trants4md, my parts source shows that a 1985 500SEL should use Bosch W8DC sparkplugs. I'd verify this with the Owner's Manual and remove the Bosch Platinum sparkplugs ASAP. Older MB engines don't run well with the Bosch Platinum plugs ... stick with the sparkplugs specified for your engine. Call a MB dealer's Parts Department if you don't have an Owner's Manual. Your problem could be caused by a variety of things ... faulty oxygen sensor, bad OVP relay, etc. Rather than throw parts at the problem, after you install the correct sparkplugs I'd have either a dealer or an independent MB-versed tech diagnose the problem. An experienced tech should be able to sort out the cause quickly and inexpensively.

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Fred Hoelzle