Diesel911 when I rebuilt the pump I am pretty certain I replaced that o ring and the valves. I bought the rebuild kit from a Bosch dealer for around $20. Before rebuilding the pump I had air in the return line pretty much all the time unless I clamped off the return line. After rebuild there is still a few air bubbles in the return line at idle but no where near the same amount. It's not till you bring the rpm up does the return line seem to have a ton of air. The car will still run fine once you get the rpm up high enough. It is just slow and sluggish till that point which is usually around 15-20MPH.

I did just put a fuel line and jar of fuel to the lift pump to test if I was getting air from the tank and the air was still there.

I am willing to try anything. I got a quote for $200 to have the injection pump tested and $1200 to rebuild it. I am only seeing used ones with no known history other than worked when pulled for 5-600. Just feel like everything I have done it seems like the injection pump is the issue but then also keep reading that these VERY RARELY fail so not sure what to do next.

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1982 300D