4.3 Mercruiser Knocking Sound

[regulator]

I have a 1989 4.3 Mercruiser that while idling in gear at trolling speed of 2.5 mph or less makes a horrible knocking sound. Throttle up and it goes away, also out of the water on the ears, there is no knock at idle. Any ideas? Thanks in advance!

[lakebound88]

Sounds like a coupler went bad. Just my two cents. I know the idling and clank back and forth wears out the teeth. Sometimes there is not much holding it there. A sudden gear change can strip it immediately. Maybe someone of a heavier mercury knowledge will step up. Good luck.

. Nick

Sent from my XT907 using Lake Ontario United mobile app

[bigCOHOnes]

It foes sound like it is gearcade related because there is load in the water and no load out of the water.. I would pull gearcase and inspect driveshaft universal joints and while in there maybe replace the gimble bearing or at the very least check the alignment. When you decide to this make sure you buy a gasket set

fir drive most likely it will get torn. Good luck!

Sent from my SCH-S738C using Lake Ontario United mobile app

[L&M]

Exhaust shutters would be my guess without hearing it myself.

[Xxx]

Can you hear where the knocking sounds are coming from, such as the front of the motor, back meaning the flywheel (starter ring gear) or the out drive, sounds to me like a wrist pin knock, if from the engine which just happens, one day nothing the next what the hell is that knocking sounds, but without pin pointing the sound we are all guessing. PAP.

[borderline 350]

Dear Regulator,

                          My best guess agrees with Hank of L & M.....exhaust shutters just below the L & R exhaust manifold bellows....Sincerely, X-Jet Boat Bill

[Tim Bromund]

Exhaust shutters was going to be my guess as well.

[Ray]

Exhaust shutters was going to be my guess as well.

mine too!

[regulator]

Thanks for the replies, such a great bunch of fellow fisherman and boaters on this site! I do know that when it happens it is coming from the back lower end of the engine. I can't hear it coming from the drive unit as it is obviuosly under the water line so would have to pull it to check that. I had the drive off last year, and the gimble, drive shaft, and u-joint all seemed to be fine, but that was last year. That all being said, what is the best way to address the exhaust shutters? Not something I will do myself, so will seek professional help! Thanks again guys and good luck and safe travels out there.

[Tim Bromund]

As far as I know, It's just what they do, my 5.0 Mercuiser has been making that sound as long as I've owned the boat.