da156mac,
There are a couple videos that show an impeller change on an RT27 & RT25SC that you can view. Google "Ranger Tug impeller change" and you should find them. Basically to gain access to the area remove 3 panels: 1 in front of the step, 1 on the side of the step and the last one under the step. If you have the optional cabin heater you will find its fuel pump attached to the panel under the step. One of the videos will show how to deal with the pump.
If your D3 engine needed the software update a letter was sent to the registered owner. I found out that I was not the registered owner for my engine when I received the letter forwarded from the previous boat owner. I had the Ranger Tug dealer where I purchased the boat contact VP and correct the ownership. Also when I went to the VP dealer to get the required SW update they told me it did not require any update. Apparently this SW update may not be well known by the VP dealers and may not be flagged in the VP database for your engine. I called VP Customer Relations in Norfolk, VA and they were able to verify that my engine did require the update and requested the VP dealer to perform the update. The VP Customer Relations person is Dana Hatton, phone 866-273-2539. Good luck!
By the way the SW update eliminated all smoke on startup.
Chimo,
I would assume that the mechanic that changed your impeller would have told you the tensioner needs replacing if there was a problem. It is not too hard to check by removing all the panels to gain access to the front of the engine. Then take a 17mm socket wrench and place it on the bolt that goes thru the center of the tensioner wheel/pulley. Rotate the wrench clockwise and the wheel/pulley will drop down releasing tension on the belt. Then release pressure on the wrench and the wheel/pulley will come up reapplying tension on the belt. Make sure the belt is still correctly on the raw water pump and crankshaft pulleys. If the wheel/pulley remains down your tensioner is not working properly and probably needs replacing. Apply CRC to the tensioner using the supplied tube to direct the fluid to the front, back and openings in the housing. It is probably not good to get CRC on the belt or pulleys, so be careful. Brian from bbmarine may provide some better advice, I am an amateur mechanic.