Merc 8.1S - port engine sounds like Darth Vader

Discussion in 'Gas Engines/Drives/Transmissions/Props' started by R-and-R, Oct 9, 2018.

  1. R-and-R

    R-and-R Member SILVER Sponsor

    54
    May 9, 2018
    2002 340 Sundancer
    8.1S Mercruiser
    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?
     
  2. juggernaut1

    juggernaut1 Active Member

    827
    Apr 19, 2015
    Perth, Western Australia
    2006 38 Sundancer
    8.1S / ZF 63V Drives
    IAC muffler got sucked in on the port motor?
     
    R-and-R likes this.
  3. R-and-R

    R-and-R Member SILVER Sponsor

    54
    May 9, 2018
    2002 340 Sundancer
    8.1S Mercruiser
    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!
     
    juggernaut1 likes this.
  4. LG111

    LG111 Active Member

    449
    Jan 4, 2015
    NW
    410
    8.1
    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: Oct 10, 2018
    R-and-R likes this.
  5. juggernaut1

    juggernaut1 Active Member

    827
    Apr 19, 2015
    Perth, Western Australia
    2006 38 Sundancer
    8.1S / ZF 63V Drives
    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.
     
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  6. R-and-R

    R-and-R Member SILVER Sponsor

    54
    May 9, 2018
    2002 340 Sundancer
    8.1S Mercruiser
    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!
     
    LG111 likes this.
  7. LG111

    LG111 Active Member

    449
    Jan 4, 2015
    NW
    410
    8.1
    Leaving them out is just fine. Their only job is to calm the breathing dragon sound.
     
  8. R-and-R

    R-and-R Member SILVER Sponsor

    54
    May 9, 2018
    2002 340 Sundancer
    8.1S Mercruiser
    The beast has been tamed!

    Two new IAC mufflers... the easiest and least expensive maintenance I’ve done!
     
    juggernaut1 likes this.
  9. rondds

    rondds Well-Known Member SILVER Sponsor

    Oct 3, 2006
    Jersey Shore
    2001 380DA
    Merc 8.1s (2008)...Hurth ZF 63 V-drives...WB 7.0 BCGD (2013), Garmin 8208 & 740 MFDs, GMR 24xHD dome
    IAC Mufflers are part of my prep for winter. I pull the flame arrestors to clean them and replace the mufflers every fall.
     
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  10. kysearay

    kysearay Active Member

    134
    Jun 15, 2017
    KY
    42 Sundancer
    8.1's gassers

    do you have the part number for the IAC mufflers - sounds like i need to replace mine?
    thanks
    chris
     
  11. R-and-R

    R-and-R Member SILVER Sponsor

    54
    May 9, 2018
    2002 340 Sundancer
    8.1S Mercruiser
    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.
     
  12. rondds

    rondds Well-Known Member SILVER Sponsor

    Oct 3, 2006
    Jersey Shore
    2001 380DA
    Merc 8.1s (2008)...Hurth ZF 63 V-drives...WB 7.0 BCGD (2013), Garmin 8208 & 740 MFDs, GMR 24xHD dome
  13. kysearay

    kysearay Active Member

    134
    Jun 15, 2017
    KY
    42 Sundancer
    8.1's gassers

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

    kysearay Active Member

    134
    Jun 15, 2017
    KY
    42 Sundancer
    8.1's gassers
    rondds likes this.
  15. mnm99

    mnm99 Well-Known Member

    Oct 2, 2015
    Long Island
    2004 340 SeaRay Sundancer
    Twin 8.1 Merc
    Did it fix it?
     
  16. R-and-R

    R-and-R Member SILVER Sponsor

    54
    May 9, 2018
    2002 340 Sundancer
    8.1S Mercruiser
    It DID! Easiest and cheapest repair of the year.
     

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