7.4L Mercruiser Knock at idle

JGarver574

New Member
May 4, 2020
7
Boat Info
1990 Sea Ray Sundancer 270
Engines
Single 454 Mercruiser
I have an intermittent knock going on with my 7.4L Mercruiser Bravo engine. It only happens at idle, and when I move the throttle up at idle it speeds up with the RPMs, then goes away. Sometimes at idle, it will go away and come back. Engine temp, level, and oil pressure are fine. Boat starts up right away with no issue. To me it sounds like it’s coming from the port side exhaust housing that comes off the block.

Video link to YouTube -
 
Try it with the drive off. I had a similar and it was a loose input shaft on the Bravo.
 
Try it with the drive off. I had a similar and it was a loose input shaft on the Bravo.

I’m a new boater, can you explain what you mean with the drive off? Like, do you mean pulling the boat from the slip?
 
The rubber may be gone off the exhaust water shutter and the metal of the shutter is tapping inside the exhaust tube
 
OMG y'all are amazing, i got the same thing going on with my 7.4. thought rod knock, then i remember that about 10 years ago on my last sea ray 7.4 i had the same sound and it was the rubber worn off the exhaust flapper. only happens at low rpm. thanks for the memories. i totally forgot about that.
 

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