Armchair diagnostics welcomed

[johnphilip]

Yesterday helping to move another boat from marina to mooring a few miles away we had fairly serious issues with the 1990's twin cylinder Yanmar.
The engine started a little reluctantly and when we revved it in neutral at one point the revs ran away and the engine had to be stopped with the stop cable. Checked the throttle cable, seemed OK and when we restarted the problem seemed to have gone away.
Cast off and just out of the berth the engine died away. Restarted OK but again once in gear the engine was dying and we hobbled out of the marina using neutral to let the engine pick up and then in gear for 10 or 15 seconds. We were leaving a trail of black smoke and sooty water. The engine was surprisingly smooth if it was just one cylinder firing. Just outside the marina the engine suddenly picked up and ran with full power and no smoke for half an hour when we killed it to go sailing.
Two hours later approaching destination we restarted and the same lack of power happened although this time it cured after only a couple of minutes.

 

[Heckler]

Yesterday helping to move another boat from marina to mooring a few miles away we had fairly serious issues with the 1990's twin cylinder Yanmar.
The engine started a little reluctantly and when we revved it in neutral at one point the revs ran away and the engine had to be stopped with the stop cable. Checked the throttle cable, seemed OK and when we restarted the problem seemed to have gone away.
Cast off and just out of the berth the engine died away. Restarted OK but again once in gear the engine was dying and we hobbled out of the marina using neutral to let the engine pick up and then in gear for 10 or 15 seconds. We were leaving a trail of black smoke and sooty water. The engine was surprisingly smooth if it was just one cylinder firing. Just outside the marina the engine suddenly picked up and ran with full power and no smoke for half an hour when we killed it to go sailing.
Two hours later approaching destination we restarted and the same lack of power happened although this time it cured after only a couple of minutes.

Could be a duff cylinder, you give extra throttle to compensate, the second cylinder kicks in and it appears to be running away, black smoke because the single cylinder is on full chat but cant make the revs. Check the injector(s) first.
S

 

[rob2]

If it is running on one cylinder, then not only is that cylinder requiring full throttle and carrying the extra load of the dead one, but the fuel injected to the dead cylinder is being pumped into the exhaust and will not fully combust, explaining the smoke! I agree that getting the injectors checked and serviced is likely to improve things, even if there is another underlying problem. If you're into DIY repairs, then be aware that most diesel engineers will advise you to have the injection pump serviced as well, but take great care to leave the shims and gaskets undamaged when you remove it, ready for re-use. The thickness of the shims sets the injector timing, if undamaged the engine will run correctly after reassembly but otherwise you'll need the fitter to visit the boat to set up the timing again by selecting shim thickness. Of course, you can refuse to have the pump serviced, but if the refurbished injectors don't solve the problem the engineers may well be unwilling to be co-operative if you haven't followed their advice. Don't be tempted to disassemble an injection pump - the professionals have jigs to position everything correctly as they assemble them - I made that mistake before getting the manual and ended up with only one cylinder firing, complete with hunting and lots of smoke.

One other thought, perhaps too obvious to be likely. These engines will self bleed if the one cylinder pumps fuel for long enough. If there is a small air leak on one injector pipe, then after standing it will not fire, but running the engine will bleed it and it will then fire, although probably with some power loss due to the leak. There are very high pressures involved in injector systems can mean that leaks are only apparent when at pressure, so one usually looks for them whilst the engine is running - often a slight mist is all that will be found, but do not get too near them. At pressure fuel can penetrate the skin! Check that the lines are snugged up tight, it may be that one didn't seal last time the lines were bled.

Good luck!

Rob.

 

[ribrage]

first thing to look at is your water seperator, if its close to full and the fuel system draws water through occasionally it will give similar intermitant symptoms

 

[LadyInBed]

Fuel starvation?
Could be caused by dirty fuel blocking filter, water in fuel, blocked injector.

 

[johnphilip]

All reasonable suggestions, if it is the injectors would it be likely to correct once the engine had been running a few minutes as this one does? Similarly if fuel problem, which could be intermittent, funny that it happens just with the cold engine. Once going properly it will run at full chat with no evidence of fuel starvation.