It may just need to be ridden hard and put away wet. My SDL ran like poo when I first started driving it. It's progressed to the point now that it fires up with the flick of a key, makes good power, and runs well cold or hot. Take it out and ride it HARD. It's about the only way to burn the carbon and soot out of the prechambers.

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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!)