Mercruiser 3.0 Outdrive Knocking Sound

[MJTurner42]

Hey all the noise was a blown head gasket. Just wanted to close this thread and thank everyone for the help!

 

[Bt Doctur]

Somehow I dont think so. The tapping/knocking is a heavy metallic sound like a very loose push rod or a broken rocker or badly bent pushrod.A leaking headgasket will not make a metallic clank like that.

 

[MJTurner42]

The shop said there was a tear between the 3rd and fourth cylinder and the compression on both cylinders was 50 PSIG.

 

[Simoniz]

Just to stick my oar in, the head gasket may well be blown, but thats not whats making that noise...

 

[achris]

Just trying to learn, how did I take out my impeller if I had water going to the muffs on the engine? Yes I know it was in the trailer position but it wasn’t in drive.

RTFM!

In reference to running on flushers, Owners handbook CLEARLY states "DO NOT operate the engine above 1500rpm"

Also, study up on how the Alpha drives work. The ONLY thing not turning when out of gear is the propeller shaft. Everything inside that drive, and leading to it (unis in particular) are all spinning at engine speed, regardless of whether it's in Forward, Neutral or Reverse. BTW, that's also in the owners handbook on flushing, always have the drive trimmed fully DOWN.

Chris........