Mercruiser 3.0 knocking noise

T

tony.lewis411

New Member
Mar 19, 2024
13
Boat Info
1989 Searay BR160. 3.0 Mercruiser
Engines
3.0 Mercruiser
I have a knocking noise and can't determine where it's coming from. Little history - I had the lower unit impeller go bad and it over heated the engine. I pulled the head and had it checked because water was getting in the oil. Head checked out good and I replaced the head gasket of course. Still getting water in the oil. I replaced the exhaust manifold gasket and elbow gasket and the water stopped getting in the oil. Ever since then I have had this knocking noise and I cannot figure out where it's coming from. I just had it tuned up and the valves adjusted and that was not the problem. It has been running like this for almost 10 years and the noise has not gotten better nor worse. Any input would be greatly appreciated. I have a video but not sure how to upload it because it's too big.
 
It's louder at low rpm and is less at high rpm
 
Are you sure it's not from high trim angles? Meaning, coming from the driveshaft universal joint.
 
No, this noise happens at engine idle, so not moving. It happens in and out if the water
 
I attached a short video
 

Attachments

  • Engine noise.zip
    4.3 MB · Views: 67
tony.lewis411 said:
No, this noise happens at engine idle, so not moving. It happens in and out if the water
Click to expand...
That doesn't negate the driveshaft - it is always spinning.

I can't seem to open the video file right now.
 
Yes the noise follows the RPM. It sounds like it's coming from the front of the engine but you know how engine noise can travel.
 
Lazy Daze said:
That doesn't negate the driveshaft - it is always spinning.

I can't seem to open the video file right now.
Click to expand...
It's a zip file, the only way I could upload it. I haven't tried to view or listen back by the drive shafts.
 
tony.lewis411 said:
Yes the noise follows the RPM. It sounds like it's coming from the front of the engine but you know how engine noise can travel.
Click to expand...
Yup, absolutely!

Try using a mechanic's stethoscope, a really long handled screwdriver or even a wooden dowel (put those last two right to your ear). When you find the offending area, it will be noticeably louder.
 
Sounds to me like something is loose slapping something every revolution maybe from the back of the engine or one of the pulleys I would just yeah use a long screwdriver or wooden pull it sounds really obvious though like it's right there in front it doesn't sound internal to me at all
 
To eliminate pulley's, run the engine (short time only) without the belt
 
With all of that work you have done on the engine have you pulled the outdrive to inspect the U joints and yoke?
 
blaster said:
With all of that work you have done on the engine have you pulled the outdrive to inspect the U joints and yoke?
Click to expand...
I had all the bellows, shift cable and outdrive water pump replaced 2 years ago. He said everything looked good there. The noise is not as loud in the back of the boat as near the motor.
 
ttmott said:
What's the oil pressure? - sure sounds like a connecting rod knock.
Click to expand...
The oil pressure seems ok but does fluctuate. At low idle the pressure will drop, higher idle it will increase. I've thought that was odd but it's been that way for some time now. I've been told it could be a main bearing..
 
Cables/impeller won't be a part of this equation. Oil pressure fluctuation like you are saying is normal.

For some reason, I still can't hear the video - probably something on my end. BUT... can you respond to my question about trim angle?
 
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