Its not that hard all you have to do is take the five bolts out of the lower unit and disconect the shift linkage, then you just pull the lower unit from the engine and you have the cover that slides onto the driveshaft remove the 4 bolts from it and pull it off and there is your impeller istalation is reveres of removal
change impeller or check for blocks in water lines
18-3085 sierra
This is how to fix the impeller on a Chrysler 25 HP outboard motor. Disconnect the coupling that's on the outside of the leg, flip the housing and twist in impeller.
The impeller on a 9.8 Mercury outboard boat motor can be easily changed. Remove the retaining bolt. The impeller will come off. Reverse the process to install the new impeller.
What year? It may require special tools.
see this site http://sites.google.com/site/bf50impellernotes/
Synthetic Rubber
Every two years to be safe
Sprinkle liberally a vile of gypsy tears
Changing the impeller on a Johnson 70 hp outboard motor requires pulling the power head and disconnecting the engine. Once this is done, an open end wrench can be used to first loosen the fitting before removal.
To fix your 25 horsepower Chrysler impeller, you will first need to find out what is wrong with the impeller. If it is clogged, you might simply be able to blow the obstruction out of the impeller. In some cases it will require replacement.
Propeller is external (outboard motor on a boat) Impeller is internal (vehicle water pump).