Rough spot around 1100 rpms, Yanmar 3GM

[A71busman]

Hello to all. I have an issue with my Yamar 3GM30 I cant seem to resolve. It idles as should around 600 rpms but when you slowly raise the rpms, when you get to about 1100 or so it coughs and shakes and sounds rough...like its starving for fuel or something. After about 1200 rpms it smooths back out and runs stable with no issues on up. This is my first diesel engine so consider me a " newbie " in this dept. I have tried bleeding the fuel sytem, changing the primary and secondary fuel filters, cleaned out the Racor separator...with no luck. Some dock buddies of mine say that diesels have a " sweet spot " in their rpm range and that this is just not mine. The service manual does not list rough idling/ accelerating in one spot only. This motor has about 700-800 hours on it since rebuild based on what the p.o. told me. Im wondering if the valves need adjustment and this is a valve issue, or if I have a clogged injector that needs cleaning or replacing? I did notice there is some black grime/ dirt built up around my # 3 injector where the return line goes to the tank but no leaking fuel is visible. Anyone have a similar issue with theirs?

[Wotname]

Welcome aboard A71, hopefully some of our diesel friends will be along soon to help with this one.

I don't know if the following would apply to a 3GM but I can tell you that the 2GM engines often (but not always) have a "rough" spot where they will shake around quite a lot. One makes sure you keep them out of that rev range. The actual revs will vary from boat to boat and is very installation dependent. My 2GM doesn't like 2,100 rpm so I keep it below 2,000 or above 2,200 and it remains happy .

[A71busman]

Thanks for the info... Thats good to hear. I can avoid the range when under way, the only issue is when im slowing down... Say coming in to my home port or going thru a minimum/ no wake zone.

[svseachange]

We had the same issue with our 3HM. It was a serious problem because the engine would cut out when we were maneuvering in tight quarters.

A fuel pump had deteriorated and put junk through the system. The mechanic ( not me) replaced the fuel pump, the primary and secondary filters and all was well. He said the original fuel pump was not intended for marine use.

We also replaced and aligned the motor mounts at the same time, one was broken, the others old.

Done a lot if motoring since and no problem... touchwood.

[mrohr]

It seems to me that every motor has certain rpms where its happy and others where things are rougher. Am I wrong?

[oldjags]

Originally Posted by mrohr

It seems to me that every motor has certain rpms where its happy and others where things are rougher. Am I wrong?

I don't think so. Everything has a resonant frequency that amplifies the vibrations at that point. It may be engine related, or boat related, or a combination of the two. And there may be several RPM points that seem to be more affected than others. Just stay away from running the engine at those RPMs.

[GILow]

I have a number of rough spots that I have to avoid on our Nissan ED33. None of them have what you describe as a misfire, but they will rattle the fillings out of your teeth.

Basically I am safe from 500 rpm to 1100, then I have to avoid 1100 to 1200, safe again to 1500, then shakes like heck till 1600, safe to around 1900 then shakes till 2000 then clears again to 2200 which is the most I have run it at, although the manual says it will do 2800, I have not had the guts to try it yet.

For what it is worth, I just had a hugely expensive rebuild of the injector pump
and injectors (after feeding a lot of water through the system) and although my wife reckons the motor is a bit smoother now, I can't feel much difference (except the thing starts) and it certainly did not affect the rough spots.

I am told by a diesel expert (not the armchair variety) that my engine is in excellent condition, and that these harmonics are to be expected due to hard mounting to the hull, as in the truck it was on big rubber blocks to absorb all this stuff.

So in short, the roughness sounds ok but the misfire/cough worries me.

Matt

[GILow]

Originally Posted by mrohr

It seems to me that every motor has certain rpms where its happy and others where things are rougher. Am I wrong?

I think you are right, and I think car engines are so well insulated from the driver and passengers that we don't realise there are these periods of roughness happening all time. Of course in (older/less glamorous) boats we don't have those levels of protection and isolation.

Matt

[Jacko]

Hi Busman don't let the (armchairs ) have you worrying about your motor, as has been said already most diesels have a rough spot and it is a bit more pronounced in the 3 cylinder jobs. My Yam has the same spot as yours and I am now heading for 4k hrs so just take it through that rev range and carry one. Cheers Jacko

[sartorst]

You just have to choose which thing on the boat you want to hear rattle, stove, boarding ladder something on bulkhead etc etc Ha

[GILow]

Originally Posted by sartorst

You just have to choose which thing on the boat you want to hear rattle, stove, boarding ladder something on bulkhead etc etc Ha

...teeth...

[svseachange]

Suppose it depends what you mean by a "rough spot". If we wait too long to change our Racor element our engine runs a little rough at about 1000 rpms.

The ten minute solution is to drain the Racor bulb a little and change out the element and bingo, problem solved. I have a bit of clean diesel on hand to keep the Racor filled with diesel to prevent air getting in the fuel lines.