Be aware that the primer pump often leaks and is a very common source of air ingress. Poke around this forum for more info, or I'd imagine someone more familiar with the 61x series engines will chime in soon with what to look for.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)