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Old 30-07-2020, 21:50   #31
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Re: Yanmar 2QM20 running issues

Read through, but didn't see anyone question what the liner sealing surface looked like.

Was there any crosshatch left? Scuff marks? A lip around the top of the liner? Grooves or pitting in the upper end of the liner?

Without answers to those questions, I'll go out on a limb and say you've got a broken compression ring in the cylinder with low pressure.
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Old 30-07-2020, 22:22   #32
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Re: Yanmar 2QM20 running issues

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Originally Posted by Wotname View Post
Hmm... I would be inclined to seperate the link between the two decompression levers and then try starting with one cylinder decompressed and see if the other cylinder fires OK. Then do it the other way around.

This might indicate if one cylinder or the other is compression challenged.
To expand on my previous post, the thinking is: good compression is necessary for initial start up and not so necessary for unloaded operation once started.

If there is enough compression to start a cold cylinder, then there should be enough for the engine to continue to run unloaded. The previous post is a poor man's rough compression check.

The fact the engine only runs for 30 secs with 3/4 throttle (presumably unloaded) suggests to me there is something more amiss than just low compression on cylinder although poor compression certainly won't help.

Looking forward to the next instalment.
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Old 30-07-2020, 22:51   #33
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Re: Yanmar 2QM20 running issues

Bit strange that it runs for 30 seconds only.
Despite the fact that your compression sounds too low I'd take Wotnames tip in post #26 & see if it keeps running on one cylinder.
It's possible you may have a fuel starvation problem as well tho the white smoke is unburned fuel.

EDIT Sorry Wottie didnt see your post #32 as I opened thread on an unrefreshed page. Still 2 opinions might be better than one.
My guess is JimB in post #31 might be on the right track.
Sadly 2 possible faults aren't better than one.
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Old 31-07-2020, 01:21   #34
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Re: Yanmar 2QM20 running issues

Update I ended up machining a new tip to fit, and then retested compression on both, 410 psi on #1 and 210 on #2.
So at this point I will be hauling out the engine and rebuilding the bottom end. Which is very frustrating but I have the time if only the parts get here quick. My pump was stuck at customs in Chicago for a month!
Thanks for all the suggestions and ideas. I should have done a better job of checking compression the first time it seems.
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Old 31-07-2020, 15:10   #35
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Re: Yanmar 2QM20 running issues

At least you've found the problem now. Pity it wasnt fuel related.
Thanks for letting us know the result so readers can all learn.
Yes the virus is a problem for shipping. Don't know where in the world you are but I cant get parts from my usual source in Asia via the usual post to NZ. Only way was prohibitively expensive.
Be good to hear what you find when you strip it down if you care to update the thread.
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Old 31-07-2020, 15:14   #36
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Re: Yanmar 2QM20 running issues

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Originally Posted by kmacdonald View Post
Its 14.7 times compression ratio IIRC.
Yes, quite right, should have used 14.7 not 14.5 as x factor.
I used to have to convert bar to psi but I didnt remember correctly.
Still doesnt match actual psi though as his compression test confirms.
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Old 31-07-2020, 15:19   #37
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Re: Yanmar 2QM20 running issues

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Yes, quite right, should have used 14.7 not 14.5 as x factor.
I used to have to convert bar to psi but I didnt remember correctly.
Still doesnt match actual psi though as his compression test confirms.
Yep. Compression ratio is actually a volume thing. Not sure exactly how its measured. At any rate, diesel needs at least 234 psi to ignite.
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Old 31-07-2020, 16:01   #38
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Re: Yanmar 2QM20 running issues

You will never get the pressure that the math will tell you, partially it’s because of leakage, but mostly because no normally aspirated engine is 100% volumetrically efficient, meaning that you won’t get quite atmospheric pressure at BDC intake stroke, there is some bit of vacuum, not much, but it’s there.
Ignition pressure is going to be variable with temperature and Cetane, higher Cetane, the easier the fuel is to ignite.

An old trick to confirm where the air is leaking out is to put a bit of oil in the cylinder, if it’s rings, compression will rise a lot, valves, not so much.
Also if you do a leak down test, you can hear where the air is leaking out.

If you do the oil trick be sure to spin the engine over a little before installing the compression tester, it doesn’t take much liquid to hydrolock a Diesel and you want to blow out the excess
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Old 01-08-2020, 17:30   #39
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Re: Yanmar 2QM20 running issues

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Originally Posted by a64pilot View Post
You will never get the pressure that the math will tell you, partially it’s because of leakage, but mostly because no normally aspirated engine is 100% volumetrically efficient, meaning that you won’t get quite atmospheric pressure at BDC intake stroke, there is some bit of vacuum, not much, but it’s there.
Ignition pressure is going to be variable with temperature and Cetane, higher Cetane, the easier the fuel is to ignite.

An old trick to confirm where the air is leaking out is to put a bit of oil in the cylinder, if it’s rings, compression will rise a lot, valves, not so much.
Also if you do a leak down test, you can hear where the air is leaking out.

If you do the oil trick be sure to spin the engine over a little before installing the compression tester, it doesn’t take much liquid to hydrolock a Diesel and you want to blow out the excess
Yes this is all correct but the point I was trying to get across was that it does not work to multiply atmospheric pressure by compression ratio to get your recommended factory compression tester reading in psi.
This was the formula the OP used to decide his compression was ok.
Obviously I didn't explain it well enough. OP thought his 200 psi compression tester reading was ok from his sick cylinder as he calculated his reading only needed to be 280. ( calc a bit awry shud be 296 )
However the required psi reading from a compression tester is much higheras is evidenced by his actual 410 psi reading from his healthy cylinder
His corrosion virus cylinder is 200psi

I tried to explain this with an example but my writing skills must be sub par & sadly OP focused on the injection pump.
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Old 02-08-2020, 15:53   #40
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Re: Yanmar 2QM20 running issues

Boyles law (P1V1=P2V2) only holds true for constant temperature. We didn't account for the rise in temp during the compression stroke and corresponding increase in pressure due to that rise in temperature or the volume of diesel in the gas.
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Old 02-08-2020, 17:05   #41
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Re: Yanmar 2QM20 running issues

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Originally Posted by kmacdonald View Post
Boyles law (P1V1=P2V2) only holds true for constant temperature. We didn't account for the rise in temp during the compression stroke and corresponding increase in pressure due to that rise in temperature or the volume of diesel in the gas.
Yes!
You've got it . Thanks for the reason but surprised one of the CF crowd with an engineering degree didn't jump in earlier to help. I only remembered the name Boyles law from school when I read your post.

Also it sounds like OP tested his injectors spray patterns on a pop tester rather than using the engines IP which may have lead to suspecting the IP.
I just assumed OP had used engine but Wotname did ask the question in post#16
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Old 09-08-2020, 09:58   #42
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Re: Yanmar 2QM20 running issues

So this past week I removed the engine and tore it down and it was pretty scary what I found! The crank journal bearing on #2 had been eaten away, there was only a third of it left.
There was damage to the crank but I found that there are 2 undersized bearings available. I gave it to a shop to see if they could polish and if not grind it to the next size down.
I spent the next few nights measuring everything and it is all in tolerance which makes me happy. The other crank bearing was installed backwards which tells me someone has been in the engine before. I believe they installed the other one backwards as well and this cause the failure along with my troubleshooting it and trying to start it. I have ordered a bunch of parts and will now have a completely rebuilt engine by the time I am done.

This engine has been a nightmare from the start and one of the only reasons I initially got the sailboat, but now I will know that it is done right and can have the confidence in it.
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Old 09-08-2020, 10:31   #43
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Re: Yanmar 2QM20 running issues

Proof is in the pudding. The rings and valves are very similar metals. If one is corroded. So is the other. Have faith. I rebuilt mine and never once had an issue with a again for years and was the most active boat in our marina. I miss that little guy.
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Old 09-08-2020, 16:08   #44
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Re: Yanmar 2QM20 running issues

Thanks for updating post, always nice to get the follow up & diagnosis
What were the pistons,rings & liners like?
I'd check the oil pump as well if you havent already in case that was a contributor to mains failure.
Lucky you can still get undersize bearings, I got no "longer available" when trying to get ones for a 1980 yammer. Ended up getting custom ones machined.
The 2QM20 has a rep for long life being a heavily built low stress reliable motor so you should be right for decades now
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Old 09-08-2020, 16:15   #45
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Re: Yanmar 2QM20 running issues

How do you install bearings backwards, don’t they have a notch in them so that they can only be installed one way?

Last post I made I meant to say that NA engines are NOT 100% volumetrically efficient, somehow the not got left out.
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