Our boat has two 5.7 liter Mercruiser engines in her. While the engine block has regular antifreeze in it, raw water (lake water) has to be pumped into a heat exchanger to cool the antifreeze as there is no radiator on these engines. This raw water is moved to the exchanger by the impellers. While we never noticed any engine cooling issues we did notice while winterizing the starboard engine that the antifreeze (now in place of the raw water) as not being pushed through with the same velocity as it was in the port engine. We suspected it was time to replace the impellers.
Before launching the next spring we moved forward with the impeller replacement. Even though we felt we may only have an issue on the starboard engine it was decided to replace both the starboard and the port side impeller. That way we know exactly where we stand as far as having good impellers in our boat.
Doing the replacement one side at a time was key. I used a paint pen to mark the positions of the mounting brackets and bolts as an aid to help me return the housing back its proper position when reinstalling the impeller housing.
Using a picnic table as our work area we were able to slowly disassemble the old impeller housing. During the process we took multiple pictures to help us remember how to reassemble.
Our suspicions of a bad starboard side impeller were true. Once disassembled it was found that a piece of plastic had broken off inside the house causing the impeller to become damaged. Again, this was not enough damage to cause an engine cooling problem but by doing the winterizing myself and by being in-tune with how the raw water is suppose to move thru the exhaust system I could tell something was not right.
In closing, I would say the most difficult part of this job was the extraction of the impeller housings. In my situation they were at the front of the engines with very little room for wrenches and water hose removal. I determined the only way to remove was to leave the hoses connected, remove the housing while still attached to the mounting brackets and drop the whole thing straight down and pull it back underneath the engine. Thanks to the help of my wife this project was a success.
Nauti Hacks & Short Stacks 