Last week i did run a closed system - connected the IP supply and return lines to a bottle of fuel and had the same issue. Idle and starting are fine just lack of power from the foamy air/fuel mix.

I have since rebuilt a fuel pump and will likely try replacing soon.

I did have a cracked line from IP rerun line which i replaced. Since it's under pressure, i don't believe air would enter there.?

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1985 300D, 250k
1980 300SD, 180K