Mike
Hi, I have a 40hp Mercury outboard that sounds an overheating alarm when revs, get over around 4500 rpm, this outboard is a US made 4 cyl 2 stroke 1996 mod,
first of all this outboard does not have a thermostat by design, so i replaced the impeller but the impeller blades were bent only and not realy damaged in any way so i am concerned that i have not fixed the problem, the engine is in very good condition for its age so i am wondering if any one else out there has had a similar problem with this engine and if so was it resolved and how.
regards Mike.
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Mike you’re impeller could be bad to where ar high rpm, the canes are collapsing and not shoving water through.

I’ve seen this in an evanrude. Where the guy didn’t spring for an OE impeller and went for a cheap ebay replacement. It would collapse . Got an original and it worked fine.

Sounds like yours is getting weak.

Good luck