Mercruiser alpha gen1 intermittent knock

[goodboy]

I have a knock coming from my outdrive... I can feel it when I put my hand on the exhaust pipe but not when I put my hand on the engine... ItsI most noticeable at idle and in gear or out makes no difference... I originally thought something was rattling against the transom, it sounds like a fishing pole hitting the transom...

It's a 91 4.3 mercruiser

Any help would be appreciated...

 

[Lou C]

Could be an exhaust flapper inside the Y pipe. They have rubber coatings on them but maybe it wore off or melted a bit if there was ever and overheat.

 

[Bondo]

ItsI most noticeable at idle and in gear or out makes no difference... I originally thought something was rattling against the transom, it sounds like a fishing pole hitting the transom...

Ayuh,..... Drain the drive oil, 'n look for metal in it,..... A chipped tooth in the upper can do that,....

Burnt up exhaust shutters usually only click at idle,....

 

[goodboy]

Thanks for the responses...

It​​​ definitely only makes the noise at idle... And is very inconsistent as can be heard in the video...

I like the idea of a bad flapper...

Don't really like the thought of chipped gears lol...

Wouldn't a gear problem have a more consistent noise?

 

[QBhoy]

Really sounds like an exhaust flapper. Often diagnosed positively by the noise going away when revs are increased faster than idle or at least 1000rpm min.

 

[goodboy]

Thanks again...

Listening to some other videos it seems like the flappers for sure...

Very happy that its not something more expensive...

My boats first year outside, its usually in my heated shop for the winter, and although i winterized it I was still worried... Looks like flappers aren't expensive... And my biggest expence this year is going to be replacing the stereo that didn't survive the freezing temps and no longer streams bluetooth...

 

[sogood]

Thanks for posting that, including the accompanying video. I was just about to ask the same question as I also have a similar noise, but more of a rhythmic "clicking" with the occasional "clack" coming from somewhere around the drive area. It's consistent under idle, whether in or out of gear. I had replaced the exhaust flappers as part of my overhaul so I don't suspect them,but part of one of the old disintegrated flappers remained absent. I had checked the "Y" pipe as best I could at the time and didn't find anything.
I've since gotten an inspection camera so I'm going to look again. I used a screwdriver as a stethoscope on various parts of the engine (5.7 Merc) and it's running sweetly and quietly. I also did the same on the leg and no noise or vibration to report there. No metal present when I drained and replaced the drive oil either.

Apart from a flapper issue, what else could this be? Thanks in advance.

 

[goodboy]

From what I've read ujoints can also make noise... And wouldn't show metal in the oil... Something to consider checking out

 

[sogood]

From what I've read ujoints can also make noise... And wouldn't show metal in the oil... Something to consider checking out

I've been doing some googling and came up with the same diagnosis. I won't be back at the boat until Thursday but can look closer. Apparently, moving the leg from side to side or up and down (but not up fully) can cause the noise to come and go, which is a pretty good pointer. Thanks for the input and I hope you get it sorted.