Can't imagine soap is going to be good for it and you have noway of knowing how much, if any, got in there.
Probably best for the tank to be pulled and flushed at a radiator shop. You'll probably need to pull the upper filler portion too and flush it out after the tank is pulled.
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Terry Allison
N. Calif. & Boca Chica, Panama
09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)
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