I have a 94 Evinrude 88 SPL. The engine hot alarm (whistle) has been an issue for years if I go much over 30mph. I finally had the impellor replaced 3 months ago and have always thought it was the problem. I was on the water last week and going about 34 mph and it went off again. I stopped , put it in neutral and raised rpm`s slightly till alarm went off(about 20 seconds). I went to Whitney this week and went across lake over 5 miles @ 28-30 mph and engine alarm never went off. I can run less than 30 mph and troll for 3-4 hours and never have a problem either. I believe it is time to replace the thermostats. I bought boat new and it has never been replaced.

I would and poppet valves.

PM sent

Thought popet valves were a mercury/yamaha thing?

I would suspect the thermostat if it hasnt been service.

Have a family member who has about the same motor that has been used in salt water extensively and has similar issues, pretty sure it has considerable internal corrosion in the water jackets.

After all the years why worry about, just drive it like you stole it

I probably will. I was on the water yesterday and kept it below 30 mph all day. When I headed for the ramp I kicked it up to 34-35 mph to see what it would do and it took about 30 seconds for the alarm to go off. I checked the stream of water coming out of the little hole on the motor and it was the same as it was when I had been trolling @ idle speed(2.2 mph).

Had similar problem with my '89 150 Yam ProV - overheat warning horn at higher rpm.

Over the years the cylinder heads/water jackets water passages had built up a crud accumulation that restricted the head's cooling water passages and holes. Some of the heads water passages and holes were blocked by the crud buildup.

I pulled the heads and soaked them in a mixture of half/half warm water and white vinegar and then scraped out the remainder of the crud from the water passages.

I believe there's also cooling system flush kits/chemicals that can be also used without taking heads off.

If water pump and thermostats are OK this the most likely the problem.

I had a old Merc EFI that would alarm periodically and it ended up being one thermostat not working.

Check pop value and thermostats

My Mercury 200 EFI was doing the same for a couple of years..frustrating to say the least. We changed multiple impellers and checked practically everywhere water flows for an obstruction..no luck. .Finally we changed the impeller housing as well and problem solved..upon inspection the old housing had become "slightly"oval in shape and evidently decreasing water pressure causing it to over heat at higher rpm..."note to self..Always check the housing when changing the impeller.."

I had the same issue on my 93 Johnson a few months ago.

My boat had been sitting for 6 years covered by my wife's piles of stuff and I decided to get it back on the water. Got new batteries and fired it up in a big barrel and low and behold, the long overheat alarm. I have had this issue two times before in the life of this boat. First time I took it into South Austin marine they told be that those boats were having issues with the alarm system, that was probably in 1995 or so. I had them fix it and what do I find they fixed it with roughly 26 years later?

They used heavy duty fingernail polish or paint to insulate one of the leads to the thermistor.

Well I cleaned all that gunk off and started it up again and no more alarm, measured the head temps the best I could with my hand and thermometer.

Check your head temps with a thermometer or even some of that temp crayon stuff. If it is overheating that will tell you right away. If not unplug one thermistor at a time to see if your alarm goes away.

I never found the source of my alarm and my boat was stuck in that low slow mode also so I started replacing parts. The thing is there is also a long constant alarm for a fuel block or restriction.

That was my cause to think I was stuck in low rpm mode. My fuel connector (female) was cracked and would allow fuel at low rpms only.

Good luck. I would just recommend a good boat mechanic to save you some headaches even though nothing is really hard to do by yourself.