sludge

note: 2000 ford ranger 2.5l 2wd mt, 275,000mi, i received the truck at 120,000mi .... i have gone quite a few oil changes to over 5k miles and/or over 10mo Hey, I am wondering if it is a bad idea to place Marvel Mystery Oil in my high mileage engine? i have never "de-sludged" my engine before...

Just bought an '86 Bronco II 2.9, automatic. It starts but runs really weak. Lots of leakage at valve covers and the one spark plug I pulled was jet black with powdery carbon deposits. There does not appear to be any evidence of water in the oil, but it is over the fill line and smells like...