4.3L Noise Help

[tylerdale101]

Hello, I am looking for some help with a noise my mercruiser is making. I can't locate it, I bought the boat with a cracked block so I sourced a used engine and rebuilt it. But it makes a noise now. The noise is a quiet rhythmic knock like knocking on a solid wall and is directly proportional to the rpms. The noise DOESN'T occur at idle, only occurs at or above 1000 RPMs. I have used a stethoscope to try and locate the noise, from the best I can tell it is the loudest on the passenger side oil pan and rear end. I have tried isolating the problem by removing belt to eliminate accessories and removed plug wire individual both didn't help. Any advice would be great!!! Thanks for your time and help!

 

[scoflaw]

Run it with the drive off

 

[tylerdale101]

Run it with the drive off

Sorry forgot to add that, I did that too. Still no change

 

[Scott Danforth]

Is it a clanging sound? if so, check your flapper

if its more of a knocking sound, it could be the balance shaft (assuming 1993 and newer). when you rebuild the motor, did you replace the balance shaft drive gears?

 

[Bondo]

so I sourced a used engine and rebuilt it.

Ayuh,..... Were ALL of the tolerances checked, 'n measured,..??

Cylinders, 'n rods checked for roundness,..??
Rods resized,..??

 

[Rick Stephens]

Be nice to know what year and how the 4.3 was configured - balance shaft or no.

Also be nice to know what all you did to it when you rebuilt the replacement motor or block. Not too many things can knock inside the pan. And all of those point to a failure of a bearing.

 

[tylerdale101]

Is it a clanging sound? if so, check your flapper

if its more of a knocking sound, it could be the balance shaft (assuming 1993 and newer). when you rebuild the motor, did you replace the balance shaft drive gears?

Yes it is a 2003 4.3l. I didn't replace the balance shaft drive gear.

 

[Lou C]

Try putting your stethoscope on the exhaust pipes where the flappers are (inside) they can sometimes make knocking noises esp if the rubber coating is worn off.

 

[tylerdale101]

Try putting your stethoscope on the exhaust pipes where the flappers are (inside) they can sometimes make knocking noises esp if the rubber coating is worn off.

I am unfamiliar with this, I haven’t seen any flappers, what do they do and where are they located?

 

[tylerdale101]

I watched some videos on flapper noise, and it was an irregular high pitch noise. Is that what it usally sounds like? If so I don’t think that is my problem, my noise is deeper and constant. It changes directly with rpm.

 

[Bt Doctur]

Only 2 thik should make a noticable knocking sound. Loose wrist pin that will sound under acceleration but not decelerating
A rod knock that will knock all the time. Try removing 1 sparkplug wire at a time to help isolate the knock

 

[achris]

I am unfamiliar with this, I haven’t seen any flappers, what do they do and where are they located?

At the top of the exhaust Y-pipe.... Look like this...

The idea behind them is that they slow (and hopefully stop) any water rushing up the pipe and into the cylinders when the throttle is pulled back quickly.

They can chatter, especially at idle on the muffs, when there's little to no back-pressure in the exhaust system.

Chris......

 

[tylerdale101]

Only 2 thik should make a noticable knocking sound. Loose wrist pin that will sound under acceleration but not decelerating
A rod knock that will knock all the time. Try removing 1 sparkplug wire at a time to help isolate the knock

I tried removing wires one by one but didn’t change noise at all.

 

[Chris51280]

Rockers or lifters maybe

 

[Scott Danforth]

first, please describe what you did when you "rebuilt" it. in detail

you stated you didnt change the balance shaft gear. what other steps to rebuild did you skip?

did you replace the cam and balance shaft bearings?
what clearance was your rods and mains?
cast, hyper-e or forged pistons?
new rods or did you re-condition your existing rods?