Yanmar 2GM20 Problem

[richardabeattie]

Who can solve this one?

Launched in the spring, a few weekends round the Solent. The usual mix of sailing and motoring. No long periods of slow running and just charging batteries. All well and I leave her on the pontoon. So that means the last time I came home I left the engine with out any obvious fault.

Next weekend, flat batteries so I attempt hand cranking a cold engine with and without decompressors and achieve nothing more than a very sore nose. (I know the nose is not the best part of my anatomy for gripping a starting handle but the starting handle thought diffferent.) At some point in this process - but maybe not soon enough? - I turned off the water intake.

Next weekend I appear with a fully charged battery. Engine turns very energetically but does not fire - nothing but black smoke. I bleed everything, change all the filters and have fuel at the injectors. Plenty of fuel in tank. Lift pump OK. Impeller impelling. I remove air filter and find no pigeon has been sucked in to the engine.

Enter an experienced Yanmar engineer. Fuel OK but no compression in the forward cylinder. Top off and forward cylinder head coked up. Valve push rods straight. Both pistons rising to full stroke so Con rods seem OK. No obvious piston ring problems - but I've left the bores full of oil to see if the levels go down at differnt rates past a damaged or stuck ring. The engineer has taken the head away to clean it up but neither of us can work out why a relatively young and I hope well maintained engine should loose compression like that.

Has the coking built up over time and jammed a valve? But why only on one cylinder? Or is the coking just the result of my repeated attempts to start it? If water got in why did nothing get bent?

All contributions gratefully received and I'll report the outcome.

 

[ShipsWoofy]

I don't think your repeated attempts to start would coke up the motor, the engine needs to get hot and fire for the coke to appear.


plus it takes years to coke an engine, or at least, It should take years!

An engine that cokes up quickly must have a poor air / fuel mixture. As you have changed / checked filters that leaves mechanical problems yes?

 

[Keith]

I would suggest the engine "may" have been overfilled with oil, this would lead to excess oil being blown out of the engine breather into the intake, where it sticks to the walls of the intake passage and turns into a thick black coke like mess eventually starving the intake to death.............i may be wrong....its been known......but drop the exhaust valves just to check.......keith

 

[snowleopard]

Odd question but I thought you got valve lifters as standard with the manual start engine, i.e. the raw water cooled version?

You can check if the compression failure is down to a bent con rod by removing the injectors and poking a bit of electrical solder wire down the hole. Turn the engine over by hand and the wire will be flattened between piston and head. It should be the same amount for each cylinder.

If you get a hydraulic lock on this engine the thing that gives is the con rod (don't ask how I know).

 

[oldharry]

If there was a lot of carbon around, theres a good chance a fragment has jammed one of the valves just slightly open, usually the exhaust valve. Kills a diesel stone dead. As to why it coked up in the first place, have the injector checked out, as it may be delivering too much fuel which is not burning off properly.

Also

 

[Capt_Scarlet]

I had a problem with my 2GM20 - became a sod to start over a period of time, and would tick over on one cylinder only. I took the head off, expecting valve trouble, and discovered much soot, corrosion-type pitting and a badly burnt valve seat - had the head reworked for new seats - problem solved, I thought. I then examined the block face and discovered a crack to the water jacket, that someone had previously repaired with what looked like overlapped brass studding (heard of this before).
Ended up sourcing a secondhand block and rebuilt this with my old engine parts and some new. Not a cheap experience, but engine properly fixed now.

So, worth examining the block face for cracks.

 

[TheBoatman]

Don't know if this will help but in the last few weeks our 2GM20 had exactly the same problem, engine slow to fire, clouds of black smoke on start up etc. I traced the fault to a lose injector, don't ask me why an injector on an engine that has been perfectly OK for years suddenly decides to lossen an injector but that was it.
When I removed the injector it was covered in carbon deposits from the bad burn. I replaced the injector and all was reasonably fine, I say reasonably because I'm not convinced that I have completely got the engine back to the A1 condition it was in, so this winter both injectors will be replaced.

Peter.

 

[savageseadog]

The repair you had is called stitching and is an accepted repair method for cast iron, if it's done right of course.