Knocking Sound

[cowpoke]

I've got a 7.4L Merc and a Bravo 1 drive on a Crownline 225BR. It has 335 hrs and is a 1994 model. I have noticed a metallic "knocking" noise near the rear of the engine (most prominently on the drivers side of the boat). I had a mechanic look at it and he believes it is a loose exhaust shutter. He was a little concerned that the sound was "deeper or more pronounced" than he recalled, but is convinced this is my problem. I don't notice any vibrations or decreased performance, but can definitely hear something loose. How common is it for the exhaust shutters to work loose or become a problem? When we first started hearing the sound, it would go away after the engine was warmed up, but now we can hear it under most idle conditions. I haven't disassembled any of the exhaust couplings to check the shutter yet, but plan to soon. The mechanic didn't feel it was an immediate concern. Would appreciate any experience/advice on this condition.<br /><br />Thanks.

 

[Walter]

Re: Knocking Sound

I'm not at all familiar with your beast...so I'll only reiterate the advice your mechanic gave and suggest you investigate the loose exhaust shutter as a first step. Perhaps the reason you used to only hear the sound when the engine was cold was because a cold engine tends to run a bit rougher and vibrates a bit more thus causing your exhaust shutter to rattle. Then when the engine warmed up and smoothed out...it didn't vibrate enough to move the shutter. Since then...maybe the shutter has loosened enough that it's noisy all the time. Hey, I don't know...I'm just tossing out some ideas here! Sounds reasonable though, eh?<br /><br />In the event what you are hearing is something more serious... I'd suggest you not run your engine much until you've been able to determine what the problem is. Hopefully it's something simple...<br /><br />Walt