Replaced the Mann filter with a new one (all that I have in stock - auto parts is a 40 minute drive each way). Also replaced both o-rings on the filter cover stem. Pressure ok at start-up but down to 2.5 by the time it warmed up to 50c. Sharp rapping noise at front end (up top - assuming valve train), worsening as pressure drops; also idling rough, appears to be missing on one of the front two cylinders. Think I will have a bite of lunch then pull valve cover.

Any more ideas greatly appreciated!

Thanks again,
Jim

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2005 C240 4matic wagon (daily driver)
87 190D - 225K (on loan)
85 190D - 312K (on loan)
2011 Subaru Legacy AWD (Wife's)