Mark...I may be able to help. I am currently outfitting a new Tayana 48, and have been in contact with a number of other t-48 owners across the country. Several of them have reported that their 4jh2-dte's have been overheating
(they have the 88 hp turbo version...is yours the same?). Anyway, I have held on to all their e-mails, and can forward them to you off-line. I'm sure they would be more than willing to help you through this...as I recall
, one of them just made some modifications to the heat-exchanger which seemed to help tremendously.
Anyway, hit me off-line if you're interested in reading their travails...