6.9 will not rev or idle right until return line is plugged

Howdy fellas, I have my Dad's 1986 Ford F-250HD XL 2WD 6.9L/C-6.
It has pretty bad air intrusion (Surges idle, dies, won't rev) until I hook another line to the return output and put my thumb over it and put pressure on the system. I replaced the line that goes from the lift pump to the frame, and right now I have a clear line that goes between the hard line to the lift pump and the hard line to the filter head, not getting very many bubbles anymore, if anything on occassional itty bitty teeny tiny little bubble. And after I plug the line, if I unplug it and put it in the diesel can or if I put the OEM line back on it will slightly surge at idle but it will still rev, but will die or try to die when put in gear. Really getting discouraged on these IDIs and loving my own 460 EFIs instead lol.

Can you post a top down pic of your engine?

Like this viewpoint:

If your fuel system isn’t totally dialed in its going to be a disaster and the biggest part of getting one of these going is the fuel system.

hacked89 said:

Can you post a top down pic of your engine?

Like this viewpoint:

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The return lines between the injectors are newer also.

I would start diagnosing problems with a good shop vac.

Are any of your return caps leaking?
After you had the system "open" by changing the line, did you bleed the air off?
You can temp replace the two return lines I circled with the clear line you already have to help troubleshooting and then bleed the system.

If you dont know how to bleed it let me know.

If it runs fine when the return is plugged... Maybe the relief valve on the injection pump is stuck open?

Did you replace the "olives" on all the hard lines?? Looks like they are leaking fuel. Also, get a new cowl gasket before you get water in the engine thru the intake (even with the filter assy installed). I placed an old rubber mat over mine since it leaked again with a new gasket after heavy rain, parked on a slight hill & bent 2 rods from Hydrolock. Early 80 years are bad about it.