I have a Mercruiser 496 Mag HO 8.1l in my boat.
When the engine is cold it runs perfect. But when the temperature rise above about 60’C (when the engine starts to get warm) the idle surging/fluctuates. The problem ONLY comes when the engine is warm.
It doesnt matter if it runs at idle speed or if I put the rpm 2500 for example, it still surging/fluctuates.
(I have videos of the problem but I cant post the youtube links here yet.)

I have tried to find threads about this problem, but havent found any concrete answears about my problem.
It also makes a air sucking sound from the throttle body. I tried to find some answer about it and read that it could be the IAC muffler.
A new IAC muffler is on its way home, ordered one yesterday.

But what can cause the surging/fluctuating idle? I cant find any good answers.
Can it be the IAC muffler causing the problem if the ”filter” is bad or gone?
Ore if the IAC is broken?
Can it be the intake manifold gasket letting air throug it?

Thanks!! /Johan

Note: I havent run the boat in the water when I tested this. Just in neutral on land at idle.

Sorry, my bad.
I dont have that kind of scanner. Is there one typical scanner you must have to test it?

When im looking in to my throttle body, there is no IAC muffler there. Its gone.
When I read threads about similar problems there is people talking about IAC and IAC muffler causing the problems and when they changed the IAC or IAC muffler the problems went gone. Therefore I wondering about the IAC muffler.

Okey! I have bought a new muffler to put in place, there was a loud air sucking sound from the throttle body.
When I look at my IAC it looks old to me.
If the IAC is broken, can it cause this problem with the surging idle when the engine gets warm?

Okey perfect thank you. No need to change the IAC then.
Is there a special scanner or dose it work with any kind of scanner?

Okey, there is place for a muffler in my throttle body.
But its gone. No muffler in the IAC either..