Mercruiser 3.0 Outdrive Knocking Sound

[MJTurner42]

I have a 3.0 Mercruiser 2002 with an Alpha 1 Gen 2 Outdrive. Took the boat out to the river this weekend and heard a knocking sound coming from somewhere in the engine compartment when ever I brought the boat over no wake speed. When in idle, revving in idle, or at no wake speed the knocking sound was not present. Anything over no wake speed there was a loud knocking sound that was clearly metal knocking on metal.

I know two times ago one of my buddies who borrowed the boat hit a dead head in the river and damaged the prop beyond repair and took the entire skeg off the outdrive. (I know poor choices in letting someone borrow the boat). The prop was replaced and a new skeg installed. The boat has been fine on two outings since then.

The previous time I took the boat out I was running on an inter-coastal waterway and had to run the out drive in 1/4 to 1/2 trim, above where it should be run at. I didn't have a choice because the tide was out and I was destroying the prop.

Anyway I now have this metal knocking sound and don't know what it is. I listened to the valve cover with a screwdriver and it was not coming from there. I am convinced it is not an engine issue because it only happens when the drive is in gear and at above no wake speed. I think its a drive issue.

Any help would be appreciated with this!

Here is a link to a clip I took of the noise: https://youtu.be/1skXiJEJdNo

 

[achris]

I haven't watched the video (my company has y-tube blocked ), but it sounds like a broken tooth on the drive or driven gears in the top box.... Drain the drive oil, check for metal, and then remove the top box cover. Have a good look at the gears....

Chris..........

 

[MJTurner42]

Hey Chris, are there any good diagrams that show the top box cover and drain plug for the drive? First time working on this outdrive. Thanks!

 

[Bt Doctur]

achris,I played it and it makes a sound like a loose/broken pushrod or rocker arm
If the sound matches the rpm setting all the time I would check the pushrods first or pull the drive and run the motor on a hose to help isolate the noise. pics and manuals are avaialable

 

[MJTurner42]

When I run the motor not in gear at high RPMs the noise is not present. That is why I am inclined to think its not a motor issue. The noise is only present when in gear.

 

[nola mike]

Ya gotta figure out for sure where it's coming from. Does it do it in reverse? Most things spin in the drive even when out of gear. My first thought was exhaust shutters...
like bt said, pulling the drive will tell you for sure.

 

[Bt Doctur]

upper unit spins no matter what, shifting is in the lower unit and affects the prop shaft . That noise sounds more like a rocker arm verses a loose U joint.but I would still pull the drive to check the oil level and condition and to isolate the noise

 

[Maclin]

That sounds exactly like a car with a loose flex plate bolt, except once a flex plate bolt works loose and starts to make noise it won't change, in or out of gear.

 

[MJTurner42]

Dropped the drive oil, no metal fragments and no water. Pulled the top cover and gears look pristine. Thoughts?

 

[MJTurner42]

Dropped the lower unit and nothing appears out of order

 

[Bt Doctur]

remove the drive and run the motor with a garden hose

 

[MJTurner42]

Stupid question, were would the hose connect since I won’t be able to use muffs at that point?

 

[MJTurner42]

Also, when I run it without the drive, should I limit the RPM? Don’t want to destroy the engine since there won’t be any load on it

 

[MJTurner42]

https://youtu.be/KagPsx_J7Kg This is a link to the motor running and being reved without the drive engaged

https://youtu.be/LqDqncLBlpg This is a link to the motor running and at idle looking at the drive

 

[Bt Doctur]

idiot in vid 1 just took out his impeller, idiot in vid #2 just ruined his U joints and impeller

With the drive removed, acquire a washing machine hose and cut it in half. Stick one end on a garden hose and stick the cut end into the water inlet hole. Turn on the water and run the engine at whatever rpm you want. . You wont hurt anything just keep the rpm at a reasonable setting.

 

[Bt Doctur]

 

[MJTurner42]

Just trying to learn, how did I take out my impeller if I had water going to the muffs on the engine? Yes I know it was in the trailer position but it wasn’t in drive.

 

[tigger3650]

Your drive shaft running from the motor to the out drive is always spinning at the same rpm as the engine, regardless if it is in gear or not. Spinning it at high rpms with the engine at full tilt is hell on the ujoints. Running high rpms like that on muffs can cause the impeller to cavitate, causing abnormal wear and premature failure.

Use a long screwdriver or pry bar up to your ear and figure out where the noise is coming from. You can also feel around the different components(DON'T TOUCH ANYTHING THAT MOVES) and see if you can feel where the sound is coming from (that sounds weird)

 

[nola mike]

And, if you've been running it that high like that your u-joints might be knocking, or ruined and knocking.

 

[Bt Doctur]

some raw water pump create enough suction to collapse a hose, burn out the impeller, lose the suction and the hose now feeds the motor. This results in a overheat when placed in the water because the pump is no longer working. Most times, anything above 1500 rpm is the max on a hose because your trying to suck more than the hose can supply
Any trim setting above the max trim limit put stress on the gimbal and U joints