On Sunday, July 13, 2014 7:59 AM, m/v MOJO <mvmojo@gmail.com> wrote: I ran into a similar problem 5-6 yrs ago on my 6.5BTA.  I bought a new impeller to replace one in the raw water pump.  Next time I took the boat out I began overheating and I noticed there wasn’t as much water coming out of the raw water exhaust.  I took apart the entire raw water cooling system looking for an obstruction but couldn’t find anything.  In talking to an old timer Cummins mechanic, he said the design of the raw water pump had changed a number of years ago and it required a “deeper” impeller.  He said this is a common problem but for some strange reason not that many mechanics are aware of it, probably because many people change their own impellers.  By deeper, I mean the distance from the front to the back of the impeller.  The diameter and number of blades is the same.  Apparently I bought one of the shallower impellers by mistake.  It fit the pump housing fine, but unknown to me it left about ½” of space between the back of the impeller and the back of the pump housing.  This significantly reduced the pumping ability.  I bought a deeper impeller and the problem was solved.    Ray m/v MOJO