Knocking sound - what is it? [video]

[morebacon]

Hey folks, I started getting this weird knocking sound last time I went out. Thought it's valve clatter so I did an adjustment (#1 was just slightly loose, but not by much). That didn't fix it unfortunately. Any suggestions what I should be checking next?

Mercruiser 3.0 Knocking Sound

I'm currently rebuilding the carb, changing the oil, spark plugs, etc. but that is part of the regular maintenance I was planning regardless. Thanks.

Here are two videos (front and back of the engine). It's much louder when facing the engine.

https://imgur.com/a/25bjoN8

 

[dubs283]

I don't know if it's the video frame resolution or me but the power steering belt appears to be loose. And the pump itself seems to be moving?

 

[alldodge]

Sounds like an exhaust leak

 

[dubs283]

Didn't see the second video initially. Agree with exhaust leak sound

Will say if the engine is warm during the video it is running pretty rough, even for a 3.0 L

 

[kenny nunez]

One way to find an exhaust leak is to squirt some automatic transmission oil into the carburetor. If there is a leak you will see the smoke.

 

[morebacon]

I don't know if it's the video frame resolution or me but the power steering belt appears to be loose. And the pump itself seems to be moving?

Good eye! It was indeed a bit loose, but the frame rate makes it look much worse

 

[morebacon]

Didn't see the second video initially. Agree with exhaust leak sound

Will say if the engine is warm during the video it is running pretty rough, even for a 3.0 L

Agree! It was cold, but running too rough compared to last season - that's one of the reasons for the carb rebuild.

 

[morebacon]

Hi folks, Thanks for the suggestions - just wanted to give an update that it was indeed an exhaust leak, the exhaust manifold gasket was blown.



Looks like the gasket itself was not the right fit, could this have been the reason?

Also, I remember reading somewhere that these green gaskets are not that great and should be avoided, in favor of the aluminum layered ones. Is that right?

 

[04fxdwgi25]

Who ever put that gasket on should be taken to the wood shed and beaten. Or at least have his / her tools taken away.

No paper gaskets on exhaust.

No gaskets that cover the exhaust ports on engine.

Wrong gasket.

 

[Scott06]

Two different styles of head ports and corresponding intakes were used on 3.0 s nit sure the transition year.
would make sure you have match head and manifold and get correct gasket

 

[morebacon]

Two different styles of head ports and corresponding intakes were used on 3.0 s nit sure the transition year.
would make sure you have match head and manifold and get correct gasket

I'm confused by this - got the gasket based on the engine serial, but seems the ports are still a bit smaller too. I'm worried because the previous gasket melted exactly at those ports that were covered:


Here's how my manifold looks:


Looks like this new gasket is what comes in the kit as well - is it OK for the ports to be smaller?

Manifold Exhaust for Mercruiser GM 181 3.0L 3.0LX 1990 and Up 806867A12 860235 [BARMC-1-860235] - $568.95 : ebasicpower.com, Marine Engine Parts | Fishing Tackle | Basic Power Industries

ebasicpower.com Manifold Exhaust for Mercruiser GM 181 3.0L 3.0LX 1990 and Up 806867A12 860235 [BARMC-1-860235] - Exhaust Manifold Assembly for Mercruiser Model 3.0LX/3.0L GM 181 CID Inline 4 cyl. (1990 and up) with Mercarb carburetor 1-5/16 X 3-5/8. Fits:3.0L GM 181 Inline 4 1990-19943.0L GM...

bpi.ebasicpower.com

I also found this adapter plate, but looks like it's for pre-1990 manifolds:

Adapter plate for exhaust manifold MerCruiser 3.0 ltr., Sweden Marineparts

Sweden Marineparts Ltd - Spareparts for your marine engine, faster, better cheaper

www.swedenmarineparts.com

So is it OK to install the gasket that I have, with the smaller ports?

 

[ScottinAZ]

I wouldnt install that gasket. Not only are you losing power due to restricting the exhaust ports, but you will likely blow the new one in short order as well. May take a bit of research to find the right gasket, but that is what needs to be done. A previous owner may have changed manifolds to the large port piece. IIRC my 1980 vintage 2.5 has the large ports, and when I changed the manifold, the gasket called for was the correct large port one

 

[Scott06]

I wouldn’t install that gasket with those ports. Would think someone swapped the head or manifold.

Maybe merc tech support can offer a solution or just order the earlier style gasket and see how it matches up

I believe the Barr kit is you can mix and match earlier and later manifolds and heads