Yanmar fuel system puzzle

[skipperpete]

I’m trying to understand how a pair of 3GM Yanmars in a Cat can both run off one tank and not draw any fuel from the other when the only connection between the tanks is a balance hose. Each engine has its own fuel tank but there are no selection manifolds for supply or return, simply the one balance pipe between port and stbd tanks.
So here’s the problem. The owner refueled both tanks to max and did a 12 hour run under power. He stopped to top up both tanks but one was still full while the other was almost empty. Here are the details...
Each engine has a dedicated tank
The only link is the balance pipe between the bottom of the tanks
Both tanks have ½” vent lines.
This isn’t one of those things where I already know the answer and intend to release details one by one to tantalize the forum,.............. I already suspect that the vent on the full tank was blocked and that caused the engine to pull fuel through the balance hose (¼”) but I’m finding hard to accept this proposition especially since the tank usage is now back to normal. Has anyone had this happen before?

[glenn.225]

Could be that the fuel returns are plumbed to one tank, coupled with the small balance hose and they hadn't had time to equalize after the long run. Possibly also some restriction in the balance hose.

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[skipperpete]

Originally Posted by glenn.225

Could be that the fuel returns are plumbed to one tank, coupled with the small balance hose and they hadn't had time to equalize after the long run. Possibly also some restriction in the balance hose.

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I traced the returns and supply lines from the engines to the tanks, definitely no interconnection other than that balance hose. I also removed the balance hose and checked the flow from each tank outlet…. All good. I just re attached the vent hoses in a better way, no dips and just an upward rise.
If my original and only theory is correct, ( blocked vent hose), whats really amazing is that the tank is so well sealed on top and along the fill hose that no air at all was able to enter and allow the level to drop.

[skipperpete]

You’re right to suggest the return line possibility, some 3 GM’s actually do return fuel to the tank if they have the self priming option( quite rare) but most GM’s return no fuel at all and only have a leakoff line from the injectors…. If much fuel comes out of that pipe, the injectors are crook.