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High pitch sound within narrow RPM range

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As part of major Merc 3.0LX repair, I replaced engine mounts, manifold, belts, gimbal bearing/bellows, sterndrive (SEI).
On the first run on the water I could hear high pitch sound in only in 3150-3350 RPM range. Not squeaking, just higher pitch sound.
What it might be related to?

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Is it diaphragm or filter? I will check if sound is originating from there.

Also, is it possible alternatoe bearings are due to replace?


Btw, there is nor indication of engine power goes down, overheating or anything else.

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Check to make sure you didn't over tighten the belt, it could be putting torsional pressure on the internal bearings of the alternator , water pump or power steering pump. Only other thing i can think of is if the drive alignment is off slightly causing pressure on the engine coupler.

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Good idea with belts. I will check it. 

I've checked alignment after installing engine back into boat. As I've mentioned, sterndrive is new. 

I do recall last year spinning alternator without belt produced some squeaking sound. It wasn't so important that time with major issueswith sterndrive, so I postponed alternator  bearings replacement. Perhaps I should do it now.

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I get to the boat tonight and discovered two things:

1 power steering fluid was low. I lost some when removed engine and forgot to add it in the fall. Maybe I added it, but when system was out of bubbles, I needed more.

2 breather hose was not attached properly to the rocker cover vent. It was sitting on the top, slightly shifted to the side. This set up might work as a whistle.


I started engine, warmed it up and briefly set 3200rpm. Haven't heard that high pitch sound. I will check it again on water.

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  • 2 weeks later...

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