What causes this knocking tapping sound?

[iwknow]

Hi engine experts!
I have a mercruiser 3.0 (181) that runs without any issue except a knocking/tapping sound near 1200 to 1600 rpms. As you can hear from the video, the sound is barely heard at idle. The sound become audible when I increase the throttle a little bit and barely audible at higher rpm. Is this a rod knocking? or something else?
I cleaned the valves with sea foam and also added it to oil for cleaning and lubrication.

 

[Bondo]

Ayuh,..... Welcome Aboard,..... Is the drive up, or down, when this happens,..??..??

 

[dubs283]

Wondering along with being up/down if the drive is turned hard over port/stbd?

On the trailer, muffs, no load? In the water under load?

Could be any number of causes from having the drive tilted up to detonation

 

[kenny nunez]

Is the noise the same with the boat in the water and in forward gear ?

 

[Scott06]

Ayuh,..... Welcome Aboard,..... Is the drive up, or down, when this happens,..??..??

My guess would also be u joints and drive is all the way up.

If drive is down you could remove drive and test with drive off

Also 3.0's are not the quietest engine always a little tappy sewing machine like on the one I had for years.

 

[iwknow]

Ayuh,..... Welcome Aboard,..... Is the drive up, or down, when this happens,..??..??

Thanks for answering! drive is up. it is most audible around 1200 to 1600 rpm, both accelerate or decelerate.

 

[iwknow]

Wondering along with being up/down if the drive is turned hard over port/stbd?

On the trailer, muffs, no load? In the water under load?

Could be any number of causes from having the drive tilted up to detonation

Thanks for answering! By "drive" do you refer to the sterndrive? if yes, the sterndrive was all the way up. Not turned to any direction.

On the trailer, with muffs, running garden hose water, no load.

I am a new boat owner, wondering why tilted drive can cause this noise?

 

[iwknow]

My guess would also be u joints and drive is all the way up.

If drive is down you could remove drive and test with drive off

Also 3.0's are not the quietest engine always a little tappy sewing machine like on the one I had for years.

Hi scott.
Thanks you very much for answering my question.
Do you know the typical symptom of u joint failure? Yes, the drive was all the way up. I wonder why the sterndrive position has anything to do with this noise.

 

[todhunter]

When the drive is all the way up, the u-joints will turn the shaft, but they're under a less than ideal angle, and they can make noise. Try it again with the drive lowered down and report back with whether the noise is still there or not.

 

[airshot]

Hi scott.
Thanks you very much for answering my question.
Do you know the typical symptom of u joint failure? Yes, the drive was all the way up. I wonder why the sterndrive position has anything to do with this noise.

The drive should never be all the way up when running an I/O, puts undo stress on U joints and can cause early failure.

 

[Scott06]

Hi scott.
Thanks you very much for answering my question.
Do you know the typical symptom of u joint failure? Yes, the drive was all the way up. I wonder why the sterndrive position has anything to do with this noise.

they tend to make a rattling when at the upper lock. just drop it down a hair and see if it goes away. They are just getting out of the range of use and at too tight of an angle.

Willing to bet if you tap it down a hair it goes away.

if it doesn't take the drive off and examine the joints not expensive to replace... can also check the gimbal. More so than the angle of the drive water intrusion is a killer for both u joints and gimbal bearing.

Everyone will tell you running it with the drive up will kill the u joints. I have had a shallow water dock on a lake cove for my entire life, so I have to put my drive all the way up or damn close to get in and out of our hoist and idle out to deep water 5-10 min each way... my last boat the u joints got replaced after 23 years so not a killer. Just keep the rpms down 900 max

 

[Bondo]

Thanks for answering! By "drive" do you refer to the sterndrive? if yes, the sterndrive was all the way up. Not turned to any direction.

On the trailer, with muffs, running garden hose water, no load.

I am a new boat owner, wondering why tilted drive can cause this noise?

Ayuh,..... With the double cardan u-joint being twisted at it's extreme maximum angle, the input shaft, 'n the output shaft are actually turning at different rpms, in various points of each rotation,.....
In other words, yer stressing the 'ell outa the u-joints,....

On strendrives, just a bump up from dead down, 'n straight ahead offers the least strain on the u-joints,....

U'joints like to spin with a very slight angle,....
Not straight on, 'n not at the extreme angles,....
Preferably with the input, 'n output shafts parallel, but not dead in-line,...

 

[flashback]

That's an interesting thought, your saying that they like a bit of angle, it seems dead straight would be less stressful. How have you come to this?

 

[Bondo]

That's an interesting thought, your saying that they like a bit of angle, it seems dead straight would be less stressful. How have you come to this?

Ayuh,..... Many years of readin' engineering stuff, 'n bein' a general Mechanic for decades,......

 

[flashback]

Ayuh,..... Many years of readin' engineering stuff, 'n bein' a general Mechanic for decades,......

Could you explain it in layman's terms? I'm all ears.. it seems like the closer to inline would be better..

 

[Scott06]

Could you explain it in layman's terms? I'm all ears.. it seems like the closer to inline would be better..

Read up on pinion angles and driveshafts. If you are into hot roding and engine swaps exactly what bondo mentions is how these things are supposed to be set up

 

[flashback]

Well I've swapped a few, I remember putting in solid lifters in a 283 Chevy. Got a few more revs and a few less miles but it was fun..

I'll google the pinion angles and see what I come up with..thx..

 

[flashback]

Well I've swapped a few, I remember putting in solid lifters in a 283 Chevy. Got a few more revs and a few less miles but it was fun..

I'll google the pinion angles and see what I come up with..thx..

I'm humble! Now I have a grasp on the geometry of this.. always wondered why there was two in such a short run...it seems they need to offset each other and a 1/2 degree is the minimum offset..thx @Scott06 for the tip..

 

[iwknow]

Update: The noise is mostly gone when i drop the drive down a little bit. Though occasionally i can still here a little bit knocking sound, it is no longer as noticeable as shown in the video.

You guys are awesome! Thanks for all the informative discussions here.