Yanmar Engines Running Rough at Idle

[Doublewammy]

Hi There, I have a problem with both my Yanmar 3JH3E engines running rough at idle. The engines now cause a vibration through the hull and deck at 800RPM, something they never used to do. About 2 years ago it was only the port engine now it is both. I have changed filters every 50hrs for the last 2000hrs, only once has there been a need to change them earlier, Indonesian fuel can do that sometimes.We have just returned from 1700nm's of average to bad fuel which we have filtered well before letting a drop hit the tanks. A lot of this distance was under motor as not much wind.
I have checked injectors, tappets, lift pumps, exhaust mixing tubes but with no real lead to my problem. We have just had the starboard fuel pump, easiest to get to, serviced but the verdict was that it was all ok. This leaves fuel lines and anything anyone can suggest.

[Albro359]

What about the engine mounts ??..have you checked them ?

[Old Snipe]

Engine mounts

Check air system for obstructions.

If you have been doing a lot of engine cruising with questionable fuel, my gut tells me it is in the high pressure side of your fuel system.

I don't mean to be rude, but some ports will pass on bad fuel with low flash points and high BS&W. Paraffin content also varies from place to place depending on where the fuel was refined. Low paraffin content can cause premature wear on the pumps and injectors/nozzles.

Have the pumps and injectors/nozzles bench tested on a actual test bench. This is the best way to see what they are doing under variable RPM's. Most good engine shops can do this fairly cheap if you do the removal and delivery of the components.

[Doublewammy]

Hi Old Snipe,
Thanks for the reply. I forgot to mention we have replaced the engine mounts. We have just collected the fuel pump and injectors back from the local pump shop, which is highly recommended for good work, it has been stripped and nothing was replaced besides seals and gaskets, tested on the bench and proven to be in good condition.
The fuel we have been using is probably exactly as you mentioned but there isn't any other type, it all comes from the same place in Sumatra. I replaced the filters 3 times whilst at sea and they were brand new when we left.
If the lines were almost clogged but allowing some fuel to pass would this affect the motor in this way. If I rev the motor to 1000RPM the shake in the motor and mounts almost disappears. It used to be like this all the time, i.e. 700RPM to 2200RPM.

[Old Snipe]

What about pump timing, has that been checked or tweeked?

[Doublewammy]

We have the diesel mechanic reinstalling the pump tomorrow, I will ask him about the timing. He is very capable so I am sure he will.

[Sailmonkey]

How old are your rubber fuel hoses? Ours developed pinhole leaks after several years that would allow air in but not fuel out. The leaks manifested themselves at idle with a rough engine, but faster and the primary fuel pump moved enough fuel volume to negate the very small amount of air in the system.


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[Doublewammy]

I will check all the hoses and clamps tomorrow when we clean the copper fuel lines. Thanks for your help.

[SVTwilight]

This is a common complaint with Yanmar. We had this problem on our 3ygm30 when it was new. There are apparently several different option of engine mount "stiffness available for each engine type. The yard had to try a couple of times to get the best result. Also, we had to bump the idle rpm to the high end of the range and it is largely better.

Good luck!

[Doublewammy]

Hi There SVTwillight,
Thankyou for your help. We changed the engine mounts last year, one was broken. We checked them before leaving Fremantle to race to Bali, they appeared fine but who knows. We replaced them with the middle strength mounts, cant remember the number, it is on the side of the mount but all 4 mounts are the same. The guys in Singapore who deal in Yanmah sized them for the engines and Sail drives. We checked them a couple of days ago and they are fine. Still look brand new. What do you think?
Cheers.

[SVTwilight]

Well it sounds ok, but I really don't know how to assess the elasticity of the mounts. I think they can harden and still appear ok visually. It seems if the rubber mounts harden then more shock is transferred into the boar resulting in or at least contributing to the symptoms that you are seeing. If none of the other suggestion pan out, trying softer mounts might be a good option

Good luck!

Originally Posted by Doublewammy

Hi There SVTwillight,
Thankyou for your help. We changed the engine mounts last year, one was broken. We checked them before leaving Fremantle to race to Bali, they appeared fine but who knows. We replaced them with the middle strength mounts, cant remember the number, it is on the side of the mount but all 4 mounts are the same. The guys in Singapore who deal in Yanmah sized them for the engines and Sail drives. We checked them a couple of days ago and they are fine. Still look brand new. What do you think?
Cheers.

[Doublewammy]

Hi SVTwillight, well we have pretty much gone over this engine from top to bottom. Things found: A steady flow of air bubbles starting at the prefilter, if we take the filter out of the circuit we have no bubbles but it does not make any difference to the way the motor runs anyway. I am looking at installing 2 inline fuel pumps. The hose between the exhaust mixing elbow and the muffler box has hardened to a point that it is not flexible anymore, we have replaced with new silicone hose and a lot of the hull vibration has gone but the motor still rocks at idle, we have upped the idle rate and this has also smoothed out the motor. I am looking into 2 sets of mounts, Do you think softer would be better than harder? I have checked the old ones were 200's and we replaced them with 150's. These are the middle of the mount types.
The fuel pump was ok but that whole process was good to go through and not to expensive anyway.
Will have to wait and see with these mounts.

[Arthur Garfield]

Probably mentioned by ealier responders-Get GOOD fuel and injecter cleaner and run these for awhile (2-3 hours).Check. If no improvement change injector tips(easy if you're familiar) or bench test for correct spray pattern.If this is 3cyl.
It has roughness.



All the Best

[DeepFrz]

Try a cetane booster.

https://www.youtube.com/watch?v=N_VJZJmApEc

[Coxswain]

Hi DoubleWhammy,

Have seen this issue in a few cats that have come back from Indo. A lagoon that I was working on had a split in the fuel pickup line in the tank, it was sucking tons of air.
It could also be a loose electrical connection, had another similar issue where it would run fine, then after a period of time constantly cut in and out... Spent hours playing with the fuel line until I noticed a loose connection on the starter motor that was the problem!!! Aaaarrggghh.
If you are in Freo area and need a hand, happy to help.


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