26sonic

can anyone tell me what the" IAC Muffler" does and what happens if it goes bad ? I have a 496 ho and was told this needs to be replaced after my IAC was found to be bad and melted!

Dave_N

It's just a little block of foam that goes in the intake of the IAC air passage. It does a good job, if you ever hear a 8.1/496 without the muffler there is a loud hissing when the IAC is even partially open, really loud when it's all the way open.

Just a note on the IAC meltdown, it's not uncommon and I believe it's due to runnning a high duty cycle too much of the time.

If you have access to a scan tool, monitor the IAC duty cycle. It should be high (99.9% duty cycle) at startup, and then drop down to 20 and 30% duty cycle at idle after a minute or two.

Dave

26sonic

What is a high duty cycle ?

Raylar

26 sonic:

Dave is correct, its just a little piece of foam that Merc puts in the IAC entrance opening in the throttlebody to cut down the IAC noise (hissing) when the IAC valve is operating. We take them out and don't replace them because they do breakdown and get sucked into the IAc and sometimes mess up an expensive IAC motor. The duty cycle Dave mentioned is a term used to describe the opening and closing rate of the IAC in operation as it is constantly moving between open and closed to adjust the amount of air the idle of the motor is feeding the engine to keep the idle stable at about 650 rpms. The IAC is controlled by the ECM.

Regards,
Ray @ Raylar

Dave_N

Hey Ray,
Thanks for jumping in. If you don't mind my asking, when you remove the muffler, isn't the hissing really annoying? Also, have you seen many IAC overheats/meltdowns? What can be done to prevent it?

Dave

Knot 4 Me

My 496 hisses at about 3,200 RPM. I bought an IAC muffler thinking mine didn't have one. When I went to install it I saw that I did indeed already have a muffler in place. I removed it and there was no significant difference in the volume of the hiss. So I removed it altogether. I installed a K&N flame arrestor which muffles the sound to an acceptable level.

26sonic

ray if this muffler is tore or pieces missing could this cause my idle to be off by 50-100 rpms ? I also have a problem when cold starting the engine it stalls the first time then is ok after that . originally the fault codes that came up were iac and map sensor . found iac melted and replaced and then came up with map sensor and replaced . don't have any more codes but still stalls out first time at cold start after sitting a few days. called merc. tech support and they said it is the iac muffler ?

monstaaa

just today i had a 38 with 496 h.o's with an itermitent stalling problem. as it turns out its the i.a.c's on both engines are at fault. if you use the laptop with merc software you see live duty cycles. you can also select to test the i.a.c and see if it is functional. sometimes the debri from the muffler plugs the line. sometimes theres a mechanical failure which will not show up as a code because electricly the unit is functional. typicaly caused by salt contamination, but not allway's. you can remove the unit and get away with cleaning it,in some cases, but use good judgement.
merc just released a bulletin about removing them( the mufflers) all together because of said issue'sof being dislodged and such.
as for the noise you really wont see much of a difference when under way, and i would think that the whistle you hear may be from the throttle blade. this will more than likely increase as the temp drops due to the thinner air.

good luck.

Whipple Charged

Its incredibly annoying if you have the hatch up. If the hatch is down, it's not as bad. This gets progressively louder as DC goes up. 100% is max opening, 0%, full closed. Typicaly, in neutral, they are between 0-20% and go to 40-60% on stock motors in gear. Supercharged motors go near 80-100% because the bigger prop loads.

26sonic

update: I removed the iac muffler from the engine with the stalling problem and it starts right up at cold start witout stalling! when I put it back in and tried at cold start the next day it stalled the first time like befor !!! why ? the other engine without problems I removed it and runs fine but after a nice run of 10-15 minutes at 4,000 rpms I put it in neautral and it idles rough ? why ?