intermitten bumping / vibration feeling from lower unit

[Augoose]

This is a Alpha 1 on a '89 4.3 Mercruiser.

Sorry if this has been covered before, but after searching, I wasn't successful in finding a solution.

We were out today and everything was great for a couple of hours. We went tubing and skiing for about 30 mins or so and after the first tube went around we were idling for a second when I noticed a vibration and a dull "bump, bump, bump, bump" coming from what I believe was the lower unit. Almost felt like an out of balance drive shaft in a truck. We immediately shut down thinking the prop was bumping off of a submerged log or some other obstruction. I dove down under the boat - nothing there. No line wrapped around the prop and it spun freely when placed in neutral. Unsure of what happened, we started it back up and slowly idled around with no sound nor any vibration. When the bumping and vibration occur, its a very heavy bumping and vibration if you can imagine that. Its not a sharp bump or a knock - it really feels like the propeller is slowly hitting a old rotten log under water is the best way I can describe it.

Everything was good for another 10 minutes or so of tubing / skiing when once again while idling around, the vibration came back along with the "bump, bump, bump, bump" sound. The sound was quick, about 4 bumps per second and it really felt like we had run aground in soft mud and the prop was just bouncing (we were in 100+ feet of water). Again, immediately shut down, pulled the dog house and looked for anything obvious - nothing.

Concerned, we slowly drove back to the dock, a 20 minute trip. No noise, no vibration, nothing - even changing speed and turning back and forth the whole way back to the dock. As we approached the dock and we slowed down to idle, the vibration and bump sound came back. Even in reverse, it made the same bumping sound and vibration. After about 5-6 seconds, it went away on its own. I put the boat on the trailer and took it to the house to inspect it.

The gear oil has been checked recently, full level, no water in it, nothing abnormal. The engine temp was steady at 150 degrees or so. The prop spins normally in neutral, ratchets when you turn it in gear one direction and locks if you try to turn it in gear in the other direction, doesn't feel like it has any abnormal play if you pull it forward and back, and at speed the boat feels and drives normally.

Recent service at the start of summer included all new bellows, new gimbal bearing, new transom seals, lower shift cable replacement, and that's about it. No leaking fluids that I can see.

I'm very careful to only operate the boat with the lower unit all the way down so as not to stress the u-joints, but could that be what my problem is?

I have the boat in the garage in hopes of identifying the issue tonight before labor day arrives.

Thanks!

 

[Bt Doctur]

Re: intermitten bumping / vibration feeling from lower unit

1 Have you ever hit anything?
That sounds like a chipped pinion gear if coming more from the lower unit.

 

[Augoose]

Re: intermitten bumping / vibration feeling from lower unit

1 Have you ever hit anything?
That sounds like a chipped pinion gear if coming more from the lower unit.

Nope, never hit anything and this seemed to start all of a sudden. Goes away and then seems to come back intermittently - but I've only noticed it at just barely above idle speed. Never felt it occur at any other speed.

Thanks for the help - any other thoughts?

 

[dubs283]

Re: intermitten bumping / vibration feeling from lower unit

don't know if i would condemn the pinion gear at this point

lets start small and work our way up

it is possible that one of the blades of the prop is bent slightly and not noticable at first glance or even to the naked eye

and you may only notice the vibe/noise at certain speeds becuase the faster/slower the prop spins may alleviate some force

has the prop ever been re-built and have you tried a different prop??

also, does the noise only occur when in gear??

 

[Augoose]

Re: intermitten bumping / vibration feeling from lower unit

don't know if i would condemn the pinion gear at this point

lets start small and work our way up

it is possible that one of the blades of the prop is bent slightly and not noticable at first glance or even to the naked eye

and you may only notice the vibe/noise at certain speeds becuase the faster/slower the prop spins may alleviate some force

has the prop ever been re-built and have you tried a different prop??

also, does the noise only occur when in gear??

Thanks for the reply!

The prop appears fine, despite having a couple small nicks that its always had. I took the prop off a few minutes ago looking for anything obvious. As for the vibration at certain speeds, it does only occur at around idle speed but it doesn't occur every time I'm at idle - weird.

Unfortunately I don't know the history of the prop and its currently the only one I have - need to pick up a 2nd / backup.

Yup, the noise and vibration only occurs in gear. I was just explaining it to someone over the phone and really the best way I can explain it is that it sounds like there is a old rotten log following me around beneath the boat and the prop is just whacking the heck out of it. Then if I speed up just a little bit, like 3-4 mph, its immediately gone. Maybe I'm outrunning the log at that point....

 

[dubs283]

Re: intermitten bumping / vibration feeling from lower unit

might want to pull the drive and inspect the gimbal bearing and the u-joints

if its been a while since service on those parts, may be one of em telling you it needs some attention

also check the engine alignment while the drive is off and the condition of the engine coupler and all bellows

 

[Augoose]

Re: intermitten bumping / vibration feeling from lower unit

might want to pull the drive and inspect the gimbal bearing and the u-joints

if its been a while since service on those parts, may be one of em telling you it needs some attention

also check the engine alignment while the drive is off and the condition of the engine coupler and all bellows

I was hoping the suggestion to "pull the drive" was not in my future as that is well past my ability! If that is the recommended course of action, then its off to the shop. The gimbal bearing was replaced in April of this year and the ujoints were also inspected. Certainly doesn't mean they haven't bit the dust since then.....

Thank you for the help - any other thoughts of things a shade tree auto mechanic could check?

 

[dubs283]

Re: intermitten bumping / vibration feeling from lower unit

if pulling the drive is beyond your scope then that is about it

take it to a reputable shop and let them handle it

i'd be curious to know if the gimbal bearing that was installed was a new style "maintenance free" one, these are not greasable, and if so the condition of it after one season of boating

you can look for yourself to see if they blocked off the grease passage with a torx head screw, it is located on the starboard side of the transom plate/gimbal housing

if there is a grease zirc there, they either didn't block it off with the new style bearing installed or you have the "servicable" bearing

 

[Augoose]

Re: intermitten bumping / vibration feeling from lower unit

if pulling the drive is beyond your scope then that is about it

take it to a reputable shop and let them handle it

i'd be curious to know if the gimbal bearing that was installed was a new style "maintenance free" one, these are not greasable, and if so the condition of it after one season of boating

you can look for yourself to see if they blocked off the grease passage with a torx head screw, it is located on the starboard side of the transom plate/gimbal housing

if there is a grease zirc there, they either didn't block it off with the new style bearing installed or you have the "servicable" bearing

I ordered the gimbal bearing from Iboats, but I don't recall which type it was. Regardless, I did pump some grease in there and looked for grease to start squeezing out anywhere - after about 15 pumps, I didn't see anything, so maybe its a maintenance free version?

Thanks again for the help. Hopefully someone might have some more ideas of things to check

 

[dubs283]

Re: intermitten bumping / vibration feeling from lower unit

without the drive off, you will never be able to tell if the bearing is accepting grease

as far as more ideas, without troubleshooting further and inspecting for worn/broken parts, you're kinda stuck

wish i could be there to lend a hand and check stuff out!!

 

[Augoose]

Re: intermitten bumping / vibration feeling from lower unit

without the drive off, you will never be able to tell if the bearing is accepting grease

as far as more ideas, without troubleshooting further and inspecting for worn/broken parts, you're kinda stuck

wish i could be there to lend a hand and check stuff out!!

Hey thanks, I appreciate it.

When the gimbal bearing fails, what are the typical symptoms? Its odd that this only occurs intermittently at idle speed and not at any higher speed.

 

[dubs283]

Re: intermitten bumping / vibration feeling from lower unit

when the bearing fails/is becoming worn, you will usually hear a growling noise coming from the back of the engine

i have also heard knocking noises similar to u-joint wear and sometimes only when the drive is in gear - i can only relate this to extra force being exerted on the drive shaft when in gear

as long as the symptoms you are experiencing are not severe i would not be worried about major damage, even with the intermittent symptoms

this is definitly something you want to get checked out before using the boat again, want to nip this in the bud

take it to a reputable repair shop that you trust and be sure to have somebody go through and explain what they find, most places/people are happy to explain

 

[Augoose]

Re: intermitten bumping / vibration feeling from lower unit

when the bearing fails/is becoming worn, you will usually hear a growling noise coming from the back of the engine

i have also heard knocking noises similar to u-joint wear and sometimes only when the drive is in gear - i can only relate this to extra force being exerted on the drive shaft when in gear

as long as the symptoms you are experiencing are not severe i would not be worried about major damage, even with the intermittent symptoms

this is definitly something you want to get checked out before using the boat again, want to nip this in the bud

take it to a reputable repair shop that you trust and be sure to have somebody go through and explain what they find, most places/people are happy to explain

Sounds good, and thanks so much for all the help!

 

[Augoose]

UPDATE! Re: intermittent bumping / vibration feeling from lower unit

UPDATE! Re: intermittent bumping / vibration feeling from lower unit

Fixed. Had a u-joint that turned to dust.

Thanks all for the assistance and sorry to doubt the suggestions