I recently purchased a boat with twin 496 HO's. Ran great, nice and strong on half a tank of fuel. Filled the tank up, and dunked it early morning, as I putted along to my lift I noticed the starboard engine was running rough and not as throttle responsive. I scanned it with a Diacom tool, and it showed no faults. Did a misfire test, and found bad plugs in cylinder 4 and 7. Changed plugs in both motors, all 16, and both passed the misfire test. Took it out, and it wouldn't plane off without giving 2 beeps a minute. Hooked computer up, NO ALARM. The only difference I can see thus far is a few psi lower in sea pump pressure. Passed the injector test, runs fine on the lift up to 2500 rpm. I even unplugged multiple sensors to make sure they would send an alarm to the Diacom. Previous owner had water pumps changed in the last 3 years, and new impellers this spring. I am at a loss as to where to go from here. Any positive feed back would be greatly appreciated!

It's likely a bad IAC.

Any chance you'd be able to suggest a way to test it? Diacom has a IAC test that you can increase or decrease it up to 100% in 10% increments. Neither motor respond to the test, but the port motor runs just fine.

EDIT
I have unplugged the IAC while running, and the engine idles rougher, and I get a two beep alarm along with a logged code.

The boat is going to need IAC valves someday anyhow. Just go ahead and buy one.

It's almost always an IAC Valve or it's the IAC Chip in the PCM.

Asnt there an IAC replacement for automotive that he can get at Advance or NAPA?

Somebody have the part number?

I should post an update. I was able to pilot the boat today, while my buddy worked the Diacom computer. Ran great all the way through the no wake zone, went to get on plane, (being a 28 ft with 496's she labors) and just as she got on plane, gave me the two beep alarm. He scanned and got a shift sensor alarm, and a neutral overspeed alarm.... I shut it down and a light bulb went off. A few weeks back when the starboard started running rough, ( before I knew it had two bad plugs) I crawled all over the motor to check all sensors. I noticed the shift sensor switch was unplugged on the starboard, but connected on the port. Well I thought "that should be plugged in!" and plugged it in. Wouldn't get on plane since. Threw the engine hatch open, unplugged that sensor, cleared the alarms, and ran 80 with almost a full tank of fuel. Going to replace the sensor next weekend after I order one. So it makes sense looking back cause the engine was thinking it was in neutral and any rpms over 2500 in neutral cause a shut down. Also I checked oil pressure, exhaust temps, water pressure, IAC duty cycle, fuel pressure, coolant temps on both motors at idle, and at 80 mph, both mirrored each other, so I am going to hold off on the IAC replacements at this time. Thank you for the reply's, I am new here and appreciate the prompt responses.

EDIT I bought this boat less than 2 months ago. The starboard shift sensor was unplugged when I took possession.

Probably not the IAC. Just replaced mine. When the IAC goes bad, usually motor will not stay started at idle speed. Also hard to start.

Its called shift in gear switch - it can even cause engine shut down and or guardian mode . The SIGS can even show on the water hose in gear or N while scanning / shift it - you watch for the change As soon as you shift - if the scan takes a while to show the position change and or does not change at all - you have problem - SIGS could be misaligned and or loose or connection problem or bad SIGS. Make sure it is tight / mounted and the connections are good. Even a loose SIGS can cause a warning beeps and or engine guardian mode..With the scan tool its one of the simplest test to look at and watch (either in gear or N) no other parameter to know other than that for the SIGS.

IAC problems and or Bad IAC can cause many problems from no start,, and or bad idle, and or hard start, and or really bad miss upon acceleration and so on plus even a engine that seems to run fairly good but has the 2 beep alarm can have a IAC problem of some sort..

Motorcraft CX1766 is the OE Part. Ford used these on 2001-2003 Ford Windstars

Confirmed Post #8: http://www.offshoreonly.com/forums/d...iac-valve.html

https://amzn.com/B000C5BLXI

http://www.rockauto.com/en/catalog/f...ac)+valve,6072