Yanmar 3GM30 only does 3/4 turn

[garymalmgren]

Good morning Val.
Well the good news is the you tried to rotate the engine by hand before start up and have probably not damaged anything., also you have created a nice brain teaser for yourself.

Looking at the parts list, I have to agree with your assumption that the culprit is not the oil pickup line. You would have to use a lot of force to dislodge that.
There is a lot of room in the lower crankcase (for oil) so it doesn't seem likely that the problem is there, BUT that is where you last worked on the engine so unfortunately it is the first place to start the trouble shooting.
Off with the oil pan, You have just done it, so you have all the tools and know the routine.
Eliminate the obvious.
Really looking forward to finding out what is causing this problem.
Gary

 

[markwbird]

In moving the oil filter to a new location you added a gizmo that mounts where the oil filter was located with hose attachments. There is a male stud that screws in there. Did you replace said stud? Is it possible that the new stud was put in too far causing it to come in contact with something inside that moves?

 

[SycloneDriver]

My guess is the decompression levers hold the valves open and for some unknown reason a valve is hitting a piston.
Try rotating it until it stops and back it up 90 degrees the release all the decompression levers and try rotating it again.

 

[Val V]

So wanted to update everyone that the problem went away but the mystery isn't solved. I went on the boat today and tried to turn over the engine again. Lo and behold it was turning just fine without anything being done.
I am completely puzzled by this but I have one theory which maybe plausible enough. It took about 2 weeks for me to get around to drilling and tapping the pan, getting new gasket and reinstalling it back. During that time the engine was sitting without a drop of oil and in temps between 25-40s here in NJ. I think something may have either rusted a little or something froze and got in the way.
When I am 30+ miles offshore I dont like knowing I have an unsolved mystery on the boat but I just don't get this one...

 

[Val V]

In moving the oil filter to a new location you added a gizmo that mounts where the oil filter was located with hose attachments. There is a male stud that screws in there. Did you replace said stud? Is it possible that the new stud was put in too far causing it to come in contact with something inside that moves?

The male threaded stud on the engine isnt touched at all. The adapter screws right in place where filter was.

 

[Paulus]

"My guess is the decompression levers hold the valves open and for some unknown reason a valve is hitting a piston.
Try rotating it until it stops and back it up 90 degrees the release all the decompression levers and try rotating it again"

Never thought of that... but then that would negate the whole purpose of this function which would be the ability to turn the engine with the decompress engaged,

This got me to think further - the decompress function unseats the valve only a little bit - not enough to cause an issue with piston travel - but the camshaft gives it a real push, to where the piston would indeed hit the valve!!!

It's known that when the timing belt goes, the pistons bust the valves on the way to engine stop.

the engine may have been resting with one of these valves pully lifted while you were working on it and when you were done, it "stuck" - maybe ice,, whatever... then, when the temps fluctuated, the sun shine wrmed things up a bit, the valve popped back in place and voila, the problem gone but mystery stayed...