I would think that the exhaust flap thing is the biggest problem here; when closed, on the lower revs, the exhausts get trapped behind it and if the engine doesnt get heated or revved enough these extra trapped fumes eventually cool off there, condensating. Meaning u get a lot of water build up there. Next time u rev enough for the flap to open u get that nice sludge spray painted on the bumper.
Golf Tee mod it is
