Cause of Oil Leak at filler cap

[Alex 2740]

Hello all,

I have a Yanmar 3GM and am experiencing oil leaking out of the filler cap on the top of valve cover. It only seems to happen at 2500 RPM and higher. I have replace the cap and O-ring and checked for cracks around the hole. Additionally I recently replaced the exhaust elbow out ouptut pipe so I dont think that it is back pressure fom those. Could it be the exhuast muffler causing back pressure or am I going down the wrong path?

 

[Maine Sail]

It sounds like a positive crank case pressure issue. Check to make sure the venting for crank cases pressure is not plugged with sludge..

 

[Tim R.]

Could also be piston rings(too much blow by for the PCV to handle). Check PCV first as that is cheaper and easier to fix.

If PCV is ok, do a compression test on all cylinders.

 

[kloudie1]

The PCV or air breather is in the valve cover (Bonnet in Yanmar talk).. It is the little 4 bolted cover on the aft end of the valve cover, with the hose going to the air cleaner plate. Remove the bolts and open the breather cover and make sure all the passages are open including the hose and the space around the partition plate inside.. May be excessive blow-by from worn rings (a clue would be that the engine oil would get really black really fast after an oil change) .. I've not heard of one of the vents plugging, but certainly, it can happen.. Good luck with it, Alex !

 

[Ross]

Or it could be a bad gasket on the cap.

 

[Warren Milberg]

Is the oil level correct when the engine is at rest?

 

[Alex 2740]

Thanks for all the feedback. I puuled the bonnet and the chamber does apper to be clear. I also checked the hose and back to the air intake and it all looks clean. I did notice that in the base of the PVC bowl the engine diagram indiates that ther is supposed to be a drilled of hole in the bottom. I have gotten in there with a magnifying glass and dental tools and I dont see any signs of a hole. Just wondering if this could be normal?

I tried to get a compression test about a month ago in another port but the Yanmar dealer had a home made tesing rig and showed a compression of 240 on each cylinder head. As I remember the spec is about 460 with a minimum starting compression requirement of 357 or so. The engine uns fine starts easy etc... so I assumed that his gear was faulty and so did he (No charge for the attempt) but I will try again where I am today.

 

[kloudie1]

Interesting.. The PCV is essentially an oil/vapor separator. There must be some provision for getting the oil back into the bonnet.. but if the oil is not leaking out of the breather and not going into the air cleaner, the breather is probably OK..
I've not had mine apart, so I don't know what the liquid oil drain passages look like.
it will be interesting to know what the compression is.. Those previous numbers are pretty low !
Good Luck with it, Alex.