No compression on diesel engine

[ianc1200]

After having the boatyard look at my engine, they say there is no compression in the first cylinder. It looks as though my boating season on the Thames is ended!

Any idea of what the possible causes are? It's a 5 cylinder, 62hp Nanni, (I think called a 5.280) & 6 years old. They've mentioned one possible cause being a cracked piston, and this means the engine out. They're going to take the head off, but any ideas?

IanC

 

[cliffordpope]

In general, posible reasons for low compression are:
cracked or burnt piston
broken or stuck piston ring
burnt out or damaged valve
cracked block
leaking head gasket

One simple pre-dismantling diagnostic test is to try squirting in some heavy oil into the cylinder and see if the compression improves. If it improves it suggests rings, if no difference = valve. But of course a major leakage won't respond to anything.
The head off ought to reveal a lot.

 

[sailorman]

wot were the symptoms prior to non start.
could a valve ( non ) clearance be the prob ????

 

[ianc1200]

The engine ran with metallic double klacking noise. A small amount of diesel coming out of the exhaust. The pipes onto the injectors were cracked, very noticable difference on No's 3, 4 & 5 cylinders, a little on 2, and no difference on 1. The pipe to No.1 injector totally taken off (for one small moment, say 5 seconds, sounded fine) but then still running with this metallic sound. Several people in the marina suggested faulty injector(s) so I was hopefull of quick resolution until today's news of no compression. Valve clearances checked, rocker arm taken off to ensure springs or push rods (if that's what they are called) not broken.

IanC

 

[LittleShip]

try squirting in some heavy oil into the cylinder and see if the compression improves. If it improves it suggests rings, if no difference = valve

Good advice and do this before anything else. If it is indeed a stuck compression ring if may free up once started and run.

Tom

 

[Ceejay]

My hunch would be a burnt out valve. Head off will be the next move whatever the cause imho
CJ

 

[ccscott49]

Sounds like its terminal, so head off first choice. Well, after the old oil trick, but if there's mechanical noise, I would suspect something worse, a stuck ring does not make a "clacking noise" a broken one might.

 

[essexboy]

If the inlet valve is not opening, due to say a broken tappet / rocker
could that not be the cause ? But I agree that head off first option.

 

[PCUK]

Due to the noise it sounds valve related, which shouldn't be too much of a job. Fingers crossed!

 

[Ceejay]

We're all waiting with baited breath.................keep us posted........ /forums/images/graemlins/smile.gif

 

[oldharry]

If there is total compression loss, then it is far more likely to be a valve than piston or ring damage. Could be nothing more than a lump of carbon holding a valve open....

 

[ianc1200]

Thanks for all the toughts, will post results here

IanC

 

[smonard]

If a valve is stuck open you would get a mechanical clacking sound, like someone tapping an anvil with a hammer as there would be a huge valve clearance. A bent push-rod could also do it. A stuck compression ring probably wouldn't make any sound.
I had an old Triumph motorbike with similar symptoms, ended up being a wee crew got sucked in and jammed one valve open. The sound was quite distinct from all the other rattles. Still got me home though.
Surely can't be that hard to get the head off and have a peek, its probably not that bad.

 

[ianc1200]

Head will come off next.

A friend rang to say he has in the past brought piston to TDC, then put 100 PSI compressed air into injector hole to see if air comes out through exhaust or inlet valves, if not it's the piston causing the problem. Only of limited interest really, needs head off whatever.

IanC

 

[Mrnotming]

Sometimes the valve springs suffer from corrosion and fail. There would then be no compression and the hanging down valve would be bouncing off the piston top. Hopefully if the angle of the valve guide is almost at right angles to the hopefully flat piston top, no damage might result and it might even be possible for a clever person to put that cyl at TDC and pop off the collets using a socket of suitable size A new spring could thus be fitted without removing the cylinder head. Did this once and it worked. The spring was cut down from another engine as a temp measure in Holyhead Sailing club by kind permission, and allowed us to have the MD7A useful on the way back across the Irish sea to Dublin.

 

[ianc1200]

Head due to come off Monday, will report what's found, thanks for the thought though.

IanC