BoatBanter.com - View Single Post - 1996 40HP Mercury outboard overheating alarm

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On Wed, 15 Jul 2020 18:10:37 -0700 (PDT),
wrote:

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.

Are you sure about the thermostat? What is the serial number?
If you haven't checked the water pressure, that is a place to start.
If it isn't around 15 PSI or more at cruise RPMs you still have a
water supply problem. Next might be corrosion in the water jacket.
There are a number of schemes to flush with salt away or just vinegar.