I have a John deere rider F915 with the same engine Yanmar 3TN66 and I also have problem with overheating. I changed waterpump, remove termostat, changes radioator, toppgasket, all radiatortubes, fan, and some other things. Problem is still there. I guess there are som constructionproblem with that engine. Later I changed to an engine from a JD 430 (yanmar 3TN72) and the problem was away! I think that the water channels inside the engineblock get stuckt from debries somehow? From what I hear Yanmar and John Deere company was never happy about this engine.
