Engine making new sound? Can I still use it?

[sadams261]

I have a boat from 2008, carb rebuilt and new spark plugs this spring. The engine is a 3L 135hp Mercruiser.

Ran fine the first few days this spring. Last week it started making this new unhealthy sound. (Attached, filmed at idle at dock ). Fluid levels are ok

Starts easily but with a bit of an initial pop (like a backfire?)

It’s very inconvenient to get to a mechanic and we are just about to start our cottage vacation. so I’m trying to understand if it is safely usable or fixable without a mechanic?

 

[dave8311]

Sounds like a spark plug blew out.

 

[sadams261]

Well that sounds fixable.

Sounds like a spark plug blew out.

Is there a way to determine which one or should I replace all 4?

 

[dave8311]

Just look at them, one of them won't be like the others.

 

[1985 Century Mustang]

If all the plugs look good, give it a compression test.

 

[flashback]

You asked if you could still use it, no, not until you find the failure.. check/change all the sparks...

 

[sadams261]

All the plugs look the same and replacing them hasn’t helped. Is my next step the distributor cap and cables or something else?

I don’t know what a compression test is so that’s not likely something that I can do!

 

[dave8311]

Cap and wires aren't going to fix that.

It sounds like a compression leak, somewhere. If not from the plugs than maybe the exhaust manifold or it's gasket.

Or, is that a metal to metal sound? Hard to tell from the video.

 

[flashback]

Yea I would have bet my first born that a spark plug was missing..
What did the plugs look like?

 

[tpenfield]

Sounds broken . . . no you cannot use it.

 

[sadams261]

Yea I would have bet my first born that a spark plug was missing..
What did the plugs look like?

This is what the plugs looked like. Normal, at least to me

 

[flashback]

If they all look like that then I don't see any problem.. but that noise is not good..

 

[alldodge]

I don’t know what a compression test is so that’s not likely something that I can do!

Remove all plugs
Disconnect distributor plugs at base of distributor

Move throttle lever to WOT
Connect compression gauge into first cylinder

Crank motor over at least 3 revolutions (a few more is better)
Right down reading
Release pressure from gauge

Remove gauge and move to next cylinder and repeat until all cylinders have been tested

Compression gauge in link, but should be able to get a loaner from local auto parts store

https://www.amazon.com/compression-tester/s?k=compression+tester

 

[Scott06]

Its compression related either exhaust manifold gasket leaking or head gasket leaking

did u do a compression test

 

[Lou C]

That sound combined with the obvious miss suggests a blown head gasket. Time for a mechanic.

 

[1985 Century Mustang]

Do you have any updates on this issue? I'm interested to know what it was.

 

[sadams261]

Only that it is beyond my ability and unfortunately it willl be a few weeks until I can get it to a mechanic. Broken just when. I most want to use the boat so I’m frustrated …

 

[sadams261]

In the end the mechanic replaced both the head gasket and the exhaust manifold gasket. thanks for all of your help, everyone