I have been chasing air bubbles in my return line from the IP for quite some time. It's not much, but it's there for sure, and if I rev the engine, the amount of bubbles increase. I am running a 2 tank WVO system where the fuel filters have been re-plumbed so they are under vacuum instead of pressure. Bubbles are only while on diesel, not VO, likely because of the higher viscosity of VO. There are no bubbles in the supply line to the diesel filter. I have clear lines everywhere so I can see exactly where the bubbles are. I have pressure and vaccum tested the diesel and VO filters, all the lines. the switching valves and lastly the IP itself and they do not leak per the tests. So the only item remaining is the lift pump, which I have not tested yet since it's a little more involved.

I am thinking of by-passing the lift pump and gravity feeding the IP with a gallon jug of diesel. If it runs without bubbles then I know the air is coming from the lift pump. Will the IP run with gravity feed? I can alway rig up an electric fuel pump if some pressure is required.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked