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Excessive fuel consumption now low temperature

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Hey Crew. 
I’ve got 2008 Monsoon with about 500 hours and I had to have the ECM replaced last after chasing a rough running condition. I did all the tune up items but every since we got it running again it seemed to burn more gas. 

Yesterday we were mostly taking an easy cruise and boat running fine and temp right at 160. We dropped folks at dock and I noticed the temperature was at about 130. Went and filled up with gas and temp stayed low. Loaded up the boat for a surf run and noticed white smoke coming from under platform. Enough that my driver and I both noticed. Boat did not struggle any and seemed to be running fine. It was the end of the day so we just shut down and called it a day. 

Appreciate any suggestions on where to start troubleshooting.


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  • 2 weeks later...

Thanks to all. I was able to replace my first thermostat in 40 years without it leaking and it is now running a solid 160. 

I was surprised how difficult it was trying to find one locally at the lake. Tried 4 different dealerships and nobody had anything in stock and was fortunate to find the only 1 that the Malibu dealer had back in Memphis.

@formulaben Any suggestions for the sudden extra fuel consumption? After resolving the tstat issue, we are just taking an easy cruise and it sure seems like I'm going through a lot of gas. I'm guessing about 20 gallons in 4-5 hours of run time and less than 1 hour of that was loaded for surfing. All the rest was running empty ballast.

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The cool engine temperature from a bad thermostat will cause the ECM to add fuel similar to a choke.  This would increase your consumption a lot, especially if the ECM thinks the engine is at ambient temperature.  Now that the thermostat is good, make sure your temp sender (the one for the ECM) is working properly.

20 gallons in 4 or 5 hours does not seem excessive to me, but you know your boat.  Your engine can easily use 23 to 25 gallons per hour wide open.

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