Originally Posted by Daibheid
Wot he said and he knows his onions.
I'd have started with the impeller in the leg. It's not difficult to replace just heavy to handle but unlike outboard lower units, you have to drain the gear oil first.
The closed water system round the block etc uses an automotive waterpump and will last a very long time in your application.
Good call on the heatexchanger but cleaning them is not easy and they can gum up with the residual waste from the raw water anode.
I've certainly seen the leg impeller in very poor condition on a friend's newly acquired AplhaOne where the leg had spent a lot of time sat unused in seawater and marine growth had taken root inside the impeller cartridge and shredded the sealing edges of a newish impeller.
Check the two anodes in the raw water cooling system, often forgotten, if they're long departed you could have a different problem with corrosion.
Let us know how you resolve it - unlike some of the leeches who post up with their problems, get good advice and are never heard from again.
Until their next problem occurs.....
Well it turns out it was the impeller in the bottom half. It looked absolutely fine, but on closer inspecting we saw that it was cracked, so basically it wasnt spinning because the centrifugal force was ju masing it against the sides and it wouldn't spin.
Changed it for a new one and all is well apart from a small gear oil leak.
For those who have had the same overheating problem.
Check your coolant level
Check your oil, change it id necessary- and filter of course.
Check the heat exchanger and make sure its not clogged up.
Check the intake at the bottom.of the drive and make sure they are clear of shells or anything blocking free water flow.
Check your thermostat, make sure it works properly.
Finally, get the boat out of the water, put it in gear. there should.be 7 bolts ro remove.2 of them hold.the anodes in place slowly lower and replace the impeller.and gaskets, make sure you replace the quad ring to seal.the gear oil
If you don't you'll end up with a gear oil leak.