Fluctuating Idle, IAC hiss, low WOT RPMs
 

Hello,

I have an '03 donzi with a 496 ho giving me some trouble. Idle rpms are fluctuating as the IAC sounds like it is contantly opening and closing. There is a pronounced hiss. When I give it a little gas to around 1800 rpms, the engine will surge to 1800 then it will drop to around 1400 and flucuate between 1400 and 1550 rpms. Engine will not hold a constant RPM under 2000 RPM. At WOT I've only been able to achieve 4400 rpm when I typically get around 4900. Any ideas? I have had two mechanics look at it. There are no alarms. Engine is not throwing any codes when hooked up to a scanner. Also, fuel pressure was tested and mechanic says it's within spec. Some say it could be the IAC valve, but would that necessarily cause a loss of RPM at WOT? I sprayed intake manifold, throttle body gaskets, and injectors with carb cleaner and didn't find any vacuum leaks. Anytime let off throttle to idle, there is a very loud hiss from the iac. I'm posting videos of problems below.

Any help would be appreciated.

Thanks



http://www.youtube.com/watch?v=43Q104u2EJw
http://www.youtube.com/watch?v=mlkpe9TzBko
http://www.youtube.com/watch?v=07NJ8JV3kmY

how many hours? does it seem to miss at closed throttle?

Boat might have 200 hours on it. At closed throttle, the exhaust note is definitely not what it is typically. If I had to describe it, it sounds like it is running cold, maybe one cylinder missing, but hard too tell.

If the IAC needs resetting, try unplugging it, turning the key on for 10 secs, turning it off. Plug it back it, turn the key on for 10 secs and then back off. Start your engine and test. That will re-calibrate the IAC. Or remove the IAC... possible blockage causing it to stay open?

A faulty IAC will cause the engine to go into guardian limiting rpm to 4300+/-. It should show up in history but may not be active at the time you scan it.