Running a fuel loop eases strain on IP and lift pump by providing warm, easy to flow oil. It also makes warm up and shutdown switchovers shorter, because you can start on a blend in your loop instead of totally purging wvo.

Apparently the 6 port solenoids are more reliable than the 3 ports though... an added benifet of running return lines.

Sorry, no advice about the bubbles.

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Cassidy
1982 MB 300DT - Running Great @ 104K!

1972 MB 220D - RIP @ ~200K (Dropped prechamber)
1992 MB 190E 2.6 - SOLD @ 145K