how much is the seller asking?

Whatever he asks drill him down at least a grand just because of the potential repair needed on the engine. Any good tech or smart mechanic would do a leak down and wet test with the reading on clyinder #3.

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1986 300SDL, 211K,Dealership serviced its whole life
1991 190E 2.6(120k)
1983 300D(300k)
1977 300D(211k)