Overheating in idle only problem on 2012 Moomba Mobius LSV

This just started at the end of last season and this year when I dropped in after performing yearly maintenance on the boat. I replaced the impeller and thermostat. I have an oem fresh water cooling FULL kit on and have the right thermostat all parts come directly from indmar. When sitting idle for long periods it will over heat as soon as I punch it to full throttle or even gas it up a bit it cools straight back down. I live in the middle of no where and there's not any mechanics that will touch this boat so I need some ideas as to what it could be so I can fix this issue. Obviously between switching riders and being in the no wake zones this could get kind of tricky if I don't get it fixed. I appreciate the input thank you!

Dumb question, but the water intake ball valve isn't closed is it?

I've never dealt with Indmar closed cooling, but in a car, this could be air in the cooling system. There could be enough air to air-lock the system, but the water pump can overcome it at higher RPM. This situation would be quite bad.

Many cooling systems in cars have a small bypass at the thermostat and can "burp" themselves, you just need to keep the coolant level full. Otherwise, you have to open the system at a high point to let the air out (like the hose connection at the top of the heater core). I'm not sure of the layout of the Indmar system, but I would start by finding the highest hose connection and just crack open the clamp and hose to make sure that it's completely full of coolant and let out any air.

This is a 2012, raw water cooled only

What was the condition of the impeller when you pulled it? The reason I ask is my buddy had a similar problem with his MC. Also an Indmar. His impeller was shot and pieces started making their way through the cooling system. He did replace the thermostat and impeller, what happened was an old piece of impeller got stuck in the new thermostat making it inoperable. Full system flush on a fake a lake solved his issue, plus another new thermostat.

I replace my impeller at the beginning of every season and they are never beat up too bad. My aprilia does the same thing where I have to burp it every time I do the coolant. I'm going to call indmar and ask them how they suggest I burp the system. I appreciate the feed back and I'm thinking it is air in the system and not a faulty water circulator. I have a perko flush valve on it as well. Any suggestions on how to burp this thing? Every motor has it's little trick you have to do to burp it properly.

Have you double checked that the impeller housing is fully sealed? This happened to me one time, and I found that the gasket had a tear, and one of the brass screws was a little loose. I started replacing the gasket each year and put loctyte on the screws.

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Originally Posted by Woody929

This is a 2012, raw water cooled only

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Oops, sorry. When I read "oem fresh water cooling FULL kit" I thought that meant the closed cooling system. There shouldn't be any issue with trapped air in the open/raw water cooling system and thus no need to "burp" it.

I would be tempted to replace the thermostat. It may not be working correctly or, more likely, is the wrong thermostat for your model causing it to not open as early as it is supposed to at idle.

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Originally Posted by JOE SUTERRA

This just started at the end of last season and this year when I dropped in after performing yearly maintenance on the boat. I replaced the impeller and thermostat. I have an oem fresh water cooling FULL kit on and have the right thermostat all parts come directly from indmar. When sitting idle for long periods it will over heat as soon as I punch it to full throttle or even gas it up a bit it cools straight back down. I live in the middle of no where and there's not any mechanics that will touch this boat so I need some ideas as to what it could be so I can fix this issue. Obviously between switching riders and being in the no wake zones this could get kind of tricky if I don't get it fixed. I appreciate the input thank you!

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I assume this is a 5.7L engine ... correct? When you say overheating, how hot is it getting? Is it setting off the alarm? Remove the cap from the overflow bottle and let the engine run till the thermostat opens and the engine comes up to temperature. }Does your thermostat have a bypass hole to allow some flow before the thermostat opens? Any trapped air in the closed system should be able to escape then.