Rough Cold Idle - Mercruiser
 

Hey guys, wanted to run this by y'all.

2006 Centurion Elite V
Mercruiser 350 mag mpi

When cold, boat has a rough idle that surges a bit and makes the boat difficult to run for a few minutes until it warms up. Symptoms are at idle only. With some throttle, she runs smooth. As soon as she drops back down to idle, rough.

Once warm, idle is PRETTY good. Maybe not perfect, but good.

So, Idle Air Control Valve? I don't want to spend $80 on parts without some confidence. Do you all concur?

I'm having the same problem with mine. Not sure what it is either. I changed iac and it didn't do anything

Injector cleaner or replace the injectors. You may be able to bring them back with fuel additives, but if the deposits are too heavy, plan on putting in new injectors. They are fairly inexpensive and easy to change (especially compared to diesel injectors). If you are not throwing codes, I doubt it is the ECU.

When was the last time you checked your plugs, ignition or the ignition wiring for moisture?

Have you checked your fuel for moisture and when was the last time you change the fuel filter. Fuel with too much ethanol left sitting too long can cause this as well as vapour lock.

Fuel injectors? Really? I'm surprised to hear that suggestion.

Ignition not been checked but I'm not sure that the symptom add up.

I put a lot of gas through it, I doubt it's excess ethanol.

Remember, its only at idle when not warmed up.

Anyone know how to test the IAC?

Did some research this evening, sounds like it could possibly be a simple clogged IAC muffler. I hope it's that easy. Will look at it in detail tomorrow evening and report back.

I thought you would have already pulled the foam out to check flow. You can check the duty cycle of the IAC during start and warm-up with a good scan tool. After a few minutes warm-up the duty cycle will be significantly lower (20-30%). The IAC will usually throw a code hard or soft, depending on the condition of the system. However, if the IAC muffler is clogged or melted, the IAC may not read correctly. You can run without the foam if you find it clogged or damaged, it will just be noisier.

Yes, really. It is frequent to have idle issues and not have them at higher RPM, just like the IAC. This is no different that you might see in an auto. Boats are usually low use engines and fuel fouling can happen (as opposed to worn out injectors). No one knows how you run your boat better than you. The same goes for regular maintenance. If you are running a lot of fuel through it, then this may not be an issue for you. For others, fuel stabilizer can be your best friend. I hope it is the simple 5 minute fix.

thanks for the response! This process would be much easier if I had to scan tool to diagnose.

The problem could also be a the ECT sensor. I REALLY wish that I could see the duty cycle to see if the IAC is struggling.

You assumed too much ;)

I'll keep the Fuel Injector thought in my mind. If nothing else fixes it before winter, then I'll pull them out and have them looked at.

All fixed. Ran great on the fake a lake once the iacv muffler was pulled

Pic of nasty muffler attached

i have same problem, I know where the IAC is, and have replaced, and still same problem. Where is the IAC muffler located?

It's inside the throttle body. Pull off the air filter and look directly where the IAC bolts onto the throttle body. You'll see it.

thx!