Getting air in system at startup 85 300SD
 

Yes I have not tracked it down as to where I am getting air only on initial start up after vehicle sat for 8hrs. when I start cold fires right up no problem but tnen immediately starts stumbleing and will not run faster then idle for about 30-60 seconds then as I am feeding some steady throttle it will pick up and run fine the rest of the time showing no signs of fuel starvation or sucking air. This is a replacement motor in the vehicle and has run like this since day 1 about 9 months ago. Does seem to be a little better when nose of car is parked downhill always clears up within a min. Then runs fine rest of the time. In the research I have done symtom seems to point to pressure control check valve on fuel return line I think. I have read that there is a spring in the check valve that I think over time does not keep the proper tension on the check valve ball and socket. I read that the spring can be removed and stretched to achieve proper tension again. The attached picture shows the banjo bolt removed from the engine side fuel line from IP. I see a ball and spring in there but do not see any way to remove the spring and stretch it longer to put more tension on the ball. Am I barking up the wrong tree here should I look else where for the problem?

may not be your problem, but when I changed my rack damper bolt on the IP, the warm up vibration was eliminated

but

on cold starts the idle struggles a little bit and goes away after a few minutes.

You've got the sealed, non-adjustable type that was introduced in '85 or a bit earlier. I got the old style off a junk yard engine and stretched it per recommendations. I doubt that this is your problem but I'd scout around for a replacement anyway.

The check valve allowing fuel to drain back and air to enter to replace it. Is one of the functions of the two check valves in the lift pump. Lift pump is cheap to clean out and re kit anyways. Perhaps the last year this is still easy and cheap to do. As soon as you get that tight you might see some fuel leakage from some point. Or perhaps none depending where the air is getting in.