trying a heavy dose of star tron plus a quart of iso alcohol and some fuel injector cleaner for a gas motor [that was cheap at big lots 1.10 a bottle] plus 2 gal of diesel fuel shake car to swill around the tank let sit and see in the morning how nasty it looks when drained
then put back the 12 gal drained before with star tron and the injector cleaner
and see how it runs

wish me luck