In our '95 355 ACMY, we have three tanks, starboard, port and aux. We understand how they work, each engine can pull/return from their respective tank or the aux tank, no issue there. Starboard side is working as it should however, we are having an issue with our port side.

When port fuel selector switch is set to supply and return to/from aux, we have noticed the port side does not actual supply from aux however does return fuel to aux.Essentially transferring port tank fuel to aux tank.

So question is... What could be preventing the port engine not to pull fuel from the aux tank? We have replaced the port supply switch(switch only) this season but didn't resolve the issue.

Fuel lines have been traced and appears to all be connected as should. We have found one line from the aux tank with a spark plug plugging it not entirely sure what this line is used for and why it was plugged in this fashion. All fuel filters replaced.

Any suggestions on what the issue could be?

Thank you,
Wifey