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Old 13-01-2015, 11:17   #1
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The compression results are in, not good. (3GM30)

So the saga continues after I rebuilt the top end on my Yanmar 3GM30. It runs on all cylinders, its a little hard to start but once its up to temperature it seems to run ok and runs under load too. I had a compression test done as a final check on my work, this was carried out by a Yanmar specialist who has an office at my marina and he sent me an email with the following info today:

Cylinder 1: 28 BAR
Cylinder 2: 30 BAR
Cylinder 3: 0 BAR

He says correct compression should be 32 BAR

Firstly, every where I read people talk about psi ratings and this guy comes back to me with BAR. after googling conversion it seems my first 2 cylinders are compressing above 400 psi? Is this right? it seems very high for an engine of almost 20 years. Can someone weigh in on the figures.

Secondly I have a hard time believing this figure for the third cylinder as it runs reasonably well. Shall I look into getting my own equipment so I can get a second opinion by myself and test any repair work I may carry out?

Thirdly after (what I would consider) a successful top end rebuild could I attempt changing some piston rings myself? Can I save money and just do one or would all need to be done? Would I need to replace the head gasket again? That would require even deeper digging into the pocket.

Thanks as always for your informative advice CF
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Old 13-01-2015, 12:34   #2
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Re: The compression results are in, not good. (3GM30)

1 BAR is 14.7
Find out where your compression loss is, I don't believe 0 BAR either, but perhaps that's his way of saying it real low.
It may be a stuck valve or something, I've had that on aircraft engines pretty often.
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Old 13-01-2015, 12:38   #3
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Re: The compression results are in, not good. (3GM30)

Reading your post again, you have had the head off and re-built?
If so make sure valves are adjusted correctly, one isn't too tight
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Old 13-01-2015, 12:55   #4
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Re: The compression results are in, not good. (3GM30)

I cant remember which threads are which, so I will just post this info again. Sailor Chic and I have both experienced cracked piston issues on that engine. Mine ran well except at idle, but started hard at times. My engine was about 8 years old at the time. Less than $2000 repaired it including removal/reinstall from the boat, new pistons and rods. (year 2000 $)
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Old 14-01-2015, 02:53   #5
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Re: The compression results are in, not good. (3GM30)

Hey cheechako
Yea I didn't realise posting to my old thread would have bought it to the top of the main thread page. I'm not sure how to diagnose a cracked piston and I'm hoping the fix will be easier than that but I'll bear it in mind if the following doesn't work:

Turn the engine over by hand with only #3 co;pressing, if i feel resistance, there must be at least some compression.

As A64 suggests, re-check valve timing, I already have the tools to do this it just takes a little time. Do you guys recommend doing this while the engine is warm?

Re-check torque on head bolts, particularly as its been running now.

I found this compression testing kit on Amazon, and I think it costs less than the co;pression test I just had done. Don't know if it will fit the GM series engines however. Anybody had any joy with this or another kit?
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Old 14-01-2015, 03:33   #6
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Re: The compression results are in, not good. (3GM30)

Quote:
Originally Posted by dtlx11 View Post
Hey cheechako
Yea I didn't realise posting to my old thread would have bought it to the top of the main thread page. I'm not sure how to diagnose a cracked piston and I'm hoping the fix will be easier than that but I'll bear it in mind if the following doesn't work:

Turn the engine over by hand with only #3 co;pressing, if i feel resistance, there must be at least some compression.

As A64 suggests, re-check valve timing, I already have the tools to do this it just takes a little time. Do you guys recommend doing this while the engine is warm?

Re-check torque on head bolts, particularly as its been running now.

I found this compression testing kit on Amazon, and I think it costs less than the co;pression test I just had done. Don't know if it will fit the GM series engines however. Anybody had any joy with this or another kit?
That tester may not be man enough for a diesel engine; it is advertised for petrol engines which have much lower compression than diesels.
8 to 1 as opposed to 20 to 1.
Check carefully before buying!
EDIT: It's only rated to 300 psi.
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Old 14-01-2015, 03:45   #7
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Re: The compression results are in, not good. (3GM30)

That compression kit is for gas motors (not high enough) and I'll bet the adaptors are for spark plug screw in. You"ll need a 500 psi gauge.
You'll need to measure one of your injectors to see what size it takes. And that it'll take the two holed clamp to hold it in place. the hose should screw into the adaptor.

http://boatinfo.no/lib/yanmar/manuals/1gm10.html#/48

Yanmar difficult to start after rebuild and only firing on 2/3 cylinders
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Old 14-01-2015, 05:58   #8
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Re: The compression results are in, not good. (3GM30)

So I'll add 'measure injector housing size' to the list of jobs when I'm next down the boat.

I've read of people making their own compression testers from old injector parts. Anybody have any success with this?
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Old 14-01-2015, 06:58   #9
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Re: The compression results are in, not good. (3GM30)

Quote:
Originally Posted by dtlx11 View Post
So I'll add 'measure injector housing size' to the list of jobs when I'm next down the boat.

I've read of people making their own compression testers from old injector parts. Anybody have any success with this?
Easy, the picture shows one I made for some Volvos. Didn't cost me anything, I had the fittings and the gauge, got an old injector body from the injector house that reworked my injectors, pulled the insides out, welded a 1/4" pipe coupling to the end of the injector body and screwed the gauge in. All the extra fittings between the black line and the gauge itself are to add length for clearance. Not very pretty though.

You can get a 500 psi gauge for less than 10.00 here in the US.
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Old 14-01-2015, 08:44   #10
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Re: The compression results are in, not good. (3GM30)

Check this web link for a 3GM30 service manual:

Download script

Regards.
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Old 14-01-2015, 10:06   #11
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Re: The compression results are in, not good. (3GM30)

As allready mentioned

No decisions until a viable compression result is established
Results on cylinder 3 ....if this is true You donīt have to worry about PSI ratings or BAR
anymore
Itīs time for estimating a change of the pistons etc
Another choice is finding a used engine with GOOD compression and trying to sell parts of the old one.
Specialized engine rebuilders should value Your engine in exchange
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Old 14-01-2015, 13:03   #12
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Re: The compression results are in, not good. (3GM30)

I'm going to bet that when the OP pulls the engine apart, he'll find #3 with broken rings and lands. That's exactly what I found on mine. Yanmar has a tiny problem with piston lands breaking in some model years of the GM series.

Luckily parts to repair are about $700 and machining, I had the block milled and a sleeve installed on my #3 was $300. Be advised that pulling the inner crack nut off requires a special tool, which you will need to make or have a welder make.

Oversized pistions are available. Stock size was $95 with rings two years ago. 10 over was $150 each. Whe I machined the one bore as the other two measured as new.

You can read my sage here: I was Bored so Pulled the Head on my Yanmar
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Old 14-01-2015, 14:06   #13
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Re: The compression results are in, not good. (3GM30)

I was lucky and my cylinder was fine... no groove. Only my bottom piston land was cracked.
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Old 14-01-2015, 14:21   #14
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Re: The compression results are in, not good. (3GM30)

A quick way to test the rings is to inject a little oil into the cylinder and re do the compression test, if it is considerably higher then most probably the rings.

You might ask the mechanic if he can do a leak down test, as that would tell exactly where the problem lies.
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Old 14-01-2015, 15:25   #15
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Re: The compression results are in, not good. (3GM30)

Any crankcase blowby? After warmup is compression any different?
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