496 quit on power reduction
 

all running great. Pulled back for no wake zone. Engine quits, no start, scan shows IAC High output, cam sensor ,cranksensor map high input... Any ideas. Looks. Like pmc burp....

Before we get rolling with this I WILL NEED your year model of the engine and the engine serial number and is your app completely stock from OEM Mercruiser ? If not stock details please. Without this info its a dart throwing contest.

Need to know if you have a tach signal when cranking the engine over ???

2004 stock 496 ho wil get serial tomorrow and check tach signal, Had weird crackling noise in engine bay, could not pin point it. Almost sounded like leaves burning, no smoke no smell..

I would check the IAC connector / connection and IAC wiring and the IAC itself for extreme overheat and or melted and for shorts. I would check all fuse portions and relays engine related and look for any melted portions and or shorts.

Your engine still cranks over is this correct ??? ---- If so look for TACH signal as well. Also while cranking I would check to make sure ignition spark is not jumping across / arching somewhere externally. ( your words crackling noise) = my guess without eyes or ears or hands on it.

Make sure ignition wires are not jumping spark especially number 7 and make sure all spark plug boots are connected and again NONE are jumping spark or split or worn thru and that applies for all spark plug wires. This possible could be one of your problems along with a IAC related problem.

ALSO CHECK THE FUSE FOR THE IAC CIRCUIT

Also check cranking voltage and does the battery cables become hot while and or after cranking. Make sure you have the safety stuff on board like a working fire ext and no gas fumes.

I still want the engine serial number because there is always the CAM and Crank Sensor to deal with. The CAM sensor is an issue all within itself depending on engine serial number.

Have you tried giving it a little throttle when starting it? If the IAC is down you may not be getting enough air to keep the engine running. Also, cam, crank, and map sensor share the same 5 volt reference power. Make sure you have a solid 5 volts on the gray wire. While your in there you might as well check the ground integrity as well, the ground wires are typically black with a tracer color on it.

Will check all. Did have an AIC connector fry on me when first started up after installing all new plugs and wires. Replaced and ran a trip or two since no issues. AIC was my first suspect but found no heat or melting....

Serial OWO40139, tach drops from 500 to 0 on start or crank, checked wires and pmc platform. Found AIC disconnected, do not remember leaving off while limping home but might have with the crackling noise. Reconnected and engine fired up. Will try another run when the tides comes up... Maybe AIC overheat? I put new mufflers in the throttle bodies.......

You have to clean and make sure the IAC cavity and long hose connected is NOT RESTRICTED especially from a old IAC muffler that could has got sucked into and or the freakin BELT DUST so all of that needs to be 100% clean and free. THESE are IAC killers and cause very HIGH IAC duty cycles and still set IAC codes along with poor running conditions, and or hard engine starts to engine stalling especially on de-acceleration .

Man my first words in my POST # 4 - " I would check the IAC connector / connection ". Sometimes you get lucky and sometimes you guess right and sometimes you are just that good. I like to think all 3. lol

The reason for the CAM & CRANK fault codes was because the engine stalled on its own while seeing RPM. FYI

wow i have been reading horror stories about the IAC and the iac muffler. i think i will be buying a spare to have on hand at all times.

at least half are owner self inflicted ^^^^^ You can search what is up here for what can cause IAC failures - I know I listed a bunch as well. If you take care of all engine related items they can last. I have some 10 plus years still running the same IAC. Not hard to do at all. Easiest first step is cleaning your flame arrestor 100 % perfectly all the time inside and out. Next is to clean 100 % the throttle body and throttle plate and of course the IAC cavity, IAC hose and of course a new IAC muffler once a year min for starters - this alone can double the life. There is more

^^^^
i have been searching this for the past hour or two now. seems like just like everything else proper maintenance makes the part last longer. one question i do have now is, i see where many people say pull the foam muffler out, and it will save the IAC as well. seems to me that it would keep dirt from getting in the IAC? what is your thought on that?

Completely wrong info. And who said that ??? I will post more later - I do have a really good post up here about the duty cycle,

The very first 496's did not have a IAC muffler and there was a reason for it, then Merc did a campaign recall for those 496 PCM's. One was for soot and emissions / to meet the emission star ratings set by the EPA compliance. I will try to find the post also.

See Merc did a recall on the 496 PCM and when you got the PCM back the IAC MUFFLER was taped to the PCM as they incorp it with the new recal of the PCM. Believe me I know all about and the reasons why after the first year maybe first 1.5 years of the 496 in production - all of them had this recall campaign.

BUP in your opinion does it make more sense to do away with the IAC tube and muffler all together and go to a small K&N type breather filter mounted at the rear of the intake on the IAC? Seems it could pull cleaner air that way.

i've seen it in multiple threads saying that the muffler foam comes apart and gets stuck in the the IAC. http://www.offshoreonly.com/forums/g...c-muffler.html this thread is where i read about it. that is why i asked.

i want to take every precaution i can to ensure my new boat lasts a long time without any type of issue. i already have a second battery going in, with a switch. to ensure proper voltage is always available.

^^^^^^ IAC muffler is a replacement item whether anyone likes it or not. Once or twice per year and you need to cut down on the belt dust so the serp belt is very important and its tension along with true running line up (alignment of the belt needs to be very true) . Everything needs to be kept cleaned is important and I already mentioned some of the items that needs to be cleaned.. Like I said I have customers with 496 Mercs with over 10 plus years on the same IAC. Why because I properly maintain for it. Yes its cost money to do so and I charge for it.

Matt we did that way back when the IAC become a big failure rate. 2004 or 2005 - timeframe ??? anyways it was the same old tune - IAC failures and they kept on increasing. One of the reason was because owners had incorrect belt tension from the adjuster bolt after removing the belt for Impeller changes. The improper tension cause alot belt dust over time. Plus not many new about the installed IAC muffler and it became completely clogged. Not many back then changed it nor seen where it was. Even some techs back then did not know about it.

What I did was more of way to see and play around with it. We blocked externally the fitting off / TB - then cut a hose to match and attached that along with a crankcase breather off the IAC block. We went to the lake all day with it. Things noticed right away - was the idle surged - we liked it but did not like it. Kind of a lumpy cam engine at idle and sound persay but not a smooth idle for docking. Next negative - it produced a louder hissing and noise than stock set up.

The PCM calibrations has changed in the later years along with the change in CRANK - X. I never tried it again after that one time with the crankcase breather nor scanned the engine. That was probably a mistake on my part not scanning it..

I will say that you need for a good range about 24 to 34% pulse width modulation from the IAC at idle from a fully warmed upped engine OEM COMPLETELY STOCK 496 Merc. I like 30 % the best in a real world but that will never stay perfectly steady. 30 % basically - thats were Merc moved up the PWM with the newer cal.

Pulled the arrestor yesterday and the muffler and throttle plate were filthy. The IAC has been replaced it looks like. The muffler looked original. cleaned the arrestor pulled the muffler to replace and cleaned the throttle plate.