "vapor lock" argument. And, as in your case, if the truck is equipped with dual tanks they can only cause problems - i.e. cross transfer issues.

There is only one return line and it is only led back to the main tank. So, if an internal leak develops in the mechanical fuel pump, any leak off can only go back to the main tank. There is no tank to tank equalizing line (as in a diesel).

If you are sucking on the auxiliary tank and fuel is going back via the return line into the main - well, it's only a matter of time until the main fills up and overflows.

[MusicMan70]

Thanks chegny. Well that makes sense now that you spelled it out for me! I will first try changing out the pump to a two port version and see what goes.