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yanmar GM10 wont start

kieronriley

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Joined
6 Jan 2004
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498
Location
mersyside
a boat in our club has a a yanmar GM10 and during the winter we had to put some oil down the air intake to start it , but it was easy enough , from the first bit of a problem it sometimes would start without the oil and sometimes it needed it .on launching it needed oil but then ran fine for over an hour. however the owner went out yesterday and i went with him to check it would start ok ,but no chance even putting oil down ,no luck so i put my hand close to the air intake ,Filter removed and turned the engine over only to get the palm of my hand covered in oil again and again . I can turn the engine over by hand just by turning the alternator ,so there dosnt seem to be any compression. the engine had been unused for over a year ,i can only think a sticky valve BUT? anyone had such a fault and can maybe offer some help it would be most appreciated. the owner is a newbie to sailing and i told him that this forum was the best aid to all sailing problems. Thanks for reading this. Kieron
 

vas

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21 Jun 2011
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Volos-Athens
a couple of bolts to remove the rocker cover on top, turn engine by hand again and notice what the valves/springs are doing.
then bring on the mechanic...
 

kieronriley

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6 Jan 2004
Messages
498
Location
mersyside
hi , and thanks for your answers, I have already removed the rocker box , the valves are moving apparently ok but don't really know how to check if they are both closing properly , but would i be right in thinking that it would be the intake vale not closing properly .
 

black mercury

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4 Jun 2013
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scotland
Make sure the valve clearances are OK. At the moment simply check that there is clearance when the valves are shut.
Turn engine over by hand until valve is fully pushed down, then rotate engine one complete revolution so that valve is completely closed. Look at the gap between the rocker and top of valve stem. You should be able to rock the rocker with your fingers at this point.
 
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penfold

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25 Aug 2003
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4,545
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On the Clyde
Other causes include debris in the valve seat, displaced valve seat, broken valve head, etc. When cranked by hand does it spin freely? What position is the decompression lever?
hi , and thanks for your answers, I have already removed the rocker box , the valves are moving apparently ok but don't really know how to check if they are both closing properly , but would i be right in thinking that it would be the intake vale not closing properly .
 
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kieronriley

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6 Jan 2004
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498
Location
mersyside
hi ,Thanks again for responding. the decompression lever is closed, and yes it spins freely. i will next check the valve clearance as i think that if the inlet valve is far to big then this will maybe point to the fault,but i do think its off with the head (to quote a French revolution cry) and se e what we can find . is there a general agreement on that,thanks .Kieron
 

RichardS

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5 Nov 2009
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Home UK Midlands / Boat Croatia
hi ,Thanks again for responding. the decompression lever is closed, and yes it spins freely. i will next check the valve clearance as i think that if the inlet valve is far to big then this will maybe point to the fault,but i do think its off with the head (to quote a French revolution cry) and se e what we can find . is there a general agreement on that,thanks .Kieron
If one of the valve clearances is too large and it is clear that the valve is not moving up and down to the full extent of its travel, you may well be able to free it off by careful use of a mallet and an old screwdriver to move move the valve up and down a little with WD40 sprayed around the stem seal. Depending on the head design. you might be able to easily remove the rocker arm and tappets and then you should have clear access.

You will know once its free and this will save disturbing the head gasket.

Richard
 

earlybird

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18 Aug 2004
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Cumbria; U.K.
Are you sure that the excessive valve clearance is on the inlet, rather than the exhaust? A possibility is corrosion of the valve stem below the guide which prevents proper closure. This could be caused by sea-water entering the exhaust passage due to a failed elbow. The GM engines suffer from that fault.
 

kieronriley

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6 Jan 2004
Messages
498
Location
mersyside
Thanks very much for your helpful replies , I have read about the exhaust valve and can see how this would cause a problem but would that create the symptom of blowing oil out of the air intake ,I’m unsure as I thought the only valve giving access to the air intake would be the inlet valve. Am I not correct in that thinking, I really do love to learn so please tell me if I’m wrong, thanks. Kieron
 

black mercury

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4 Jun 2013
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Location
scotland
Thanks very much for your helpful replies , I have read about the exhaust valve and can see how this would cause a problem but would that create the symptom of blowing oil out of the air intake ,I’m unsure as I thought the only valve giving access to the air intake would be the inlet valve. Am I not correct in that thinking, I really do love to learn so please tell me if I’m wrong, thanks. Kieron
Yes, unless the exhaust is blocked, which I don't think is the case with your engine.
It is possible that when squirting oil in through the inlet valve the oil has maybe mixed with dirt in the cylinder head on its way into the cylinder and this dirt is now stuck across the valve face preventing it from seating. Before removing cylinder head I would use some brake cleaner or similar and spray it down the inlet, getting it close to the valve as possible. Turn engine over so that the inlet valve is open so the spray can clean the whole valve.
 

Tamarside

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19 Jun 2020
Messages
165
In the primary post you mention you had to introduce oil into the engine to start. This an old trick to increase compression due to worn bores. stuck rings. Although you seem to have some valve problems now, I would diagnose a lot more than is obvious.

As a minimum, remove the head and get a valve and seat job, This would also give you access to the top of the piston to assess possible further problems.
 

JumbleDuck

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8 Aug 2013
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23,588
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SW Scotland
My guess is that the exhaust port in the exhaust port in the head has rusted through. Sorry. Get your credit card ready and brace yourself for the lubricated fist of Yanmar's spare parts pricing policy.
 

Tamarside

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19 Jun 2020
Messages
165
My guess is that the exhaust port in the exhaust port in the head has rusted through. Sorry. Get your credit card ready and brace yourself for the lubricated fist of Yanmar's spare parts pricing policy.
Can be bored and sleeved. How would that spurt oil from the inlet?
 

Buck Turgidson

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10 Apr 2012
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Zürich
As has been pointed out. The symptoms are no compression and air/oil ejected from the inlet. Occam's razor folks!
 

Caraway

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11 Aug 2019
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4,119
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England
Expose the rockers. Hold the rocker in contact with the valve stem and give it biff with a hammer. If the valve is stuck it could release. You will notice if it does. Make sure the rocker sits back in the socket in the pushrod top when you have done.

You may have loosened the valve, or not, but unless you are very hamfisted you aren't going to break anything. You often have to tap valve caps to release the collets when dismantling engines.
It'll take no time and if it hasn't any effect you are going to be removing all that gear anyway. You just have to keep dismantling until you get to something that is wrong. Inspect everything as you remove it.

It's hardly a tricky job.

 
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