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Help!! Port Exhaust Manifold Overheating - Monsoon 340

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Hey Guys/Gals 

We were wakesurfing weekend before last and started to smell a slight burning rubber smell coming from the engine compartment. Upon closer examination, the exhaust coupler connecting the stainless elbow from the Port side manifold was very hot. Using my laser thermometer, I was able to determine that this side was HOT (like 250-300 degrees) while the Starboard exhaust manifold was only 130-150 degrees. After letting the engine cool we were able to slowly make our way back to the dock. 

My initial thoughts were: 

A) there is something obstructing the water flowing to this exhaust manifold 

B) There is a cylinder over fueling causing the catalyst to overheat. 

At this point, I pulled the spark plugs and found them all to be in great shape, with no signs of fouling so I don't suspect this to be the problem... but more on that later. I had heard recently about the upgraded cooling schematic for the older malibu's so I went ahead and installed the reroute kit but my problem remains. 

Assuming that the catalyst could have been damaged from high heat,  removed both stainless exhaust down pipes and found that Port catalyst was 90% gone... at this point I removed what was left from the port side and removed the Starboard catalyst completely. As well, I measured the volume of water leaving the port/ starboard exhaust manifolds and it was identical... leading me to believe there isn't some type of obstruction. 

Unfortunately my problem persists, at idle the port exhaust manifold is nearly 150 degrees while the starboard side only reads 90 degrees. EVEN if I had an over fueling problem I cant imagine it causing the manifold to heat up so quickly... with no catalyst in place. Anyone dealt with something similar before?

Thanks in advance!


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7 hours ago, Christian said:

 I had heard recently about the upgraded cooling schematic for the older malibu's so I went ahead and installed the reroute kit but my problem remains.


Interesting if you have the Re-plumb kit.  How long have you been running this boat with the re-plumb kit and never had an issue AND I hate to ask, but are you sure you ran that replumb kit correctly?  There have been a few threads of people double checking on here to make sure they ran the kit correctly.  Have you added anymore weight listing the boat more than normal?  I know you checked water flow and both ports are the same but you could still have an obstruction causing limited flow.  Did you check your impeller and transmission cooler.  Do you have a sea strainer?  If you have limited flow at the beginning of your raw water flow, your port riser is the last to get cooled.  Could be that your starboard riser is getting "just" enough flow to cool it but as it gets around to the port the water is hotter and limited, not enough to cool it?  Only thinking out loud here...

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Thanks for the reply! 

I have checked all the above, and have put almost 30 hrs on the boat with the Re-plumb kit. Impeller is new this season as well. 

Thinking I may have just found my issue this morning. It appears that there were some Very small plastic debris that has lodged itself where the water coming off the port riser flows into the stainless steel elbow. The boat is idling with two very cool manifolds (with one SS elbow off). Hoping I can get these bits of plastic out of there! 

Will provide an update once I can test on open waters!

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Those elbows were originally plastic.  They probably melted in a previous overheat. 

You may be able to put plastic ones back on now that you have installed the cooling kit. 

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