I had the same issue earlier this year and just changing the O-rings was not enough for me. The major source of air leaking in for me was the short fuel line from the fuel heater to the pre-filter. I'm not sure how air was getting past the new O-rings on the old one, but replacing entire thing worked like a charm.

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Scott C.

2006 E320 CDI (120k miles)
FOR SALE: 1998 E300 Turbo Diesel - Black w/Tan Leather - Euro delivery (236k miles)