Mercruiser 3.0 strange sound at low end acceleration

[qrntna]

Hi all,

I have been struggling with a strange sound coming from my 2003 mercruiser 3.0 (serial begins 0m64). I bought the boat last summer and this problem started near the end of the year. Took the boat out first time this year today after changing all the oil and spark plugs in off season but getting some of the same issue. I have read a LOT of threads around here but have not been able to find one I can confirm is like this. Thanks in advance for your help!

1. In video attached, the sound is a kind of hollow tinny somewhat high-pitched rattling
2. Only seems to happen in forward, in between idle (which is quiet) and full throttle (also quiet)
3. It does not happen all the time - maybe about half of the times after I start boat and put it into forward gear, but has typically started in the middle of trips out, not right when we first start it for the day
4. After the sound happens upon acceleration, I can usually shift back between neutral/reverse/forward a few times and get it to engage and not make the sound; if successful, then it won’t make the sound again so long as the boat stays on and in forward gear
5. Not sure if this is related, but sometimes when trimming while driving we will hear this or a very similar noise which begins right when we begin trimming up (my sense is it happens more for up than down but not positive on that)

Any thoughts on what could be causing this? I am struggling to find mechanics with availability here in Tahoe and would appreciate any opinions on how concerned I should be in the meantime.
Thanks much,
Matt

 

[Scott06]

I wonder if it is detonation. Id see if the spark plugs look lean, you can find some photos by goggling how to read spark plugs. If they are looking lean set the ignition timing and check fuel delivery, fuel filter verify fuel pressure and rebuild carb

 

[alldodge]

What maintenance has been done?
Gear lube changed?
Is the lube at correct level?

 

[qrntna]

Thank you both.

I did change the gear lube in the fall before I put it away for winter. Putting in the recommended amount of gear lube at the lower unit left the level at the monitor in engine bay on the lower side of the “add” marker, but not by much. After running the boat for a few hours yesterday first time out since the change, I see the level dropped a little bit further, so I added more to get that in the operating range.

New spark plugs just put in this week look pretty OK to me from what I have read online about reading them. The ones I took out were all quite black - I don’t know when they had been replaced last. Here is a few pictures of the new plug I just checked in case helpful to get an opinion here.

Beyond the oil, gear lube, spark plug change I have done, I am not aware of any other recent service. According to a mechanic from late last summer I do need to eventually replace the shift cable. The kill associated with the shift was getting triggered when shifting, and the adjustment in the cable was tapped out, so he disabled the kill temporarily. I’m not sure why that would lead to this sound being made, but that is my only other thought on service history.

 

[alldodge]

Plugs look ok
Pull the top vent plug from the drive just enough to see if clean gear lube comes out

 

[qrntna]

Gear lube does look pretty clean (at least after removing some grime from around the vent plug). The boat has only been out once since I changed gear lube in the fall.

 

[nola mike]

Hard to diagnose noises on video, but sounds like a gear grind almost, or can of marbles? Wondering if it's a bad alt/water pump/ps bearing? Could disconnect any relevant belts and turn by hand

 

[nola mike]

Does it make noise on muffs?

 

[qrntna]

Does it make noise on muffs?

I have never been able to get it to make the sound on muffs. Seems to only happen when engaged in forward.

I had considered whether it could be related to the alternator. When we bought the boat we hadn’t realized the serpentine belt was broken in the bottom of the engine bay (realized it a dead battery later...). The noise didn’t start until after the belt was replaced BUT what stumps me on it is the noise doesn’t seem to be primarily associated with when that belt is spinning. It specifically kicks in when the boat is accelerating in the low end, not wide open. And sometimes when that belt runs it’s perfectly quiet.

It does sound to me kind of like a gear grinding mechanical type of noise, but I don’t know if that’s worth much. Maybe a bit like marbles in a can but more of a high-pitched tinny screeching sound than most other videos I have come across.

 

[nola mike]

The serpentine belt is always spinning. Try disconnecting it, spin the accessories by hand, and run it a bit without the belt and see if it still makes the noise

 

[alldodge]

Gear lube does look pretty clean (at least after removing some grime from around the vent plug). The boat has only been out once since I changed gear lube in the fall.

Agree lube is good.

 

[bbook83]

I have an older 3.0 that made a similar noise that would come and go as you have described. It went away when I removed the remains of the exhaust flappers and replaced them.

 

[Rick Stephens]

Yup, as bbook83 said, exhaust flappers that get a little overheated will burn the rubber off and they will make some of the strangest tinny sounds as they rattle when running near idle. Your plugs look great..

 

[nola mike]

Yup, as bbook83 said, exhaust flappers that get a little overheated will burn the rubber off and they will make some of the strangest tinny sounds as they rattle when running near idle. Your plugs look great..

Idk, that's awfully loud for burnt flappers

 

[qrntna]

Wanted to close the loop here in case it can help anyone else -

I eventually brought it by a mechanic who offered to look at it for free. It was just a loose alternator belt. Feel like an idiot for not being able to figure that out myself! Humbling I guess…

Also shout out to the mechanic who charged me an arm and a leg to originally diagnose and replace the (then missing) belt, only to leave it loose to put me on this rabbit hunt

you learn something everyday I guess!

 

[Scott06]

Wanted to close the loop here in case it can help anyone else -

I eventually brought it by a mechanic who offered to look at it for free. It was just a loose alternator belt. Feel like an idiot for not being able to figure that out myself! Humbling I guess…

Also shout out to the mechanic who charged me an arm and a leg to originally diagnose and replace the (then missing) belt, only to leave it loose to put me on this rabbit hunt

you learn something everyday I guess!

Well at least it was something simple. Unfortunately on the 3.0 the belt gies through the front engine mount.