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Mercruiser 496 IAC valve

J

jimnlouisiana

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My mercruiser 496 (inboard) on port side is giving me a warning that the IAC idle air control is inop. It definately won't idle, but I also noticed that just prior to getting that alarm smartcraft display the top end RPM is 1000 RPM less than it used to be. I normally could get 4900 RPM at firewall power but it is down to 4000 RPM at full throttle. Could the IAC be affecting the top end power?
 
The IAC only controls Idle. It has no effect on the top end. They go bad often. I carry a few in my service truck.

As for the RPM loss, It may be time to replace all the fuel filters and make sure your wheels are clean before you do anything else.
 
Is your engine horn going off periodically? Just wondering if Guardian has kicked in for some other reason and is limiting RPM. Do you have Smartcraft gauges with the little LCD text display on the bottom, if so, any message?
 
jimnlouisiana said:
My mercruiser 496 (inboard) on port side is giving me a warning that the IAC idle air control is inop. It definately won't idle, but I also noticed that just prior to getting that alarm smartcraft display the top end RPM is 1000 RPM less than it used to be. I normally could get 4900 RPM at firewall power but it is down to 4000 RPM at full throttle. Could the IAC be affecting the top end power?
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Yes, Having a bad IAC of some kind could drop your normal 100% "guardian available engine power" down to 90% available power lowering your wide open RPM's.

In some cases you could hear a warning horn go off as well.
 
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