My engine is running like a dog
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.