97 4-Winns Horizon RX, Volvo Penta 4.3GL/SX PLKD (Gray) - fluttering sound

[turnersc]

A couple of weeks ago I noticed what I can best describe as an intermittent fluttering sound coming from the motor (see link to audio file below). I also noticed that the temps were running within the normal range, but not always dropping down when the boat was idling or when we were moving through the water. The temps were running in the 140-160 range.
Last year I replaced the impeller on the raw water pump after the old impeller came apart. I then pulled all the hoses to make sure none of the vein's were clogging up the water passages. I also replaced the thermostat at this time. The boat ran and cooled fine for the rest of the season.
So after hearing the fluttering this year, I've checked the belts and tried to listen to the raw water pump, alternator, circulation pump, power steering pump to see if it's a bearing going bad and the sound does not seem to be coming from one of these. The more I listened it really seems to be coming from the engine toward the stern. I then pulled the plugs and they all looked normal for their age and uniform in appearance across all six, so I don?t think it?s a cylinder issue. I?ve got new plugs and plan to put them in today.
Here is a link to an audio recording of fluttering sound [url]https://drive.google.com/open?id=0B7VRkaxKFwLNVjRsbk56RDAwYW8[/URL]
The flutter starts right off the bat and cuts in and out. It stops from :07 seconds to about :18 seconds. It?s the loudest in the 20?s thru the end of the video.
So I am not sure where to go from here. Any suggestions would be greatly appreciated.

 

[Maclin]

Welcome Aboard!

I couldn't play the video, but will put a guess out there for you. It could be the exhaust flappers. They are there to keep water from whooshing back up the exhaust after an abrupt stop at speed. Later on Volvo stopped putting them in, this was around '98-'99 I think. They would be in the top of the exhaust Y-pipe. You can get at them by removing the large rubber exhaust hoses that are between the riser and the Y-pipe. On some engine configurations some opt to take the riser off to have more room.

In case that it what it is, there is a Volvo bulletin on how to remove them and plug the holes for the hinge pins, as Volvo started thinking they were not needed in later models. You can also replace them. If they are bad and do not look like they are intact then they are at the bottom of the Y-pipe, and may be accessible by removing the drive, and fish them out that way. What happens is the engine gets run with little or no water flow through the exhaust and the rubber parts melt away. Or just aging.

 

[turnersc]

Thanks for the welcome and the great advice. Sorry about the link to the recording not working. I called it a video in the post when it's actually an audio file, if that makes any difference. I've never heard of this issue and it makes sense based on where the sound seems to be coming from. I looked up the exhaust flapper issue in the forums and there is a wealth of info and discussion on them and what to do as well as the service bulletin you mentioned. I'll grab a dowel & stethoscope and see if I can localize the flutter to the riser/y-pipe area or the drive. I don't think it's running hot enough for me to be overly concerned about using it a little, but knowing the source of the flutter gives me a bit more peace of mind. I've got a 15 year old who just learned to waterski and he's getting impatient with me as the summer draws to a close. Plus we live on a small lake so the motor is never run for very long. I'll plan to get into the drive to look for the flapper parts when I winterize it, which in Michigan is not that far off. Thanks again for the help and I'll post an update regarding the ultimate resolution of the issue.