Help with Diesel Diagnosis

[robertsapp]

OK all you expert gearheads, here's a scenario for you:

A Yanmar 3GM30F has a pronounced knock at idle. The knock is coming from number one cylinder. Flipping the decompression lever on number one while running makes the knock go away, opening numbers two and three has no effect.

The engine starts instantly and shows no smoke while running, so compression appears excellent and there are no apparent fuel problems.

The knock goes away as RPMs increase, and the engine hums (ticks, actually) like a well oiled sewing machine at cruise RPM with no evidence of knocking.

A rapid increase in throttle from idle does not aggravate the situation, nor does a rapid decrease in throttle from cruise RPM cause the knock to occur.

What do you think? I may be just wishfully thinking, but based on what I believe I know about engines, if the knock was coming from a main, crank, or wrist pin bearing or from piston slap, there should be some knocking at cruise RPM, or during a rapid increase or decrease in throttle setting. The fact that the only time knock is evident is at idle leads me to believe it's a late injector. Opinions? Other things to try to diagnose?

 

[Jake N Eggs]

It is usually pre-combustion that will cause a fuel knock. That is the ignition is too early. This could be caused by over fuel, possibly by a defective injector, or it could be the fuel is igniting early. It's not usual for a diesel as it needs the compression temperature to ignite the fuel. In some cases, when a diesel has a heavy carbon build up in the combustion chamber and on the injector, the carbon can glow red hot and cause the fuel to ignite slightly early.

If you think that this might be a possibility you could run the engine hard and under a lot of power. This could clean the carbon out of the engine.

Also check your valve clearances. If the intake valve does not have the correct clearance the valve could be held slightly open causing the combustion noise to sound like a knock.

Good luck.

 

[Steve Dion]

Robert:

Many sailors believe that they are stressing the Yanmar by running them at 2800-3100 rpms. This is actually the recommended operating RPM range. I would agree with Jake. Run the hell out of the engine at 3000-3200 RPMs and see if that helps. Low idle speed can also cause knocking so you may want to check your actual idle speed (need to check RPM speed with something other than the engine tach).

 

[rtmccue]

Injector

Try to switch the injectors . Some times they leek and this would couse a knock. See if the noise follows the injected

 

[anchorclanker]

Diesels dont get fuel knock like a gas engine. Except for hot compressed air, there is nothing combustible within the combustion chamber until the diesel fuel is injected, and it combusts virtually instantaneously in the plus 1000F atmosphere. Prior to the injection, there should be nothing in there to burn.

I would try swapping injectors to see if the knock follows it, you could just have a bad injector, might not be spraying properly. If it doesnt, you likely need to start looking internally.

Check the intake carefully. If any engine oil is somehow leaking in, it could perhaps only be reaching that one cylinder, and would fire off early, and possibly heavily and creating your knock.

Your correct that generally a knock should sound on a sudden pull back of throttle, but not always, and they have fooled a lot of really smart people. Its doubtful its the injection pump, but I wouldnt rule anything out until you find the cause, and hopefully before it turns itself into scrap iron. Sometimes a cracked or broken piston will make more racket at slow speed, but again, if thats the cause and its not caught the piston could break up and bust your engine something incredible. If its cracked horizontally below the rings you could still have good compression. Those are hard to find. Basically, if the knock doesnt follow the injector, start taking things apart.

 

[robertsapp]

Diesels dont get fuel knock like a gas engine. Except for hot compressed air, there is nothing combustible within the combustion chamber until the diesel fuel is injected, and it combusts virtually instantaneously in the plus 1000F atmosphere. Prior to the injection, there should be nothing in there to burn.

I've read that in situations where an injector pops a bit late, the fuel enters the cylinder a little later in the compression cycle than design, finds a higher temperature and pressure than it would normally, and basically explodes rather than burns, producing a quicker pressure pulse and thus a knock. I'm rationalizing that this could possibly cause a knock that goes away at higher RPM and doesn't exhibit any of the classic symptoms of bearing knock. If that's what's happening, I agree that the problem should follow the injector. Replacing all three has been on my to-do list for some time, but I'm hesitant to do it now that the season has started because I know it's going to be "one of those jobs" that turns out to be ten times harder and take 20 times longer than the manual says. The reason for my pessimism is that the previous owner allowed a seawater drip from the vented loop to drip on the engine for probably years, and even though I've cleaned everything as best I could, the injectors look like they're going to be corrosion welded in place and I could have to take the head off and take it to the shop to have the injectors removed. That's something I'd rather do over the winter.

But the engine purrs at 2900 RPM all day long with no trace of smoke, so I'm guessing that whatever is going on can't be too serious. I'm just curious if anyone else has seen a situation with a pronounced knock at idle that completely goes away at power, and is not aggravated by sudden throttle changes.