Engine intermittent rattling and smoke - diagnoses please!

[skyflyer]

Yanmar 3GM30. Fine all last year apart from very rough running (vibration) at low RPM; laid up for winter, and on basis of same advice from several different sources, decided to have injectors recon'd. All the winterisation, service and re-commissioning was done professionally - I was busy on other jobs!

Last friday we ran it on the hard (with water connected!) and all seemed fine apart from a brief period when there was a rattling noise and a slight drop in revs - but no vibration. This went after about 30 seconds. The 'on the spot' diagnosis was air in the system from incomplete bleeding (fuel filter had just been replaced). Engine ran smoothly then for about 15 minutes.

After launch later that day I have only motored a very short distance to my mooring and into and out of the marina, briefly, until today when i went for a bit of a run, (to test the new a/pilot rather than the engine) but never above 2000rpm

The rattling noise returned - sounds like theres a handful of small nuts and bolts in there - and as well as an RPM drop i noticed the exhaust was giving off clouds of smoke, bluey white rather than brown, which I take to be unburnt fuel? I was reluctant to open up the throttle, if there was misfiring. There was no obvious vibration however.

Two questions arise:
1) what is happening?
2) could it be connected with the work done on the injectors?

I have some theories but would welcome other thoughts - especially as my detailed knowledge of diesels is somewhat limited!

Thanks

 

[daveyw]

Sticky valves? Can't think why though.

 

[coopec]

I just googled "diesel rough running blowing smoke"

There's quite a bit there relating to the problem but not necessarily marine engines (maybe that is not relevant)

 

[essexboy]

I would suggest one of the injectors is "hosing" that is not atomising the fuel properly, but squirting fuel in. Ask to see them testing them. Try not to run it too long in that condition as it puts a big loading on the engine.

 

[RichardS]

I would have thought that a serious injector fault would have been more likely to cause black smoke rather than blue but the white could indeed be fuel.

Bluish smoke is usually caused by oil. Has any work been done on the top end? Has the rocker cover been removed? Might be worth checking the clearances before sending the injectors off for testing again as that's not a difficult job and might explain the rattling if that does sound metallic.

Are all the injectors screwed in properly?

Richard

 

[ianj99]

After refitting reconned injectors to my 4108, I had a lot of knocking and loss of power (idle rpm dropped as well). It was a poor seal on no 4 injector seat, and also a bit of air between pump and injectors.

 

[skyflyer]

Well here's the strange thing - having limped back to my mooring last night, tried again this morning and everything sweet as a daisy - up to 3000 rpm and not a murmur.

Bit of a nuisance really as intermittent fault is always going to be a nightmare to pin down.

 

[Birdseye]

Rattling engines can come from all sorts of things other than the engine - sympathetic vibration in metal fittings near the engine, exhaust pipe hitting things with engine pulses, broken engine mount and engine touching bearers etc. So before you leap into action, you need to do some careful thoughtful investigation. Just too easy to imagine difficult and expensive issues as being whats wrong. For example one engine rattle I had turned out to be the boarding ladder sympathetically vibrating against the backstay bolts

On your latest trial were you on your mooring with the engine running at 3k revs whereas the problem was with the shaft rotating under load at 2k revs? If so, it suggests the problem was triggered either by shaft rotation or load. If so is there a drag on the shaft that is overloading the engine and causing both smoke and rough running?

If there is no problem with the drive system then the problem is in the engine. All a diesel requires to run is compression, fuel and air. Air shortage is unlikely so the next place to look is fuel, starting with the tank ( is the pipe blocked by crud / bug?) and working forward through the filters to the injector system.

personally I would happily spend time pottering til I solved the issue. But it sounds as if your time is limited and you have little practical knowledge so get a mechanic in.

 

[Jim@sea]

Do a Cylinder Balance Test. i.e with the engine running slacken off an injector nut, the engine should slow down, (then re tighten) then do the rest, if you have a faulty injector when you slacken that one off the reduction in engine speed will be less pronounced. Also indicates which the faulty injector is. If they all slow down to the same amount then the fault is elsewhere.