Have an early d2 3j. When we purchased the machine it needed the pony tuned up and of course starter pinion work. Fixed everything and the pony runs and starts the diesel great. While warming up prior to starting the diesel it will create about 15 pounds of oil pressure while cranking. Main engine starts and oil pressure is usually around 25-30 cold with the engine slightly higher than idle and roughly 20 at idle. Now the problem. After about 5-10 minutes run time it will have ZERO at idle and I would say maybe 5 at full throttle. New oil filters, 15-40 rotella and a quart of power punch oil additive. I know the d3400 engine has lower than normal operating oil pressure but not zero. I've changed the oil pressure guage, bypassed the aux. oil cooler and turned the thumb wheel on the oil pump all the way in and still no change. The previous owner had the same oil pressure problems and he claims to have changed the main and rod bearings with obviously no success. I can see some loss in oil pressure on a engine that is at full operating temp but this thing doesn't even get the oil warm! We do have a brand new oil pump that I can swap but not sure if it will make a difference. The thing runs great and no extra noises. Any ideas on the pressure loss? Thanks any help would be greatly appreciated.

Just because the bearings have been changed doesn't mean clearances are right.
I'd try running straight 40wt to see if that makes any difference before tearing into it.

remember 15w-40 is the thickness of 15 when 15 is cold and the thickness of 40 when 40 is hot.
but that is weird.
as OM has suggested i would try mono-grade oil.
maybe the previous owner did a half-arsed job and didn't use plasti-guage(its a fine plastic strand that was devoloped for easy checking of bearing clearances on car engines by the repco company for diy use it allowed checking without special measuring tools to use you placed a piece between the journal and bearing shell and tightened the bolts to spec, undo the bolts remove the piece of gauge and compare it to the table on the back of packet if the reading is within manufacturer spec reassemble the bearing assembly and tighten to spec repeat with all journals if not the clearance needs to be remidied) and bearing blue to check the bearings on assembly(looking for even contact), ordered the wrong shells or didn't get the crank ground maybe all three.

normally a engine gains oil pressure with rpm. and yes they lose a bit when hot. can you check the pressure relief valve it may be faulty.

That is one heck of a swing on oil pressure from cold to warm. I don't have a manual for that engine but is it possible someone tried to convert the oil filter from bypass to full flow and mucked it up? My bet is that something is going on with the relief valve somewhere in the system. With no oil pressure you are in danger of spinning a bearing and or flattening the crank. A new oil pump cannot hurt but usually oil pumps last "forever" as they have an abundance of lube. On the 3J is there a relief valve in the oil pump?

When you learn to provide tractor s/n I'll look into it.

Thank you for the responses. All good ideas. Yes it has a major swing in pressure! Like mentioned low oil pressure doesn't bother me but NO oil pressure does. Its so strange like the pump quits pumping oil. Probably explains why the oil doesn't get warm? It has the field add on kit I believe. 2 filters, one stacked on top of the other. When I bypassed the oil cooler all I did was plug the holes in the filter housing and remove the spring and valve. As for the relief valve on the pump I think its just the thumb wheel thing? Maybe I can remover it and build some shims. 3j568 is the number on engine.

The full flow filter option was not available until 3J3445-up. Relief valves opening just results in non-filtered oil, has no affect on overall pressure in either the by-pass or full flow system. Is the spring behind the thumb knob intact and not rusted away? Still seems like excessive bearing clearances to me and rear main leakage kind of confirms that.