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Old 25-12-2013, 18:12   #1
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Decompression required to start 3GM30 Yanmar

If it has been more than 24 hours since the engine started, It will not start unless I decompress the 3 cylinders and then flick the levers. I have changed the fuel filters )both primary & secondary) but without benefit. There are no fuel line leaks that I can see. Any ideas??
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Old 25-12-2013, 18:28   #2
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Re: Decompression required to start 3GM30 Yanmar

The starter may not be able to turn the engine over fast enough to get good compression. Could be caused by weak battery, bad battery connections, starter malfunctioning (dirty), starter brushes worn, solenoid contacts burned. Those are some places I would look.
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Old 25-12-2013, 18:48   #3
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Re: Decompression required to start 3GM30 Yanmar

So next time you have the engine running, pull each decompression lever one at a time and see if there is a difference between cylinders. If there is no change or not much of a change on one cylinder, that cylinder has in issue, Rings, injector or valves. Then next time you start the engine just decompress the one BAD cylinder.

If all are good each cylinder will sound like the next. Then

I found that if one cylinder has bad compression, it will prevent the engine from starting. Could also be a bad injector or miss adjusted valves.

Mine in the end was broken rings . From discussions here, there was a bad batch of yanmar 3gm30's from the early 90's that tended to break piston lands at relatively low hours. Mine did at 1100 hours. With luck yours will be a bad injector or valves needing adjusting. Also check for leaks in the fuel system.
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Old 25-12-2013, 18:51   #4
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Re: Decompression required to start 3GM30 Yanmar

Did the engine turn over prior to decompressing it ?. If it did was it at normal cranking speed?.
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Old 25-12-2013, 19:21   #5
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Re: Decompression required to start 3GM30 Yanmar

Quote:
Originally Posted by DeepFrz View Post
The starter may not be able to turn the engine over fast enough to get good compression. Could be caused by weak battery, bad battery connections, starter malfunctioning (dirty), starter brushes worn, solenoid contacts burned. Those are some places I would look.
Thanks Friz, I will check these things. However I have been crossing both starter batteries (cat with 2 engines) but will check starter connections. Starter will be a problem as am in Sabah.
thanks
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Old 25-12-2013, 19:22   #6
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Re: Decompression required to start 3GM30 Yanmar

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Did the engine turn over prior to decompressing it ?. If it did was it at normal cranking speed?.
Yes turned over andf cranking speed was normal.
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Old 25-12-2013, 19:43   #7
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Re: Decompression required to start 3GM30 Yanmar

If you have been starting on "both" batteries, try each separately. You may find you have a weak battery. If so, combining them will cause the good battery to discharge into the weak one, and you then are trying to start the engine with what is essentially a big (combined), weak battery without enough juice to crank to detonation.

Happened to me.
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