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RPM surge from 800 to 1200 at idle


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I am at my wits end and need some help. I’m located in Lafayette, LA which is south central Louisiana without a dealer within 2 hours drive. We bought our boat new in 2006 Chapparal 220 ssi with a 5.0 GXI-f engine. It has been maintenance free until last year. This year has me stumped. For the last couple of years it was easy to start cold then had to give it throttle to start when it warmed up so this year changed the IAC which others proclaimed was the problem in other posts. I replaced it simultaneously with a new problem which was start  cold engine idle at 800 rpm then rpm increased after 30 seconds to 1200 rpm.  Tried to run the boat but would not achieve much more than 2,800 rpm.  Thought maybe TPS so changed that no improvement. Engine is running a little rougher at idle. The engine will rev above 3000 rpm in neutral. Are there any diagnostic devices close by or advice on what to look for?

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16 hours ago, nautical53 said:

I am at my wits end and need some help. I’m located in Lafayette, LA which is south central Louisiana without a dealer within 2 hours drive. We bought our boat new in 2006 Chapparal 220 ssi with a 5.0 GXI-f engine. It has been maintenance free until last year. This year has me stumped. For the last couple of years it was easy to start cold then had to give it throttle to start when it warmed up so this year changed the IAC which others proclaimed was the problem in other posts. I replaced it simultaneously with a new problem which was start  cold engine idle at 800 rpm then rpm increased after 30 seconds to 1200 rpm.  Tried to run the boat but would not achieve much more than 2,800 rpm.  Thought maybe TPS so changed that no improvement. Engine is running a little rougher at idle. The engine will rev above 3000 rpm in neutral. Are there any diagnostic devices close by or advice on what to look for?

Sounds like a bad IAC right out of the box. It happens. Also, the little servo motor can be damaged easily if the pindel gets pinched on carbon during installation. If they fail on the electronic side, then you will get an engine sensor alarm, but mechanically, no such luck. If you have the old valve, seal it tight with epoxy and install it temporarily as a test. If you end up with a low, but steady idle, then I suspect your new valve is the problem. W

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WIngnut, I have the old IAC but do not follow regarding how or what to seal it(?) tight with epoxy. 

1 hour ago, Wingnut said:

Sounds like a bad IAC right out of the box. It happens. Also, the little servo motor can be damaged easily if the pindel gets pinched on carbon during installation. If they fail on the electronic side, then you will get an engine sensor alarm, but mechanically, no such luck. If you have the old valve, seal it tight with epoxy and install it temporarily as a test. If you end up with a low, but steady idle, then I suspect your new valve is the problem. W

 

 

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When closed, the throttle plate stop screw establishes a low idle and the the ECM takes immediate control of the IAC valve to bring the idle up to the specified number. Even with a plug wire off, the engine will idle at the program specified RPM, which will vary depending on ambient air temperature, engine coolant temperature, manifold air pressure, and perceived RPM based on input from both the crank and cam position sensors. It also compensates almost instantly when boat is put in gear or returned to neutral. So the answer is the IAC position is based on it's input signal from the ECM, but it is seldom if ever 100% closed. Just a fine adjustment tool for idle. If it hangs open or searches due to mechanical bind, it causes a variable, virtual vacuum leak and idle quality really sucks. W

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