Hi,
Just my opinion....I haven't worked on a 75D....
Unless the flush valve was really spraying water up under the hood I kind of doubt that the small leak you describe would cause an alarm or de-power. However, if things up top got wet it might. You'll be wanting to stop that at any rate.
I think your plan of action is a good one except, if you have any doubts about that water pump (as in it hasn't been serviced in a long time) I would probably not wait. If it hasn't had a complete pump assembly replacement, you will want to consider doing that as opposed to just replacing the impeller.
(Items 1,2,3,4,5,6,9,27,28 and 34 in the link below)
A pricy endeavor if purchased individually but the kit (item 2 in the link below) can save you some money and hassle.
Still expensive but worth about $2k in peace of mind with that expensive outboard.
If and when you disassemble the waterpump look very carefully at the liner's internal walls. Even a relatively new liner will show wear in the area the impeller vanes ride on. A new impeller installed in even a low hour liner will wear rapidly to match the liner wear pattern. This decreases efficiency and life of a new impeller directly proportional to the extent of how badly the liner is worn.
Then, inspect the liner to plastic housing fit. It should be almost exact with little to no movement of the liner if you try to rotate it inside the housing. Close inspection of the contact points in the housing that prevent the liner from rotating will typically reveal signs of "hammering" from the forces involved. That hammering and associated damage to the housing only gets worse the more movement there is.
Always...ALWAYS, replace the drive key when installing a new impeller. Just carefully comparing a used one to a new one explains why. The key is made so as to wear instead of causing wear in the mainshaft drive slot. An old, worn key can move and "trip" in the slot. It can cause you plenty of grief.
Not saying that the pump caused this, just pointing out things I've seen over the years. I once saw a statistic that said about 70% of all catastrophic IC Engine failures begin with a compromised cooling system.
One last thought is that recently, I have seen two cases here (albeit on 60hp models) where the on board water separator has seemed to be the culprit for triggering an overheat alarm and de-powering the engine. Not sure how you would go about troubleshooting that though. Maybe simply try unplugging the connector with engine off??
I believe both of those threads resulted in replacing the separator to fix the issue but don't hold me to that as my memory is faulty at times.
Hope you get it sorted out.
