My en­gine is run­ning like a dog

Practical Boat Owner - - Ask The Experts -

QThe Yan­mar 2GM20 en­gine in my boat has had an in­ter­mit­tent prob­lem since I got the boat four years ago.

It can be run­ning rel­a­tively smoothly for many hours of mo­tor­ing at sea or ma­noeu­vring in port un­til, when restart­ing the en­gine, the prob­lem be­gins. This prob­lem may con­tinue for an un­de­ter­mined pe­riod of time and then, after some sail­ing and for no ap­par­ent rea­son, go away again... un­til the next time.

When I say it runs ‘rel­a­tively smoothly’ I mean there’s a con­stant vi­bra­tion, but gen­er­ally ac­cept­able for a two-cylin­der mo­tor.

The prob­lem man­i­fests it­self as a se­ries of soft jolts from in­side the en­gine or gear­box, run­ning at as lit­tle as 1,000rpm un­der load. It sounds like a ‘wuff wuff’, ap­prox­i­mately once per sec­ond. Each ‘wuff’ is sup­ple­mented by an ad­di­tional ‘im­pact’ that can be felt all over the boat. In­crease the revs and is be­comes more in­tense. Edgar Liepins, Latvia STU DAVIES REPLIES:

It sounds as if there’s an in­ter­mit­tent mis­fire on one of the cylin­ders. The ‘wuff’ puz­zles me – but could be ex­plained by a stick­ing valve.

The ‘im­pact’ felt is likely to be the cylin­der that is fir­ing tak­ing the load as the gover­nor tries to com­pen­sate for the loss of power as the other one mis­fires. So we need to find out if one of the cylin­ders is mis­fir­ing.

First get all the tools needed to slacken off the in­jec­tor nuts, along with some thick pieces of cloth to ab­sorb diesel. When the noises start, get into a good po­si­tion and slacken off the in­jec­tor nuts in turn. Be care­ful: diesel from the pump is un­der huge pres­sure of around 3000psi and can pen­e­trate the skin – which is why you need cloth around the in­jec­tors to ab­sorb the diesel.

As you ‘crack’ the in­jec­tor nut one of two things will hap­pen. If the en­gine sounds like it it’s go­ing to stop then you know that’s the cylin­der that’s work­ing OK. If it makes no dif­fer­ence then you know it’s the bad cylin­der.

Now you can fo­cus on the cause. Is it a stick­ing valve? Is it a bad in­jec­tor?

I’d take off the rocker box first and check all is good in there. Look for ex­ces­sive clear­ances on the bad cylin­der’s valves or signs of dam­age. If pos­si­ble do this after shut down when it’s been run­ning badly.

If all looks good then pull the in­jec­tors and get them checked, take note of the one off the bad cylin­der and pay par­tic­u­lar at­ten­tion to that.

Edgar Liepens has a Yan­mar 2GM20 en­gine like this – but it’s run­ning rough

Newspapers in English

Newspapers from UK

© PressReader. All rights reserved.