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Old 08-07-2010, 19:01   #1
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Advice on Sick 3GM30

I purchased a sailing catamaran with two Yanmar 3GM30s.

The starboard engine has around 3000 hours on it and has a 55 Amp alternator. This engine runs quietly and well.

The port one is hooked to the hot water tank and a Balmar 120 Amp alternator and has around 3300 hours. I would expect the additional hours were at idle making hot water and/or 12 volts. This one sounds very bangy at less than 2000 rpm and will eventually stall out if it is reving at 1200. It does drive the boat well at high revs.

A number of mechanics have suggested that I have it rebuilt. If this were a gas engine, one might think that it is varnished up and a light honing of the cylinders and light regrinding of the valves might everything OK.

What the wise thing to do? replace it with new? or reconditioned? or do the rebuild? etc.

I hope to cross oceans in this thing.

thanks in advance,


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Old 08-07-2010, 19:26   #2
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Going on what you have written check tappets not likely to be main problem but is always overlooked and takes not much time and next to no money. I would then have a good Diesel injection guy check, compression + pump timing + have the injectors tested and at the very least serviced. I would not run the engine much until you have had these items checked as damage may be done to a serviceable engine that only needed some external repairs. 3000 hours is not much if the engines were looked after so run slowly. The not wanting to run under 2000rpm makes me feel that the fuel system or valves could possibly be the real culprit.

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Old 08-07-2010, 19:47   #3
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Pull the exhaust elbows off and examine them carefully for blockage.
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Old 08-07-2010, 20:26   #4
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How much money and time do you have?

You boat looks to be 6 or 7 years old. Boats 25 years old have similar engines that run well, as does your starboard engine.

For mine it's a question of time and money. If the money is no problem, and you're short of time have the engine pulled, sell it on Whatever and have a new one installed. End of problem.

If you have plenty of time and no money I'd suggest assuming that the bores had glazed and act accordingly.

There have been suggestions that changing to a straight mineral oil and running the engine hard is the way to go. Other on this Forum will know way more than me.

The worst course of action in terms of both time and money is to chase unknown faults at umpteen dollars an hour and expensive parts with a mechanic. I'd suggest do it yourself or don't do it. It might be expensive lessons but by the time you've got the problem sorted you could be as good as some mechanics. A nice way to go cruising.

There is, after all, the starboard engine if the port goes kaput.

Do keep in mind that my advice is worth exactly what you're paying for it.
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Old 08-07-2010, 20:42   #5
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Before investing in a very expensive rebuild I would do some basic maintenance. If the banging doesn't get worse at higher speed it is not very likely that something is coming loose inside the engine. A bad bearing would be very apparent.

Low RPM performance issues can be related to timing. You say its "bangy" and won't idle well. Bangy is a had thing to troubleshoot over the internet. Here are some things I would do.

- Adjust the valves on both engines. Probably hasn't been done in like forever. Valve clearance will affect performance, especially at idle. At low rpm excessive clearance can be heard as a mechanically "clackety-clackety" noise kinda like gears not meshed properly.
- Check the exhaust mixer - How many of those extra 300 hours were at low power where carbon build up could be worse.
- set the suspect engine in neutral to the rpm where the "bangy" is prevalent. Set the other engine to the same rpm. Examine both engines visually. Is the suspect engine shaking on its mounts? - Maybe new mounts are in order.
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Old 08-07-2010, 22:36   #6
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While I would check (clean) the exhaust elbows as a matter of service blocked exhausts usually effect an engine as the RPM increases and have lesser of an effect at idle, usually!
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Old 09-07-2010, 02:19   #7
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Try changing the injectors over you may surprised,cheap and easy too.
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Old 09-07-2010, 06:44   #8
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I had the same problem in my cat with two 3gm30fs. Having two engines focusses attention on the lesser one. In my case the stbd engine was hard to start and clattered exactly as you describe.

Replacing the injectors solved the problem.

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Old 25-07-2010, 00:15   #9
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Pulling and sending in an oil sample will tell you if there are and expensive problems that break things and make holes it the block, about $75 now. also run the engine to the rough RPM and crack the fitting for each of the injectors indivigually, there should be a noticable change when you have the line cracked. If no change you have a bad injector, injectors can be bad but still workable, send in for check and rebuild.
diesels can be hard to set the pump timing without a very good mechanical background.

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3gm, 3gm30, yanmar

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