Knock Knock - Who's There - Hopefully not a bad piston bearing

[brodie123]

Hi all;

Mercruiser 250 L6 4.1L

When checking my base timing (6 degrees) which was fine, I dropped the RPM as low as it would go.

A knock became evident. Sounds more like a TICK than a knock actually.

Brought motor back to ~800 rpm (or less maybe) and the knock went away.

I pulled 1 spark wire at a time, seems removing cylinder 3 stops the knock.

Trying to trace the sound, I swear it's coming from the front - timing chain area.

I'm obviously hoping I don't have a crank bearing going. Any thoughts here or am I in for a rebuild?

 

[Scott06]

Tick is probably valve train related maybe adjust the valves. If it’s coming from the front it’s likely the timing gear drive or water pump. This and it’s smaller sibling the 3.0were never the quietest engines. A tad sewing machine like in my experience

 

[brodie123]

Thanks. So I can aim my efforts before I start tearing it apart. If it was valve train, collapsed lifter, bent rod or valve lash, would it not be more audible when I take the breather (oil cap) off? That was the first thing I checked - it did not change the sound.

Also, if disabling the 3rd cylinder (removed spark plug wire) and the noise goes away - does that still point to the valve train?

thx!

 

[Scott Danforth]

on the inlinner

exhaust flappers tick and rattle
stock timing gears tick and click
lifters have a bit of a tick

start with a mechanics stethoscope

flappers should be inspected yearly
lifter noise is easy to diagnose and correct.
timing gears require the removal of the pan and front cover to verify
bearing noise requires the removal of the pan to verify.

however generally if pulling a plug wire removes it, its tied to that cylinder. either lifter noise, or bearing noise. hence the stethoscope

 

[crazy charlie]

Before you start a BIG job,try something simple to possibly isolate where the noise is coming from.With the motor idling ,take a wood broom stick.Place one end on a part of the motor or exhaust ,place the other end of the broom stick in your ear.I would start with the exhaust outlet tubes where the flappers are.If you place the broom stick in the problem area you will hear it loud and clear when you put the broom stick in your ear.This method has never failed me or anyone I know.Post results.Charlie