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Old 29-07-2020, 18:33   #1
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Yanmar 2GM20F - treatment for blowby

My Yanmar 2GM20F runs well and starts easily, but I have noticed that there is significant pressure from the crankcase when the oil fill cap is open, same thing when the breather tube is opened up, so I can tell that its not blocked. I don't have a problem with the dipstick popping out, so its not extreme pressure. I'm not burning any oil and there is no smoke other than a little at startup.

I've also noticed that the engine cannot reach max RPM - I don't regularly need more than 2900 rpm because that already drives the boat above hull speed, but I do open it up to WOT once per day to blow out the exhaust. The bottom and propeller are clean. Based on what help I can find, I expect that I am getting some blowby on the rings. I haven't tried to figure out how to do a compression check. From other posts I've read regarding blowby, some have added some rislone or marvel mystery oil ( to the engine oil) in the chance that it might help the rings seal. Some say that nothing should be added to the oil.

Anybody willing to weigh in on these symptoms and whether one of those products is worth trying? And if so, how much to use/how long to use it before replacing oil?
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Old 29-07-2020, 20:06   #2
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Re: Yanmar 2GM20F - treatment for blowby

First off need to know if it could reach max rpm before?
How many hours on the motor & how long have you noticed the blowby?
Did the problem occur suddenly?
Doesn't sound bad if it's not a smoker, starts easy & doesn't use oil.
Can't comment on those additives as no experience sorry.
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Old 29-07-2020, 20:40   #3
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Re: Yanmar 2GM20F - treatment for blowby

More questions

Does it rev to max in neutral?

When was the last time the exhaust manifold and injection point was checked for any exhaust restriction?
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Old 29-07-2020, 22:04   #4
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Re: Yanmar 2GM20F - treatment for blowby

That model usually has a vent from the tappet cover into the intake manifold to relieve any crankcase pressure.
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Old 30-07-2020, 03:49   #5
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Re: Yanmar 2GM20F - treatment for blowby

Answers to your questions....
It used to reach 3300-3400 under open throttle. Now I'm getting 3100-3150.
In neutral, I get 3300-3400, but need to double check. Certainly its higher than under load.

This did not happen suddenly, but it is getting a bit worse. I think has been progressing over 2 years but I only just noticed the pressure from the crankcase. The engine has no hour guage. My use is approximately 50 hrs per year. Over the 5 years I've had it. It's a 1986 and I suspect it has never been used heavily. I avoid running at idle for any long period of time.

I removed the exhaust elbow and checked it for restriction and corrosion in March. No problems found. It was last replaced 5 years ago.

The vent from the tappet to intake manifold is what I checked first for pressure. Then I confirmed the same thing when oil fill cap is removed
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Old 30-07-2020, 04:44   #6
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Re: Yanmar 2GM20F - treatment for blowby

Iíve heard marvel mystery oil but never tried it. Since it sounds like it runs clean agree with trying something before drastic measures like rebuild
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Old 30-07-2020, 07:06   #7
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Re: Yanmar 2GM20F - treatment for blowby

Thanks for answering questions as it helps to narrow it down.
I assume if it has an aircleaner that is ok.
Next is throttle linkages are they free & have full range of movement?. Try manually opening the throttle with cable disconnected.
Is fuel filter clean & no air in system?
Have you ever set the tappet clearances?
No prop shaft issues? i.e. stern gland not getting hot?
Have the injectors ever been cleaned?
Those are things you could check before doing compression test.
I'd be surprised if the additives do anything if you regularly run it up to 2900 rpm. My guess would be that their only chance of working would be if rings are gummed up.
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Old 30-07-2020, 07:55   #8
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Re: Yanmar 2GM20F - treatment for blowby

MMO won’t hurt but likely won’t do much either.
I second the valve adjust, be certain they are not too tight, valves wear tight, too tight loses compression lowers power and burns valves.

How many hours on the engine?
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Old 30-07-2020, 08:03   #9
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Re: Yanmar 2GM20F - treatment for blowby

Not burning oil ...so it's likely not ring wear. The 2GM and 3GM have suffered from cracked lower piston lands. Mostly 3GM that I have heard of. Same basic engine though. It sounds a bit like that.
You need a thread-in fitting to check compression on a diesel. I had one made by the mechanic who diagnosed my cracked piston. We shared the cost 50/50. Both my 3GM failed. One was replaced under warranty, the other failed at 1600 hours. What year is the engine?
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Old 30-07-2020, 08:11   #10
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Re: Yanmar 2GM20F - treatment for blowby

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Old 30-07-2020, 08:36   #11
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Re: Yanmar 2GM20F - treatment for blowby

Had a friend that was high enough in the Moble corp that he has access to their labs.

MMO was #1 diesel oil with a colorant and oil of wintergreen added to change the smell.

I would check with the outboard folks for solvents that free rings .

Use it then change the oil.

If it does not help enough pull one injector and borrow a bore scope to see if the cylinder bore still has hone marks.

The head can be pulled if needed to get rid of a burnished cylinder problem,
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Old 30-07-2020, 09:16   #12
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Re: Yanmar 2GM20F - treatment for blowby

Quote:
Originally Posted by SSAsailor View Post
My Yanmar 2GM20F runs well and starts easily, but I have noticed that there is significant pressure from the crankcase when the oil fill cap is open, same thing when the breather tube is opened up, so I can tell that its not blocked. I don't have a problem with the dipstick popping out, so its not extreme pressure. I'm not burning any oil and there is no smoke other than a little at startup.

I've also noticed that the engine cannot reach max RPM - I don't regularly need more than 2900 rpm because that already drives the boat above hull speed, but I do open it up to WOT once per day to blow out the exhaust. The bottom and propeller are clean. Based on what help I can find, I expect that I am getting some blowby on the rings. I haven't tried to figure out how to do a compression check. From other posts I've read regarding blowby, some have added some rislone or marvel mystery oil ( to the engine oil) in the chance that it might help the rings seal. Some say that nothing should be added to the oil.

Anybody willing to weigh in on these symptoms and whether one of those products is worth trying? And if so, how much to use/how long to use it before replacing oil?
I'm afraid you are approaching rebuild time.
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Old 30-07-2020, 09:23   #13
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Re: Yanmar 2GM20F - treatment for blowby

I had the same issue with my 2gm20f. The issue was a kink in the return hose from the cover to the intake manifold. Not much of a kink but enough to cause a block and create pressure in the crank case an pop the dip stick
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Old 30-07-2020, 10:02   #14
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Re: Yanmar 2GM20F - treatment for blowby

Youre not excessively burning oil...no smoke.. just a few hundred RPM drop at WOT? Have you cleaned the contacts on the tach sensor? My 2GM20F has similar issues but Im also loosing about 1/2 cup of coolant every 4 hours of running. I keep extra oil and a full jug of coolant on the boat.

Ive checked the breather as well to see if its open and functional... I was surprised that it was quite dry inside instead of wet oil. Maybe I should pull the tube off of the outlet and run the engine to see if Im getting airflow out of the breather.
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Old 30-07-2020, 10:17   #15
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Re: Yanmar 2GM20F - treatment for blowby

Is this engine considered obsolete ? I notice no-one seems to offer an overhaul kit for it, whereas you can get a kit for a 3gm.I don't know if the liners,pistons,valves etc are compatible or not. Blow by is going to degrade the oil much faster than normal wear. I would try to determine if the liners and pistons would work because if it would you can get yourself a practically newnengine for $600+ some labor.
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