5.9L. I called Cummins Service did PM on it. They looked up the engine number I gave them and said it is a 5.9L. Lol. Two printed brochures I have state two different things. 5.7 and 6.7. I am creating a Master Quick Reference Checklist with all the specs as I go along and make certain it matches this coach. Hey, thanks again for all your expertise!!! James