Mercruiser knocks but only after 3,000 RPMs under load

[backonthewater]

Hi all. I’m back on the water after a couple of decades of being away. It’s good to be back but I’m having an engine problem. I bought a used boat and my 2005 mercruiser 5.7 ltr developed a knock (I attached a brief video) It sounds high in the engine to me. The knock only appears at about 3,000 rpms under load. Below 3,000, it’s smooth and has power. I can’t reproduce the sound if I run it up to 3,000 rpm in throttle only mode - it only appears under load. What do you think?? Thanks!!

 

[alldodge]

Sounds like a fan blade is rubbing. Look at the alternator and other pulleys. Take the belt off and see if there is any play

 

[Scott06]

Sounds a lot like this video I saw, and rpm seems to be similar -

in the comments owners states it was the head gasket that blew after and overheat. Do you see any evdience of water on the spark plugs

 

[alldodge]

Scott you might be on to something, maybe preignition

 

[QBhoy]

Do those models have a balancer shaft and wheel ? Can’t think.

 

[Bondo]

Do those models have a balancer shaft and wheel ? Can’t think.

Ayuh,.... The balance shaft is only in the mid-late model 4.3l motors,....

Detonation sounds likely,......
Dump the fuel filter's contents into a clear jar, 'n check for anything but clean fresh gasoline,.....