Yanmar 3gm Low Idle Knocking / Slap

[laika]

I've got a 2004 Yanmar 3gm30f with about 2000 hours on it. Good, reliable engine. Bought the bought about 2 years ago and the low idle knock has always been there. Doesn't matter if its cold or hot. Engine purrs at anything above 1400 RPM. I most always run it between 2400 and 3000 RPMs. I changed out all the injectors last year and had the valve clearances checked. Had to pull the head to get a sticky injector out and the cylinder walls looked great. A little carbon around the rings, but the mechanic I had come look said the thing looked great overall. The engine runs great, and I've only had your standard fuel delivery issues in last two years, but still there's the knock at low idle. Was hoping it would go away with new injectors and/or valve adjustment, but no dice. Is this typical for a small diesel?

I suppose the next step would be running out for a stethoscope to try to pin down the noise, but just not sure if I'm chasing a ghost here.

[model 10]

There is so much knocking and clatter in a diesel , not sure a stethoscope would pin anything down. I would think a knock at cruising rpm would be your main concern. Piston clearance at low rpm makes a bit of noise but goes away as cylinder pressure increases and sets the rings

[MARINE ENGINNER]

laika - do you have this noise when you engage your gearbox?

[MARINE ENGINNER]

you should check also faulty injection timing of the fuel pump and pressure

[laika]

Thanks for the replies Guy and ME. If it's not typical, it does seem like I could probably get away with the knock long term, but I'd love to get the engine running as smoothly as possible. I've posted a YouTube video of it running at idle..

Yanmar 3gm30f Knocking at Low RPMs - YouTube

Marine Enginner, the knocking occurs when the gearbox is engaged, but it also knocks just the same in neutral. It's just a matter of RPMs. Get up above 1500 and she smooths out. Also, the knock seems to go away briefly after throttling down from high load, but returns after idling for a few seconds.

The fuel injector pump is really the last thing in the fuel system that has not been touched since I got the boat. Thanks for the tip. I've been trying to avoid looking at that part of the system, but maybe it's time to get the timing checked out.

Thanks again.

[roverhi]

Want to trade, it's way quieter than my 3gm30f with a lot fewer hours on it than you've got on yours. Sounds like normal diesel noises. They are designed that way so you won't run them at idle for very long.

[Cotemar]

Yup. Had the same knocking noise at ide with my Yanmar.

Easy fix.

[Cotemar]

Easy Fix.

One of the motor mounts is loose. Just tighten it up.
Noise is from the motor mount washer hitting the loosened bolt.

[Liam Wald]

Yanmar shop and owners manual says idle RPM should be 850.
My 3gm30f doesn't like 850. With idle set at 950 the engine is very smooth and knock is reduced considerably.

[Liam Wald]

Originally Posted by Cotemar

Easy Fix.

One of the motor mounts is loose. Just tighten it up.
Noise is from the motor mount washer hitting the loosened bolt.

Really good point. Very good idea to check motor mount bolts every couple of hundred hours.

[DotDun]

Originally Posted by laika

The fuel injector pump is really the last thing in the fuel system that has not been touched since I got the boat. Thanks for the tip. I've been trying to avoid looking at that part of the system, but maybe it's time to get the timing checked out.

If it's timing, it's most likely a stuck plunger and early fuel delivery. Not fixed by an actual timing procedure. One has to assume the timing was correct when new and as long as nobody has mucked with the shims under the pump, it's still correct.

Remove the pump and have it cleaned/calibrated by a professional. Use the same shims under the pump and put it back together. Make sure the governor linkage is connected correctly, check it twice before startup. (there is an inspection hole, remove the plate and check it)

Beware that diesel will varnish just like gas if things sit too long.

[Rakuflames]

Originally Posted by laika

I've got a 2004 Yanmar 3gm30f with about 2000 hours on it. Good, reliable engine. Bought the bought about 2 years ago and the low idle knock has always been there. Doesn't matter if its cold or hot. Engine purrs at anything above 1400 RPM. I most always run it between 2400 and 3000 RPMs. I changed out all the injectors last year and had the valve clearances checked. Had to pull the head to get a sticky injector out and the cylinder walls looked great. A little carbon around the rings, but the mechanic I had come look said the thing looked great overall. The engine runs great, and I've only had your standard fuel delivery issues in last two years, but still there's the knock at low idle. Was hoping it would go away with new injectors and/or valve adjustment, but no dice. Is this typical for a small diesel?

I suppose the next step would be running out for a stethoscope to try to pin down the noise, but just not sure if I'm chasing a ghost here.

I have a new engine that sounds like yours except that it's a 20. The manual clearly states that you can expect some knocking at certain RPMS that will be different for each engine and it recommends just running the engine above or below that RPM.

I'm no expert on diesels, but this is true of my engine, and it's 1500 RPM for my engine. I also run mine between 2400 - 3000 although its redline is 3600 and any time I'm running the engine for any length of time I do take it up to 3500 per the certified mechanic's instructions.

[MARINE ENGINNER]

laika your engine sound is fine .About the idle rpm don't increase too much from recommended because of the gearbox engagement

if the knocking noise was while you had engaged your gearbox in idle then your problem would be damper plate or very low idle rpm

[laika]

Thanks everyone for your help.

Sure enough, there was a loose motor mount which after tightening seemed to reduce the slap considerably. Still hear it, though it's softer and more intermittent. Maybe I just didn't torque the bolts enough. I was able to tighten down on all but one with just a hand wrench. Aft starboard mount was really loose.

I'm sure the fuel injector pump hasn't been serviced since new, so I might go ahead and get it serviced anyway this winter. Recently changed out the injectors and the lift pump, so what the hell.

I changed idle speed from 800 to 875 last year because the high amp alternator really liked to bog it down when it kicked in.

Thanks again all.