Strange Knocking Sound Mercruiser 454 (7.4L)

[tpenfield]

As you may know from my 'Stringer & Bulkhead' thread, I launched my boat today. As I was prepping the boat, I ran the engine on the muffs for a while.

I noticed a strange 'low level' knocking sounds that I have not heard previously. I was not the 'ticking' noise that you generally get with a valve being out of adjustment, lower in pitch and a bit more subdued. I do not really notice that the sound continues when the engine is revved up.

I also remember hearing this slight sound when I ran the engine on a test stand a couple of weeks ago. I did not think too much of it then. . . it seems more pronounced now that the engine is in the boat.

Here is a vidoe of the engine idling in the boat today . . .

What do you all think the noise is from???

 

[tpenfield]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

Just for reference purposes . . .

Here is a video of the engine running a few weeks ago . . .

Not sure if the noise was there on the test bed, but I think it was.

 

[bajaunderground]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

Sounds like an exhaust reverb...

I have nothing more than automotive experience to base that on! Have you listened with a stethoscope? The noise is obvious, but to me (again, see above) doesn't sound "mechanical" I can hear the same rhythm in both videos...hopefully that's what you mean?

 

[tpenfield]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

Sounds like an exhaust reverb...

I have nothing more than automotive experience to base that on! Have you listened with a stethoscope? The noise is obvious, but to me (again, see above) doesn't sound "mechanical" I can hear the same rhythm in both videos...hopefully that's what you mean?

Yes, same rhythm, but seems to be more pronounced once the engine is in the boat. I am hoping it is not a valve related noise . . . when I rev the engine, it does not seem to increase, etc.

 

[Scott Danforth]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

its pronounced more in the boat because the boat is now resonating like a drum.

 

[26aftcab454]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

sounds like sweet music to me--healthy!

 

[tpenfield]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

Thanks for the comments so far. I think that it may be the exhaust hose on the starboard side. Both hoses are in tough shape internally, so I plan on replacing them.

 

[tpenfield]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

A quick update:

I replaced the exhaust tubes and the low-level noise seems to have gone. The old tubes were in rough shape, so I think the 'exhaust reverberation' theory was correct . . . probably the exhaust sound was bouncing around the torn inner layer of the tube, eyc.

Here is what one of the old tubes looked like on the inside.



And here is a video of the engine idling (no "low level" noise).

The engine does not struggle at the top end (WOT) either, as it did before, so that's an improvement.

 

[bajaunderground]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

Nice and simple fix...a rarity on a boat!

 

[joewithaboat]

Re: Strange Knocking Sound Mercruiser 454 (7.4L)

I bet that was a relief.....