This video shows the impeller replacement process on my 1997 Mercury 15hp 2 stroke outboard motor. The process is pretty straight forward. To get to the impeller requires removing the gearbox. This is done by removing the shift shaft retainer, the lock actuator then the 3 bolts holding the gear case on. The gearbox should then slide out, sometimes needing a bit of persuasion, a couple of taps usually does it.
With the gear box off I normally use an adjustable work bench to hold the gear case whilst remove then replace the impeller. The next step is to remove the four pump cover bolts then the pump cover itself revealing the old impeller. When removing the impeller watch out for the two nylon washers and key pin as you may need to reuse them depending on what replacement parts you purchased. I was planning on just replacing the impeller but once removed I could see that the lower gasket also needed replacing.
The rest of the parts were reused after been cleaned. You then replace the impeller then reassemble the unit using plenty of marine grease then torque the bolts to required tension. The gearbox can then be slid back into the outboard and bolted in place. Next it's the shift shaft retainer to refit, this often goes in easier if you wiggle the throttle grip at the same time whilst pushing the retainer. The last task before testing is to re attach the lock actuator.
Now give the outboard motor a test, I use a tank of water, you can use muffs if you have them.
I did go on and perform a full service on this outboard which I will upload at a later date.
Mercury Outboard Motors: www.mercurymarine.com/en/us/engines/outboard/
en.wikipedia.org/wiki/Mercury_Marine