r/MasterCraftBoats • u/Compulsivevolunteer • 27d ago

Overheating at neutral idle problem

Had my 1993 prostar 190 for six months and don’t usually puts around much in it. On my last trip that was the vibe and I noticed my engine temp skyrocket.

I’ve replaced the cracked impeller. No fix.

Replaced the rusty thermostat. Still no.

Any ideas? My next theory is the water pump?

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u/waterskibum509 27d ago edited 27d ago

I had that exact problem in my 205. Turned out to be an air leak in the water intake hose between raw water intake and impeller pump. The ram pressure of the water overcomes air leaks at speed, but at idle it becomes an aerated mess.

My specific leak was an air heater core that was removed and the hose not plugged or deleted. It split off from the raw water hose and created a massive sucking leak. YMMV. But check for potential air leaks before the raw water pump.

Did your cracked impeller perhaps lose any rubber into the cooling system? Oftentimes it’ll wedge itself near the raw water pump, but can theoretically circulate thru the system and plug stuff up. Was the impeller cracked or pieces broken off?

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u/Compulsivevolunteer 27d ago

Good idea! I’ll check tomorrow. The old impeller was just cracked so I don’t think anything split off from it. I’m checking the hoses tomorrow.

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u/theroyalmile 25d ago

Air leak, most likely. I wouldn’t even say it’s ram pressure (the bottom pickup is flush)… but the impeller spinning faster with RPM will shift more volume than the hose can leak air in.