It is an Idle Air Control Valve,  and apparently it is very important to a motor with the Mercruiser PCM 555 .....and if you ignore the little bastard, it will let you know in manner quite disturbing.

If you have not checked your IAC lately, read below and perhaps save yourself some headaches.

Sunny day, idling off the coast of Waikiki checking out some turtles and dolphins.  I hear "2-beep" warning.  I hear it again one minute later.  Raise the hatch.  Smoke plume from the back of my port motor.  Smells like burned plastic.  Shut the motor down, smoke clears.  Check things out...no visible problems.  Fluids full, temps good.  Restart motor.  2 Beeps every minute.  Idles fine.  Head back to a lagoon to hang out and swim for a few hours.  Restart motor, beep...beep.  Still there.  Pull boat out of water.  Flush motors, clean boat.....beep....beep.  Engines running or not, 2 beeps per minute.  Put boat away for the week.  Come back the next weekend and scan the motor with my Rinda.  IAC, fault 152.  Clear faults.  IAC Fault does not clear.  Stick my head back there and look at the IAC.  It looks bad.  Paint flaked off and the connector is melted...fused to the pigtail harness.  Crumbles in my hand.  Holy SHIT!  I was incredulous.  The plastic was melted as if it were on fire.  I needed a new IAC and a new harness.  Fortunately, Merc makes a replacement pigtail with instructions and merc approved heat-shrink butt connectors in the package.  Several weeks later new parts arrived and I set about replacing the IAC and the burned out connector.  Fault beeps immediately gone and IAC duty cycle tracks as normal on the Rinda.  I actually feel lucky that I caught it before I ran the motor too much longer.  There have been cases where a burned IAC fried the PCM.  Below are some pictures for your viewing pleasure.

My Z06 wrote on May 27^th, 2014 at 4:16pm:


Not bravery. Judgment.  Boating in the ocean presents a different set of environmental factors that a reservoir simply does not have.  You make decisions based not only on the mechanicals of the boat, but the sea state, water depth, proximity to the reef and weather.  Float there and fret about what might be going on with one motor and you may just find your vessel on the reef, inside the surf.  The beauty of twins is that extra motor to get you and your crew home safely and your investment back on her trailer to fight another day.

What's the deal with boats and IACs?  I hear of them failing so frequently people are carrying spares with them.

wow G that sucks!!  Glad you and your crew was able to get back safely!!

I'll see your IAC's and raise you a top end rebuild!!!!  LOL

Fingers crossed.

Let's definitely not make this a competition, we all lose

Bob has got me for a coil wiring harness connector this week, we'll see what else. Glad I've been running the heck out of my boat to find these little bugs before we're running hard out there.