I have noticed when I turn my engine off, there is always an air bubble that forms in one of my fuel lines. It may be the return line but i'm not sure. Is it normal to have an air bubble in there?

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2006 E320CDI- current dd
1999 E300TD - retired
Wilmington, NC USA

You are prob referring to the short line leading form the fuel thermo (in the head) to the prefilter. It is normal for that line to drain back at shutdown.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)