Engine knocking heavily on start-up

[macd]

Difficult to understand why anyone needed to post anything after #9. It is nothing to do with glow plugs, injectors, oil, air, water jackets or any other fanciful theory, it is simply that when the engine starts from cold it runs very slowly, less than 500 rpm unless the 'throttle' is opened a little. Diesels simply do not like to run as slowly as this and make knocking noises in protest.

+1. Some of the 'suggestions' seemed to be running slowly, too.

 

[sailorman]

Even more important that the injectors are working properly and giving a good vapour then.

I had a Citroen BX that had done 230k mls without touching the injectors it did 45 mpg day in day our no matter how it was driven

 

[Spyro]

If you take any notice of Yanmar's stipulations, you'd not be using a multi-grade oil as you do.
It is possible that your engine is suffering from oil depletion on load-bearing surfaces - use as proposed an SAE 30, straight mineral oil, they tend to keep a more effective lubricant film on surfaces than modern synthetics.
SAE 30 CDI/SP. though an outdated description is the only one Yanmar offer.
All major oil producers have such oils in their range - unlikely to be on easy offer as the prices are too low to attract resellers.
It is possible that the injection pump timing is a little advanced.
Certainly do not consider adding another additive to your engine.
In the indirect cooled version, the 2GM is good for about 10K hours, as any Yanmar specialist will testify.
All horizontal twin engines are, basically, unbalanced, so you may have to live with the problem.

It's a 2gm20. it's not a horizontal twin.

 

[Spyro]

If your engine is jumping around a lot on start up the knocking could also be your shaft hitting the stern tube. I experienced that when I first got my present boat. If you've been able to start it from cold with no throttle you have been very lucky.

 

[macd]

It's a 2gm20. it's not a horizontal twin.

I think he also meant parallel twin. Horizontal twins can have perfect primary balance, or poor balance, depending on configuration.

 

[Tammany]

Official starting procedure is full throttle in neutral then back to half way as this engages the fuel cold start settings. All yanmar GM's shake a bit on cold starting up because as mentioned they have no glowplugs to preheat the cylinder, they are just that sort of engine. It's the back to basic nature of the engine that's most appealing as their is bugger all to go wrong electrically. I suspect after a few mins running when you switch off and then restart it starts fine with no shaking?

 

[JimC]

Yes.

 

[vyv_cox]

Yes, if I start mine from cold without remembering to apply a tiny bit of throttle it knocks for about ten seconds, then settles down. Once warm it never does it.

 

[ghostlymoron]

I always found that my 2GM20 started better with 1/2 to 3/4 throttle. Dropped down to tick over as soon as it fired.