Urgent Diesel Engine Question

[Dockhead]

What would make a marine diesel engine, fresh water cooled, naturally aspirated, suddenly overheat when driven over 2,200 RPM?

At 2,200 and below it behaves perfectly normally. Cleaning out the heat exchanger didn't help.

Some ideas which you can laugh at if appropriate:

1. Sensor reading incorrectly? (But how can that be, if it reads normally at lower engine speeds -- why would it suddenly shoot up?)

2. Raw water flow impeded somehow? Impellers have been changed recently; strainer has been checked.

3. Heat exchanger shot? Engine is not that old, less than 3,000 hours, heat exchanger was recently cleaned (but I didn't see it done, so the job could have been botched).

4. Exhaust back pressure?

I was a pretty good gasoline engine mechanic at some point in my youth, but I am pretty ignorant about diesels (something I will be trying to change).

Your comments will be greatly appreciated.