Mysterious diesel fuel leaks, multiple tanks at same time

After filling all six tanks (200 gals each) we left for a day-long cruise in what were boisterous conditions. We returned in calm conditions about 15 engine hours total and using perhaps 100 gals. On return I found diesel fuel slowly leaking from both sets of three tanks, seemingly from the top area of the three tanks, on each side. The vents appear clear, and no signs of fuel having ever come out. These tanks are 5250 aluminum replacements for "black iron" tanks about 20 years ago; all hoses and fittings replaced at the time.) The slow leaks continue 3 weeks after our return (while the engines are off, at rest--all caught in absorbent pads.) Thinking the tanks somehow got pressurized I opened all 4 fill caps without seeing any change in the leaks.

I can't access the tanks without removing lots of paneling surrounding them , making it impossible to see exactly where the fuel is coming from.

I haven't changed my supply/return valves since they were installed. Does anyone have any thoughts about what's causing the leaks, immediately after filling, before I remove all the paneling and visually check what I can't see?

If it started after you filled them up, maybe the leaks are on the top where the dip tubes or gauge penetrate the tank? Can you hang paper towels around the tanks and then see where the paper towels get wet?

Might be an engine that returns a lot of fuel like a DD.

The fuel will be warmed by passing thru the engine multiple times , expanding it, testing seals..

A top leak is likely, if hard to repair , just fill the tanks 3/4 full and forgetaboutit!

Hawkeye said:

...I can't access the tanks without removing lots of paneling surrounding them , making it impossible to see exactly where the fuel is coming from...

Click to expand...

Do you have a borescope? They’re pretty handy for looking into areas that aren’t readily accessible. All it takes is a 0.33” hole. If you don’t and decide to get one I’d spend the extra money for better quality. We have $15 and $65 borescope. The picture quality is significantly better on the $65 one. This isn’t ours but similar. Good luck!

https://www.amazon.com/Industrial-Y...?keywords=HD+borescope&qid=1572954706&sr=8-52