2001 Air Nautique GT40 351 is overheating at idle?

Matt,
The crack in the manifold will not cause the overheat. The water flow is going the other way and is exiting the engine at that point. Check all your hoses at every connection and tighten the hose clamps with a socket wrench. Check your strainer and make sure it's seal is good. Unlike a auto engine, the marine with a raw water system is not pressurized. There's just enough pressure from the RWP to get the water through the system. Sorry to hear about the crack. Post a picture - it may be repairable by brazing or you will need a new one. Some have even had good luck with epoxy. If repaired, the manifold should be pressure checked to make sure the wall between the water jacket and the exhaust cavity isn't cracked as well. No winterization or was there water still in the manifold?

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