Mercury 40 HP Outboard overheating alarm over 4000 rpms on 24' Aqua Patio Pontoon

[bbrad99]

I inherited a 1994 24' Aqua Patio Pontoon boat from my father. The boat runs fine going slow to medium throttle. When I go full throttle or anything past 4200 rpms, after about 30 seconds I get a constant beep that won't stop unless I bring the throttle back down to idle or very slow. Running slow around the late for hours I have no problems. Had it out in the 100 degree weather running medium speed and never had an alarm. Only going fast.

Boat is peeing strong the whole time. I'm told my dad had the water pump replaced last year. I'm also told he had the same problem but never went fast so he didn't worry about it.

I have read online that one theory could be cavitation causing the boat to not get enought water when going fast. The pontoon does have a steel V shield in front of the motor. I can't say for sure if this motor is original with boat. Is it possible that when going fast it rides higher and is not getting enough water some how?

Don't know if I shold try to replace water pump again. Maybe replace thermostat ( but I would think if its bad it would overheated at med speed) or maybe this cavitation has some merit.

Thoughts? Any tests I can do to help determine problem?

Thanks much

 

[serico]

Re: Mercury 40 HP Outboard overheating alarm over 4000 rpms on 24' Aqua Patio Pontoon

I don't have a lot of experience with outboards, but have worked on machinery for years. Based on that disclaimer, I'd suggest that you tie the boat to a dock, and run it in neutral at 4200 rpms. This will sound terrible. But, if the engine can run at this speed under load, it can probably run at that speed at high idle. Let the engine run like that for the length of time that it usually takes for the alarm to come on. At the same time, check the temperature of the overboard water discharge. If, after the usual period, the alarm goes on, and the overboard water isn't hot, then one might come to the logical conclusion that you got a defective alarm that goes on after X number of minutes.
If the alarm DOESN'T go on, then it seems apparent that the alarm is signaling something when it goes on with the engine under load, and you'll have to figure out what that "something" is.
I have no idea of what the logic loops are in the alarm circuitry.
Hopefully this will be of some help.

 

[oldman570]

Re: Mercury 40 HP Outboard overheating alarm over 4000 rpms on 24' Aqua Patio Pontoon

When on the trailor where is the cavationplate of the motor in relation to the bottomof the Von the toon? If the motor is not in the water far enought at hig speed, it will not get enough water flow inorder to pump and cool the motor as designed. The cavation plate of the motor needs to be at or just below the water at WOT. JMO
Toonz that size are not speed demons.

Oldman570