Rough Idle on Merc V8 inboard

[MrG]

Hey, I have posted about this on previous threads but wanted to update with more info. I have a 2575 QCW with a Merc 350 v8
We have had a rough idle for sometime that is an intermittent problem. We’ve done all the normal things like fuel, injector cleaner and tuneup, spark plugs, and even replace the “Idle air filter” this is the small air filter that takes in air while the boat is under idle. Most recently we removed all of the fuel injectors and tested them, and they were all within spec.. we were hoping that we had a dirty fuel injector and it would present itself, but this doesn’t seem to be the case. I was wondering if anyone else knew of somewhere to check the Mechanic seems to think that we might try swapping out a motor control unit or ECM.

 

[BigKahuna]

Hey, I have posted about this on previous threads but wanted to update with more info. I have a 2575 QCW with a Merc 350 v8
We have had a rough idle for sometime that is an intermittent problem. We’ve done all the normal things like fuel, injector cleaner and tuneup, spark plugs, and even replace the “Idle air filter” this is the small air filter that takes in air while the boat is under idle. Most recently we removed all of the fuel injectors and tested them, and they were all within spec.. we were hoping that we had a dirty fuel injector and it would present itself, but this doesn’t seem to be the case. I was wondering if anyone else knew of somewhere to check the Mechanic seems to think that we might try swapping out a motor control unit or ECM.

When we had our I/O we went through a period when we had a rough idle. We did everything you did but when we changed the distributor cap and spark plug wires that smoothed everything out! Worth checking out!

 

[Remediation]

If it uses the flat distributor cap, they are notorious for going bad and aftermarket caps can be bad out of the box. I have been through that problem a couple years ago.

 

[SEMPERFI8387]

If it uses the flat distributor cap, they are notorious for going bad and aftermarket caps can be bad out of the box. I have been through that problem a couple years ago.

I’ve also heard certain color caps have more issues than others. As a mechanic I’ve never tracked that theory but it makes sense as a “virgin” plastic would be stronger than a recycled plastic.

 

[69jbjohn]

Don't forget spark plug wires.........................ohm them out , or replace.