Schizophrenic Yanmar 1GM

[eebygum]

Thanks in part to expert advice from this forum we had a lovely sail from Lytham-St-Annes down to the Menai Straits on Saturday which took about 10 hours under sail and then motar-sailing...... however at various points in the trip the Yanmar definately got a bit moody !

The engine would purr along nicely for anything up to 2 hours put then periodically for up to 2-3 minutes shake violently on the engine mounts and make a right racket ! only to disappear as quickly as it appeared. It lasted probaly about 10 mins in total for the whole duration of the trip.

It did not appear to be dependant on the tack, the load on the engine or the state of the swell or angle of heel and continued even when the revs were cut back.

Any suggestions on what could cause such an intermittent problem, if there was a problem with the engine mounts then surely it would shake and rattle vioelently all the time ?

If the engine mounts had to be changed as a precaution can this be done when the boat is in the water ?

Thanks in advance.

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Age is a question of mind over matter. If you don't mind, it doesn't matter.

 

[snowleopard]

a few suggestions:

1. resonance. does it happen at a precise rpm and go away if you speed up or slow down?

2. occasional misfire resulting from muck in the fuel, air leaks or some such. being a single cyl it would jump about a bit under those circumstances.

3. something round the prop. if you have a rope cutter it would struggle until the cutter cleared away the obstruction. the torque from the prop would be transmitted up the shaft to the engine.

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[vyv_cox]

Sounds like typical reaction to a plastic bag around the prop. Seems very unfortunate coincidence to have struck five or six plastic bags in one trip, I've only picked up that many in the past 15 years. Maybe something else, such as weed. Was there a noticeable amount around?

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[brian_neale]

I have had something a little similar in the past on a 1GM, when it was air in the fuel. In my case, though, it would run roughly for maybe 10-20 seconds, recover a little if you throttled back, and then stop! Bleeding the thing was a quick cure, but it does not sound like the problem with yours, and the fact that it fixes itself sounds more like an external effect than a basic engine problem.

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[Jools_of_Top_Cat]

I was wondering if the prop was cavitating, (don't know correct spelling), maybe a gust of wind into the sails causes a speed change that had an effect.

I think you will have to motor without sails to eliminate this. This is only my opinion, obviously you were there and would know if this might have been the case.

The mounts on the 1GM seem like sponge, I am not happy with mine, my flexible couplings must be taking a hammering when you see the engines at tick over.

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[vyv_cox]

Soft mountings

This is the modern philosophy. Engines are mounted on very soft rubber to minimise transmitted noise. Yanmar make harder mountings for cases where there is a problem, this basically is those cases where the shaft contacts the stern tube. I went through all this when I fitted my 3GM but unfortunately it didn't cure the problem.

Single cylinder engines can look pretty frantic on tick-over, you might be able to reduce the movement by a slight increase in revs, but there is no problem if you get no additional knocking noises.

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[jleaworthy]

Almost certainly your engine is drawing a small quantity of air into the fuel somewhere between the fuel tank and the lift pump. It is then building up in the system until displaced. The erratic running happens as the air passes through the governor. Remedy is to check and tighten all joints and then bleed the system and try it! Good luck - I know how frustrating this can be.

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[oldharry]

These intermittent faults are the worst to find, as there is nothing there to work on, and no way of knowing whether you have actually solved it!

Firstly - eliminate what it CANT be: if its intermittent then its highly unlikely to be anything like engine mounts which are either right or not right. Its also unlikely to be a basic mechanical fault in the engine or drive train. It could be a fouled prop, but in my experience this rarely puts itself right without further intervention, and for a prop to clear itself so many times in a day - well whats your secret?!

Its unlikely to be an air leak in the fuel line if the engine is running consistently OK the rest of the time - leaks rarely seal themselves in diesel lines - they just get worse....! Micro bubbles of air in the fuel from an air leak dont usually affect the on-load performance: the commonest symptom is that the engine refuses to tick over properly, and keeps dying. Partially blocked filters can have the same effect.


Resonance etc: well I won't say it isnt, but usually that sort effect appears fairly consistently under certain conditons of engine speed/load etc.

So that leaves us with the variable faults that it might be: intermittent fouling of the fuel line with muck or water being stirred up in the tank and causing partial blockage of the lines and filters? In which case you were lucky not to suffer total engine failure, which is the usual end result. Check the separator for fouling.

An intermittent fault in the engine? faulty injector pump, injector, or sticking valves seem to me the most likely culprits, causing late, partial, or missed ignition and making the engine run rough until the sticking component clears itself and allows normal operation.

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[Evadne]

I had similar symptoms last season before more drastic ones appeared: see the Sick Yanmar thread adjacent. It could be a sympton of any number of faults but sticking to what I've had experience of:
water in the fuel, Diesel bugs (they go hand-in-hand), cracked exhaust elbow, damaged lift pump (caused by water ingress..).
Subsequent damage can be caused to the valves, rings, head and/or injector., so keep chasing it.
Good luck


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