Even though you have had the tank professionally cleaned, if the fuel pick-up tube was not removed it may have a mesh filter fitted to the end, remove it, they can be a cause this, better to let the primary filter catch any particulates. Also check the breather/overflow line for blockage, mud daubers or other insects find them ideal places to live.

Finally, most on-engine mechanical fuel pumps on small diesels (engines originally used for small tractors with above engine fuel tanks and marinized for marine use) are only good at 'lifting fuel' over 10" or 12" or so, if your fuel tank is well below the engine I would recommend supplementing the on-engine mechanical pump with a low pressure electric pump.

 

It seems you have eliminated most everything.

After the engine quits after the long run and you pump for the 5 min run is that all you get over and over, just 5 min at a time? Does everything reset after the engine sets and cools over night.

I would take a close look at any rubber hose on the suction side of the lift pump for air leaks and loose fitting. Maybe a hose was removed from a fitting by cutting and left a small leak. Maybe a loose clamp.

I would be looking for something that might change position or size from the heat or viberation of an engine running for 4 hours.

 

Thank you both. I'll let you know what I find.

 

After cleaning my tank I replaced tyhe access panel by using a thin bead of silicone. The panel screwspicked up the silicone creating a tiny pin hole sized ball which b ecame lodged in the fuel petcock.Under heavy load the ball would get sucked up and block the fuel line causing the engine to return to idle thereby eliminating the problem . Of course the next time it was under load itwould happen again. It's a longshot but disassemble the fuel petcock and check it out

 

I wouldn't assume the problem is only on the delivery side even if you pumping makes a temporary difference. I would install a fuel pressure gauge. That will tell you if it is fuel delivery or in the injector pump. You can buy a cheap fuel pressure gauge for around twenty dollars.

 

Good point Careless....although I suppose I could just pull the output side line off the pump when it's happening and see what gives. Thank you for the beneficial outside look - too often you stare at the wall so close you can't tell there's a mural on it.