Yanmar engine question

[peter2407]

3GM30F with c. 30% lower pressure in the centre cylinder than is expected as per the spec. Rather than take the engine out and do a refurb/rebore, what other things should be looked at that might save that being necessary?

TIA

 

[vyv_cox]

Other factors might be tappets too tight so that a valve is not closing fully, bent or burnt valve, debris on valve seat, crack in cylinder head, failed head gasket. Anything below that, e.g. piston, rings or bore would require the engine to come out.

 

[jakeroyd]

Not much really.
It's possible the valve clearances may be wrong on that cylinder.
I would suggest checking these just to be sure.

After this you could try running a little oil into the cyclinder and checking the pressure once more.
If the reading increases this would point to rings or possibly the cyclinder bore.

Are there any signs on the outside of the engine that the cylinder head gasket is blowing on that cylider or that the head is not flat near that cylinder?

Is the injector seated correctly ? (unlikely since i assume it was removed for the compression test)

What is the spec? Do you mean it's 30% down on the lowest spec value or down relative to the other 2 ?
If the engine is running OK it may be possible to just live with the low compression?

Going back to rings , is there any sign of blowby contamination of the sump oil ? Ie dilution of the oil or a rise in oil level ?

 

[peter2407]

Thanks Vyv. I did have the injectors refurb'd when I initially had the problems with the engine that turned out to be due to the exhaust elbow being full of crud.

 

[robmcg]

As already eluded to, I would suspect a coked up exhaust valve not seating properly. Quick inspection, relap and you may be cured.

 

[toad]

As already said if the valve adjustment is ok take the head off and do a paraffin test on the valves and pistons, the other possibility is a bent con rod so check the height of the pistons. It may be possible to do this with a piece of solder wire in the injector hole depending whether it has a pre combustion pot or not.

 

[charles_reed]

Normal GM Yanmar problem - slight head-gasket leak into the water gallery or between two cylinders
Usually, on the 3GM, at the back of #3 cylinder.
When it happened to my 2GM I was told it was the most common problem on all the GMs and had to have the head machined to get rid of the score mark.