98 Mercruiser 5.7 350 knock...

[ahasiec]

Was at the boat yesterday and when I was getting ready to start the motor, when cranking, I heard a knocking noise, one knock and thats it...when the motor was running I ran back there to see if I can hear it and there was nothing...shut the motor off and turned it on again and no knocking...

I turned the motor off again, waited about an hour before starting and it knocked again...but once again while running in idle nothing as far as kncoking coming from the motor...

I did not want to take it out and have the marina removing the vessel from the water to take a look...any thoughts??

Piston knock? Head gasket?? Risers??
Mind boggling that when running there was no knocking...

 

[Don S]

Re: 98 Mercruiser 5.7 350 knock...

Next time you shut it off after running for a while (on muffs or in the water) before you try to start the engine. Remove all the spark plugs and using a remote start button, turn the engine over and watch the plug holes for any signs of water. If there is a little water in one cylinder, it will knock (and not hydrolock) and then dump the water out the exhust on the next stroke. Then, no more knock.
That would indicate risers, manifold or the connection is leaking. It WILL get worse.

 

[ahasiec]

Re: 98 Mercruiser 5.7 350 knock...

By manifolds, do you mean exhaust manifolds??

Thats what I was thinking, maybe they are clogged with something....when the motor was in idle, I ran the RPM's a little to see if there was any knocking and nothing...

 

[Don S]

Re: 98 Mercruiser 5.7 350 knock...

Leaks is what gets water into the cylinders, not clogs. Clogs cause overheats.