I had a 1973 450SE that did this. It would get so bad during the warmup cycle it looked like the car was on fire from the rear. It would foul #3 plug about once a month. I replaced valve stem seals on that cylinder, and the problem persisted for about another three days, then just went away. I drove another 50,000 miles before selling the car.

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1968 230S Automatic, Elfenbein
1975 O309D Executive Westfalia Camper Bus, Blau/ Weiss
1972 280SEL 4,5 Dunkelrot
1966 VW Type 34 "Grosser" Karmann-Ghia
1963 VW 1500 Variant Pearlweiss
1969 VW Variant Automatic, Perugruen
1971 VW Squareback Automatic, Clementine Orange
2001 E320 4Matic Wagon- Our belated welcome to the 21st century! Polar White
1973 280SEL 4,5 Sliding Roof "The Bomb", Dunkelblau.