Sick Yanmar

[Capt_Scarlet]

I have a Yanmar 2GM20 that suffered from poor starting last season. It would only fire on 1 cylinder when cold.
I removed the cylinder head last weekend, and discovered heavy pitting in the head and piston of one cylinder, and the piston could be wobbled in the bore by about 0.5mm - the other cylinder was fine. Also, the pitted one has far greater carbon deposits on it.

I reckon I need a new piston and rings in the duff cylinder, and probably new valves.

My questions are:
a) What was the likely cause?
b) Am I likely to get away with just a piston and ring change, or is a rebore required (no obvious scoring seen).

Thank you.

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[snowleopard]

it's just possible you have had water in the cylinder which bent the con rod causing reduced compression.

the way to tell is to take a length of multicore solder, bend the last few mm at right angles and stick it down the injector hole. when you turn the engine over by hand the wire will be crushed. do it for both pots and measure the width of the flattened bit. if i'm right the duff cylinder will produce a narrower result.

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[Birdseye]

I had a problem with my 2 gm which was the result of corrosion in the exhaust elbow allowing sea water to splash back into the head then causing corrosion there as well ultimately leading to complete oil loss and failure.

Your problem is clearly not to be solved by sticking new rings into a rebored block. That would cure the symptom not the cause. The engine needs examining by a Yanmar dealer or at least by a reputable diesel engine reconditioner.

Sorry to bear bad news, but what you describe is not a routine minor problem.

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[pete]

What was the condition of the valves and valve seats on the faulty cylinder I had a starting problem on my 1 GM 10 and it had burned out the exhaust valve seat, after having the seat re cut and fitting a new valve the fault stopped only to reappear about a month later ( exhaust seat again) whilst the head was then being fitted with new valve seats the machinist noticed that in the bottom of the seat hole the metal was black and on prodding it with a pointer we were into the water jacket it seems the head had gone porous above the exhaust valve and was driping salt water on to the valve and burning it out it ment a new head but solved my problem (this is about two years ago), the head for my engine was just over £ 300 - inc vat and delivery and included valves, springs, ect Thank god it was not a vol£££vo.
Pete

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[Jools_of_Top_Cat]

how old was your engine when this happened?



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[Evadne]

My 1GM7 has just had the same problem -burned out exhaust valve, pitted head, etc. Symptoms the same as described: poor starting, wouldn't idle. Source of water was believed to be the cracked elbow, the engineer who was doing it for me recommended checking the inside of the exhaust elbow every 2-3 years, they often don't last more than twice than that apparently.
Other sources of water (and I've had them all!) are blocked or faulty siphon valve and insufficient filtration (i.e. water coming in from the tank). The latter is usually accompanied by bacteria. Once the rings, valve and/or pre-ignition chamber gets damaged, the symptoms seem identical regardless of the source of the water.

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[pete]

Hi Jools
The engine was around 11 years old when the head had to be replaced, the guy I bought the boat off had it profesionally serviced from new but when I checked the engine anode there was very little left, perhaps it was due to this or perhaps it was an inperfection in the head casting from new
PS the engine looks almost like new with excellent compresion 580psi when I checked it with the new head.
I have since converted it to fresh water cooling !
Pete

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