Merc 8.1S - port engine sounds like Darth Vader

R-and-R

Active Member
May 9, 2018
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Boat Info
Boatless
Engines
Boatless
2002 340 DA

At idle, and up to 1200 RPM, I hear a air hissing/suction noise at 1-3 second intervals. Loud enough to be heard with the hatch closed, very noticeable with the hatch open. The “breathing” rate increases from idle up to about 1200 RPM and then goes away.
Becomes less noticeable as engine warms up, but doesn’t go away completely.

It’s also accompanied by RPM fluctuations of 50-100 RPM.

Port engine only, sounds like it’s coming from the air intake.

I think it’s been doing it since I purchased (in June), but didn’t think much of it at the time.

Any thoughts?
 
IAC muffler got sucked in on the port motor?

Sounds like a possibility. I didn't think that would cause the surging RPM, but after reading a few other threads here, that seems to be a common symptom.

Thank you for the tip!
 
Pull the air flame arrestor off. At the top of the round intake chamber is a small square intake. If the foam muffler is gone or dirty replace it. Push it all the way into the square opening. This is the cheapest part on the entire engine. Like 2 dollars. It's about the size of a sugar cube and it tames the fire breathing dragon sound down to a pure.
I think this is the part number.
863829
If the rpms continue to fluctuate and you hear that change in rpm. Then you may also need to clean or replace the IAC. Order new gaskets first before you pull and try cleaning first. A bad gasket will make matters worse. Clean the pin inside. I used carb cleaner and then a quick shot of wd40 and moved it back and forth until it was like butter. My rpms are smooth and stable a year later.

 
Last edited:
Sounds like a possibility. I didn't think that would cause the surging RPM, but after reading a few other threads here, that seems to be a common symptom.

Thank you for the tip!

My Merc tech says to pull them out and leave them out - he's seen to many IAC failures when they block up. I leave mine in but change them annually when I service the motors - so far so good.
 
Thanks for the help everyone.

Sounds like IAC muffler is another item to add to my annual engine service list. Probably the cheapest item on the list!
 
Leaving them out is just fine. Their only job is to calm the breathing dragon sound.
 
The beast has been tamed!

Two new IAC mufflers... the easiest and least expensive maintenance I’ve done!
 
IAC Mufflers are part of my prep for winter. I pull the flame arrestors to clean them and replace the mufflers every fall.
 
The beast has been tamed!

Two new IAC mufflers... the easiest and least expensive maintenance I’ve done!


do you have the part number for the IAC mufflers - sounds like i need to replace mine?
thanks
chris
 
do you have the part number for the IAC mufflers - sounds like i need to replace mine?
thanks
chris
Hey Chris,
Well, I've been logging part numbers for everything I've purchased since buying the boat earlier this year...except for these little guys. I went to my local Mercruiser Parts shop and the guy didn't even look them up. He just went and got them from his stock bin, they cost about $1 each.
 
Hey Chris,
Well, I've been logging part numbers for everything I've purchased since buying the boat earlier this year...except for these little guys. I went to my local Mercruiser Parts shop and the guy didn't even look them up. He just went and got them from his stock bin, they cost about $1 each.


Wow, thanks alot - I will check it out = my port engine just started this!!
thanks
 

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