I've never cleaned a tank in-place, it's easy if you remove it from the car first. The method I used that worked well, was to put some paint stripper into the tank, then a large handful of lag bolts. Cap/cork all the openings, then shake the tank vigorously, all around, from all angles. It's a real upper body workout! Then shake it around some more. Drain and repeat until it drains clean. Rinse and dry thoroughly. Then put some POR15 or special tank sealer product inside, and shake until inside is thoroughly coated. Let the excess drain, and let the product dry for a few days. Tank is cleaned, sealed, and ready for a few more decades of service.

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1998 E300 turbodiesel

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