Bravo III Rattle uner load

[shawnmd]

I have a 96 Chap with a 5.7lx and Bravo 3. I just put it in the water today. The boat ran great for the first hour at a slow speed (about 7 knots, 2000 rpm), then when I increased throttle to about 3000-3500 while pulling my son on tube, I began hearing a loud rattle (like marbles in a jar, not a nice sound). At first it sounded like a rocker arm rattle, but it was coming from the drive. I stopped, put it in neutral and revved the engine to 3500, and didn't hear any rattle, sounded perfect. I put it in gear again and went to 3500 under load and the rattle was back. I then trimmed the drive up a bit and the rattle stopped under load. Since a trimmed the boat that time, the rattle has for the most part stopped. I can still hear it very faintly under load.

Again, no noise in neutral, and trimming the drive seemed to have quieted the rattle some. Just before putting the boat in water, I changed drive fluid and greased the gimble bearing. Just last year, I replaced the drive bellows.

Does this sound like a gimble bearing going, or a universal?

 

[alldodge]

Does this sound like a gimble bearing going, or a universal

Yes either one or both

 

[CamaroMan]

Hi - having the same issue, installed new bearing and ujoints, same thing. havent tried revving in neutral but surely the bearing/joints are spinning either way? unless in water with prop load i cant see how testing on muffs will make a difference?

 

[shawnmd]

Hi - having the same issue, installed new bearing and ujoints, same thing. havent tried revving in neutral but surely the bearing/joints are spinning either way? unless in water with prop load i cant see how testing on muffs will make a difference?

I never thought of that. Even at idle (revving to 3500), the shaft is still spinning, so why wouldn't it make the noise then? I did use a different gear fluid this time. I used Sierra Hi-Performance Synthetic Blend Lower Unit Gear Lube. I only ever used Mercury High
perf fluid. Could that be it?

 

[alldodge]

so why wouldn't it make the noise then?

Because the drive has no load on it. The prop puts a more of load on the parts

 

[Scotvl]

I never thought of that. Even at idle (revving to 3500), the shaft is still spinning, so why wouldn't it make the noise then? I did use a different gear fluid this time. I used Sierra Hi-Performance Synthetic Blend Lower Unit Gear Lube. I only ever used Mercury High
perf fluid. Could that be it?

I would say maybe because I thought you weren't supposed to use anything except the mercury high performance gear lube which I think is synthetic.

 

[shawnmd]

OK, I may have found what is causing the rattling noise....One of the lift ram anodes was missing a bolt, and was hanging on by a thread on the other bolt. At least I hope that is what is rattling. I did some testing tonight, and from what I can tell now, the rattling doesn't dissipate when turning left or right or trimming the drive up or down (like I thought). It just comes and goes at high rpm under load. Sometimes it does it, sometimes it doesn't. So I am really hoping its that rattling anode.

 

[shawnmd]

Update......The ram anode was not the problem. I also pulled the drive and replaced the Gimball bearing. The universals looked perfect and turned freely. Put the boat back in the water, and the noise continues.

I should mention, before all of this, and simply for preventative maintenance, I just replaced the manifolds and risers this past spring. Could that work have created an issue?

So I have determined that lifting, lowering and turning does not affect the noise, and it only occurs under load, at about 2800 rpm and up. I have attached a video of the noise. Sorry about the wind noise at the beginning. You will notice that there is not rattle until i raise the rpms, right around the 16 sec mark.

https://youtu.be/xRLldix9kmE

 

[alldodge]

Guess my ears or speakers are not that good but I'm not hearing anything that would concern me.

 

[shawnmd]

Guess my ears or speakers are not that good but I'm not hearing anything that would concern me.

Have to wait until the 16 sec mark, can definately hear it....

 

[shawnmd]

I am thinking it is detonation, and may be related to the knock sensor (disconnected or faulty).

 

[Bondo]

I began hearing a loud rattle (like marbles in a jar, not a nice sound).

Ayuh,.... Probably detonation, induced by bad fuel, or a lack of it,....

 

[harleyman1975]

I agree I think it is detonation or "ping".

 

[shawnmd]

Well, we have a winner. Did a compression check on all cylinders, 6 were between 130 and 150 psi. However, cylinders 4 and 6 were 25 and 30 psi. Squirted a bit of oil in both, and didn't see any major difference in compression. Hoping it's just a head gasket......

 

[CamaroMan]

Yeah - I resolved it too, timing mark was incirrect on mates boat. Used a piston stop to verify - set timing and it went away.. timing shd be around 8-10 degrees..