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Old 10-06-2016, 05:04   #1
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Yanmar 1GM10 Bad Compression

I have had trouble all season starting my Yanmar 1GM10. Turns over well, plenty of fuel, suspected an air leak. Eventually brought in a mechanic who diagnosed it with no compression. Seems odd because it ran well last summer with only a couple of months of inactivity, although I was concerned with a lack of power at the time.

I'm looking for a starting point.
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Old 10-06-2016, 07:54   #2
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Re: Yanmar 1GM10 Bad Compression

Originally Posted by Latitude 28 View Post
I'm looking for a starting point.
You might trust the mechanic.
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Old 10-06-2016, 10:19   #3
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Re: Yanmar 1GM10 Bad Compression

A diesel needs 3 things to start. Fuel, compression and enough compression pressure to raise the air temp to above ignition temperature. You loose compression from valves, rings, stuck open compression release and the engine turning over too slowly. If you have a compression release, I would check that 1st. If the mechanic is right and you have no compression then it's a major opening, like a stuck open valve or totally worn out rings/cylinder, burnt valve, valve way out of adjustment. If it's low compression, it can be rings contracting while sitting in cold weather.
Removing the injector and squirting oil in the cylinder will help seal the rings. Heat will help starting. Either a block heater (best) or heating incoming air can help the engine reach ignition temperature.
If you've been using starting fluid it could be broken rings. Ether raises compression above normal. If too much is used the very high pressures will break the rings leading to even more lost pressure.
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Old 10-06-2016, 12:47   #4
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Re: Yanmar 1GM10 Bad Compression

Hard starting is the first sign of low compression. Of course you could have an air leak from a banjo fitting. Not that uncommon. Check the valve lash first and banjo leaks, and fuel filter(s) and if that's fine, then pull the head and do the sailorchic leak down test.

If it is low compression, it's probably broken rings. Yanmar has had some quality control issues over the years. What usually happens is a ring land breaks and that breaks the ring(s).

But if you do the work yourself, it's not too spendy. New piston and rings is $100-150 and $200-$300 for machine shop, plus bearings and head gasket, timing cover gasket, seals, etc. Probably $500-$600 in parts and machining.

You can read about my adventures rebuilding a yanmar gm here

If the engine is an older GM the sleeves are replaceable. Late model gm's did not have sleeves. Though I had one hole bored and sleeved. So it can be done. NOTE: Older GM's and newer GM's had different piston sizes.
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Old 10-06-2016, 13:02   #5
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Re: Yanmar 1GM10 Bad Compression

That engine should be a no brainer to overhaul. New rings at least. Check the piston closely. Disassembled I wonder if you couldn't carry the block home by just lifting it?
"I spent most of my money on Booze, Broads and Boats. The rest I wasted" - Elmore Leonard

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Old 11-06-2016, 08:17   #6
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Re: Yanmar 1GM10 Bad Compression

Low compression, check compression release.
Next, remove injectors. Squirt small amount of oil into cylinder, then re-check compression. If markedly improved, it's the rings. If not, the valves.
Broken Yanmar ring lands usually the result of long deferred injector maintenance.
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Old 11-06-2016, 10:49   #7
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Re: Yanmar 1GM10 Bad Compression

If mechanic did not remove injector and connect a high pressure gauge he is guessing about compression. I have a compression gauge with Danfoss dummy injector connected to it, you pay shipping and it is yours.
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1gm10, compression, yanmar

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