Engine knocks at Idle

Warren Milberg · Nov 28, 2016

How old is the fuel in the tank? If the oil is old then the fuel is probably old, too. Change the oil and filter yourself -- this is an easy DIY job that can be done in a few minutes. But I doubt that would cause the knock you mention. Another thing to think about/check would be valve lash, another task you could do yourself or have a qualified mechanic do for you while adjusting the valves. Many owners never think about this task -- and come to regret it at some point.

Don S/V ILLusion · Nov 28, 2016

How many hours on the engine?

KD3PC · Nov 28, 2016

cruddy injector

Roland5048 · Nov 28, 2016

Is the knock actually coming from the engine? The reason I ask is that my 3YM20 with 550 hours on it will rattle a bit upon cold start up and bang the shaft a little against the stainless stern tube. Sounds like the engine is ready to disintegrate. Reving a bit settles it down and all is good after it warms up. Just a thought.

Benny17441 · Nov 28, 2016

MOTOR MOUNTS. All these engines have an inherent roughness at idle which is tamed by the motor mounts. If the mounts are worn or broken the engine will shake and exhibit a rough idle. You cannot visually inspect a motor mount to determine if it is good or not. It would be unusual for a 2003 boat model to have badly worn motor mounts but one could be broken. Have seen a propeller line wrap stop an engine dead and split a motor mount. Cannot diagnose online but this would be another possible cause to look into.

Mark Maulden · Nov 28, 2016

First step: Find a laser tach and verify correct engine speed..

MitchM · Nov 29, 2016

"It would be unusual for a 2003 boat model to have badly worn motor mounts but one could be broken? " NB: yan mar recommends replacing motor mounts every 5 years. (hardly anyone does.) our 80 seafarer had a new 2 GM20 F engine installed in 1998. ( we checked the mounts and p shaft alignment at the coupler yearly. ) by 2005 we noticed some unacceptable vibration. the mounts could not be properly adjusted as they had compressed to the limits of the adjusting nuts. so we replaced them.

that being said , unadjusted engine mounts would not cause knocking at low idle. excessive vibration from bad mounts would be noticeable at every rpm in the operating band. i'd more guess that dirt in the fuel tank from a dirty filter and from a cruddy tank bottom is blocking an injector at the low idle rpms. at low rpms less volume of fuel is being pumped, so the lack of fuel available to one or more injectors causes a knock. at higher rpms fuel flow is still restricted but the effect much less noticeable.
when you drain the fuel tank be sure to clean have a professional clean the Asphaltine muck out of the tank bottom .

Don S/V ILLusion · Nov 29, 2016

john mendolia said:
I have to check.

It's nothing more than a wild guess what may be wrong unless you can give us some additional information including engine hours at a minimum. A boat of this era almost certainly has an engine hour meter.