So I had new plugs put in my 06 496 the other day. It was running rough and loss of some power. It solved the problem and the boat runs great. I noticed that it idles high in gear now. Its to high like 1000 rpm by the tach and it is noticeable when trying to dock or maneuver. It ides down when I put in neutral and goes back up after a second of going into gear.

I researched on here and it appears that the idle is set by the ECM so I cant change it. Any thoughts?

Make sure your flamer arrestor is clean, and you could take the IAC out and clean it. You likely could have your IAC starting to go bad on you. The IAC controls your idle, based on with the ECM tells it to do. You NEED to get this fixed. It sounds like you are shifting out of gear at 1000 rpm, and that's not so good.

Go buy a spare iac and keep it with you just for future problems. They aren't known for longevity.

Here is a tip for the IAC on PCM marine engines. Many marine techs do not even know this info. For one many think this form of a IAC is tested / scanned and controlled in counts. It is not plus there is other tests that you can do and one of them of course is a visual, make sure the IAC does not have any protrusions, if so it is bad, also make sure the IAC air hose and its connection are properly solidly connected and in good shape from one end to the other. Here is the big tip - normal resistance for the IAC is 10.1 ohms, you can test them with a volt meter. All Mercruiser specs for ohms testing is at 70 degrees so try to be as close to that air temp for ohms testing for any sensors. Again another big tip and I can not tell you how many professional techs do not know this, especially about testing sensor ohms as far as air temps goes.

The IAC (valve) is a 12v sensor / circuit powered by the main power relay = (MPR). The PCM grounds the IAC valve to open in which allows more air to enter the plenum. The PCM ungrounds the IAC valve to close in which reduces the air flow into the plenum. This strategy of the IAC control is called pulse width modulation (PWM) not counts. it is measured in a form of a percentage. So 100 % of pulse width modulation (PWM) means the valve is fully 100 % open. As 0 % (PWM) means the valve is fully 100 % closed. The PCM & power from MPR basically tells the IAC what position to have the valve constantly to control proper idle at all times.

I figured, hopefully this helps about how the IAC works plus helps testing it and how it can cause idling and off idling problems.. Also a bad IAC and or its circuit can cause hard starting or even a no start condition.

Ok so some IAC testing is in order. I know that leading up to the problem of it running not so well I did some IAC testing, cleaned the flame aresstor, and IAC little muffler. I noticed today that I have forgot to put the muffler back in but I wouldnt think that would do it. I was also assuming that it isnt a cable issue because I would assume it would not make a difference whether it was in nuetral or in gear if it was. There is nothing on the drive cable that affects rpm, right? I will check all mentioned see where I get.

So I am heading out to replace my IAC at the marina. Anyone know off the top of their head what size star or allen the two bolts are?