OMC tank leak. Chasing a ghost?

[Spartans]

Hi all. Happy 4th. Here’s a zinger for ya all.

Warning- this is a long drawn out thread so go get a beer and sit down.

Anyhoo, got two older OMC tanks (80’s- single line - btw) that leak from the fuel outlet when the cap is shut tight. Won’t do it when cap is turned to the first stop (where the cap rattles but won’t come off).

I rebuilt the pickup unit on each tank - so that’s new o rings for both pins including the little bushing on the front of the one pin that has the brass seat. Replaced sending unit to tank gasket with NOS gasket. Pins move freely and does not leak at tank connection when I attach a hose and transfer fuel.

I also replaced outer cap gasket and the cap closes tight with resistance- (note if people don’t know there are two rubber gaskets sandwiched between a piece of metal in those caps.

So - if I am wrong here great - I need to be educated but I always thought these tanks are sealed until the connector is attached which allows fuel to flow because the little pins let air to enter into the tank.

My issue is that these two tanks dribble fuel from the outlet with the cap closed tight which leads me to believe it’s sucking air from somewhere.

Now that I have bored you with the background I have two questions:

1. What I am missing? Could it be the sending unit gasket that is leaking? Is it the cap?

2. These tanks are supposed to be sealed until the connector is attached. I am thinking that if it’s getting air in to allow fuel to flow then I should be able reverse the process to apply some amount of positive pressure to the tank and look for leaks - similar to a BBQ. I am thinking of applying 1-2 psi of pressure into the tank and look for leaks with soapy water.
Does anyone see an issue with this approach? I am at my whits end here.

Added phot of cap and sending unit for clarity

Any advice is appreciated. TIA

 

[OldOMC31]

Just went through the same thing myself. Bought a few sets of orings to replace on multiple leaking tanks.

Your valve retainer, brass washer on the bottom photo, looks distorted like a funnel or cone. This washer is staked in place, and the aluminum needs to be moved with a punch prior to removing this washer. Then once you replace the oring behind it, (OMC #0332488) the washer needs to be set back in then aluminum re-staked to hold it in place.

Think you need to replace the brass washer and oring again. Figure on buying a few core plugs and orings for the back side release valves as well as youll need to drill out at least the core plug on the back side of the brass washer to get a new oring in here.

If the cap on the disconnected fuel tank is tight, and the tank heats up with daily fluctuations in temperature, or leaving it in the sunlight, the tank will leak as it becomes pressurized, doesnt need to let air in to leak as you suggest. The other pin lets in air, once fuel line hose is connected. This is how your tank equalizes the vaccuum inside your tank when your fuel pump is pulling fuel out of the tank. If your orings/retainer valves are not sealing, your tank will leak when there is fuel in it, disconnected from hose, and the cap is shut.

 

[Spartans]

Thanks for the information about how the tank works. I was told my an old OMC tech that if the tank is holding any amount of pressure (only up until the relief pressure which he thought was in the range of 25-30 psi if I remember correctly), the pressure relief valve is working. I originally thought that was the issue but clearly is wasn't. I also was not clear on the tank's operation.

I went back through and removed the valves, o rings and the front o ring and brass washer. Your are correct in your observations - it looks like I was overzealous with the punch and yes it was deformed. I replaced with new.

Turns out that the o rings that the valves seat against were not set deep enough and were not bearing on the shoulder inside of the pickup unit. So, in essence, the valves were passing the o rings and not sealing when the springs were reassembled in the channels and the welsh plugs installed.

I replaced all the o rings and the brass washer taking care to only stake the brass washer at the little indentations on the pick up unit and only hard enough to seat the washer. I also used a smaller punch.

In the end, the repair worked and the tanks function great. It was clear that I was not attentive enough to review the positioning of the valve o rings.

When I did it the second time I used one of those lighted magnifying desk lamp to see down the passages and ensure the o rings were seating properly.

Thanks