2001 4.3 Gi rough idle, occasional (brief) RPM drop at cruise

[GHBAY]

Hi all

picked up my boat from the shop recently after having additional work done. The motor was running like a top when it when in, now it is running very rough at idle, and I have noticed on 3 occasions, a very brief drop in revs at cruise- as if I am pulling back the throttle a couple notches and immediately putting it back up. The engine was pulled out to perform the latest round of hull work.

Starts just fine. new plugs, oil, fuel filter. all new fuel, fuel lines, etc.

I was told my IAC probably went bad (randomly) and that would explain the idle issues. In fact, plugging and unplugging it while at idle had no effect at all. I am not sure that explains the hiccups while at cruise though.

What else might explain both symptoms?

Motor has 300 hours.

any advice/input would be helpful. for a great number of reasons, I have zero confidence in the shop that had been doing the work at this point and want to avoid taking the boat back there to be diagnosed and fixed.

thanks

 

[alldodge]

Hi all

picked up my boat from the shop recently after having additional work done. The motor was running like a top when it when in, now it is running very rough at idle, and I have noticed on 3 occasions, a very brief drop in revs at cruise- as if I am pulling back the throttle a couple notches and immediately putting it back up. The engine was pulled out to perform the latest round of hull work.

Starts just fine. new plugs, oil, fuel filter. all new fuel, fuel lines, etc.

I was told my IAC probably went bad (randomly) and that would explain the idle issues. In fact, plugging and unplugging it while at idle had no effect at all. I am not sure that explains the hiccups while at cruise though.

What else might explain both symptoms?

Motor has 300 hours.

any advice/input would be helpful. for a great number of reasons, I have zero confidence in the shop that had been doing the work at this point and want to avoid taking the boat back there to be diagnosed and fixed.

thanks

No Volvo dude but here is my opinion if it was a Merc. When folks pull engines there is no telling what was accidentally pulled off during the process. Look all over including under the engine. My first thought would be a vacuum line was pulled loose. The IAC should effect the idle, and since it's not again I'm thinking a vacuum leak. When an IAC fails most often the engine will not start unless throttle is given to give it more air. If the vacuum line is off your fuel pressure regulator you could have to much pressure which can cause some of you symptoms.

 

[Bondo]

Hi all

picked up my boat from the shop recently after having additional work done. The motor was running like a top when it when in, now it is running very rough at idle, and I have noticed on 3 occasions, a very brief drop in revs at cruise- as if I am pulling back the throttle a couple notches and immediately putting it back up. The engine was pulled out to perform the latest round of hull work.

Starts just fine. new plugs, oil, fuel filter. all new fuel, fuel lines, etc.

I was told my IAC probably went bad (randomly) and that would explain the idle issues. In fact, plugging and unplugging it while at idle had no effect at all. I am not sure that explains the hiccups while at cruise though.

What else might explain both symptoms?

Motor has 300 hours.

any advice/input would be helpful. for a great number of reasons, I have zero confidence in the shop that had been doing the work at this point and want to avoid taking the boat back there to be diagnosed and fixed.

thanks

Ayuh,.... Water, 'n Crud in the fuel,.....

Check the Contents of Yer filters,....

 

[John2037]

pretty sure once it's running the IAC won't do anything if you unplug it... IAC's do get dirty, so you could pull it and clean it up and while youre at it the rest of the TB. Also, theres a clip holding the FPR (fuel pressure regulator) - pull it and inspect the FPR. ^ probably crud in it. If it sat for awhile, you could have fuel pump issues too.