Yes I did consider that too. If it has a nice even build up on the inside it could act like insulation and block the heat transfer. Guess if the outside is spotlessly clean and belts are good and the right fan is on engine next step would be a good internal cleaning. Believe the other thread had suggestions on ways to do that. Any chance a D4D would have a reversible fan? Had a machine many years back that was over heating and found one of say six blades was facing the wrong way.