Advice on tracking down the source of a knock

[robertsapp]

I have a knock coming from my number one cylinder on my 3GM30F. I know it's coming from number one, because when I open each of the compression releases, the knock quits when I open #1. From what I can tell, the knock is evident at idle, and appears to get better (reduce in intensity) at higher RPMs. It's a sharp, clear knock, and not a dull thud. There's no unusual smoke coming from the exhaust. Based on what I've read, I think I'm dealing with a late or sticky injector that's spraying late and giving me an ignition knock. I know that you can also get knocks from the connecting rod bearing, piston pin, or crank bearing. I'd naturally prefer that it be coming from a late injector. How do I tell for sure?

 

[Warren Milberg]

While I think you may be on the right track in regard to thinking the problem is injector-related, I suggest that you first make sure of the quality of your diesel fuel, to include its cetane rating. If you have old fuel with a low cetane rating, that may be the culprit here. I think your engine, like my 2GM20F, requires a cetane rating of 45. Most of the diesel sold in my area has rating of 40 so I'm using a cetane booster. I also triple filter all the fuel I put in the tank. I'd give this a try first and see if you resolve or reduce the problem. I also think that the Ultra Low Suphur Diesel (ULSD) that we are all using now may lack lubricity as the sulphur content of the fuel has been reduced from 5,000 ppm to 5 ppm. So give some thought to adding a lubricity booster to your fuel as well.

 

[sailaway78]

Begin with the easiest

I would start with the easiest things to check first. While you may very well be right in the source of the noise, the first thing I would check is the valve clearances, and rocker arms. An extremely loose valve will cause quite a clatter, as can a bent or damaged rocker arm. Do you have an oil pressure guage? A bad rod bearing would more than likely show up in lower oil pressure. Those Yanmars are basically bullet proof, so I would not be overly concerned with the lower end bearings or wrist pins.

 

[robertsapp]

Re: Begin with the easiest

Over the winter I pulled, repaired, and replaced the fuel tank (I posted several messages about my experience with aluminum fuel tanks) and took on fresh fuel. A cetane booster sounds like a good idea, no harm in trying that. I adjusted the valves about six months ago. It was the first time I tried it, so it's possible I didn't get it exactly right, but would a valve issue make a knock that sounds like slapping two heavy wooden dowels together extremely hard?

 

[sailaway78]

Valve Lash

An extremely loose valve adjustment can make a lot of noise. Don't think you have a problem with injector timing, as this would effect all three cylinders. How many hours does this engine have on it.

 

[robertsapp]

Re: Valve Lash

I don't think it's injection timing, because you're right, it would affect all cylinders. When I mentioned a late injector, what I mean is that it pops off late, as in it may be sticky and may need to be rebuilt. I have to say I honestly have no idea how many hours the engine has on it. The boat is now almost 16 years old, there's no engine hour meter, and when I bought the boat approx. 18 months ago, when I asked the PO how many hours it had, he said "Lots." Good news though is that it fires immediately, shows no smoke, uses very little oil, and runs smoothly and strongly. It just has this annoying knock from #1 at idle and low speeds.

 

[Alan]

I would definitely check your valve clearances first but a poor injector can certainly cause this kind of thing. To check the injector swap it with one of the others and see if the sound follows. If it does you've found the culprit.

 

[greg17887]

Re: Valve Lash

If you wish to find out if its the injector then switch one from #1 with one from #2 if its the injector the knock will move with it.