|
Surging idle Mercruiser 496 Mag HO 8.1
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. |
Sometime you need to give the folks time to answer
https://www.offshoreonly.com/forums/...problem-2.html In the other thread you have coolant in the oil, If there is a intake problem this can also impact idle and running. The motor needs to be hooked to a scanner to get some readings Since it starts after warmed up, I would be leaning toward the MAP. Once the motor warms up the ECT tells the PCM to lean the fuel mix. The IAC and muffler will have next to no effect at higher rpms. |
Originally Posted by AllDodge
(Post 4709681)
Sometime you need to give the folks time to answer
In the other thread you have coolant in the oil, If there is a intake problem this can also impact idle and running. The motor needs to be hooked to a scanner to get some readings Since it starts after warmed up, I would be leaning toward the MAP. Once the motor warms up the ECT tells the PCM to lean the fuel mix. The IAC and muffler will have next to no effect at higher rpms. 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. |
IAC mufflers can get clogged up depending on the micron rating, Ive heard its ok to run without them but they were put in place to muffle the sound of the IAC motor, Id replace it.
|
Originally Posted by F-2 Speedy
(Post 4709697)
IAC mufflers can get clogged up depending on the micron rating, Ive heard its ok to run without them but they were put in place to muffle the sound of the IAC motor, Id replace it.
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? |
The sucking noise means the iac is working, install the muffler and it will quiet down, you need a scanner to trouble shoot this correctly as AD alluded to
|
Not all years had the iac muffler. If it had one it might have been sucked into the iac.
|
Originally Posted by F-2 Speedy
(Post 4709706)
The sucking noise means the iac is working, install the muffler and it will quiet down, you need a scanner to trouble shoot this correctly as AD alluded to
Is there a special scanner or dose it work with any kind of scanner? |
Originally Posted by underpsi68
(Post 4709707)
Not all years had the iac muffler. If it had one it might have been sucked into the iac.
But its gone. No muffler in the IAC either.. |
| All times are GMT -5. The time now is 02:15 AM. |
Copyright © 2026 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.