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03 Sunscape high idle issue

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Just got an 03 Sunscape LSV 21 with monsoon 335. Put in the water for the first time today. When stationary the rpms are between 1150 and 1250. When put in gear rpms drop to 850. Not sure what to do to correct as I’m new to boat engine maintenance. Is this something that can be adjusted diy. 

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I'd check the IAC.  They are relatively cheap to replace if you use off-the-shelf GM parts from your local parts store.  You could pull yours and see if it even works first.  To replace, tell them what it is off of, but start the search with a 1999 GM Suburban 350 and you'll find a match.


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I have often found that high idle on cable driven throttle body EFI engines are caused by the cable not being adjusted properly at the throttle body, or at the helm throttle control, or an issue with the helm throttle control.

A problem with the IAC or an intake vacuum leak can also cause high idle speed, but verify that the tachometer reading is accurate.

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Agree with @csleaver. Check the throttle body to ensure the butterfly actuator arm is resting against the idle set screw as first item of business. Idle should be around 650 rpm warm. 

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