Originally Posted by barry123400

Remove the return line that exits the relief valve on the injection pump. Immerse the end in a small container of fluid. The fuel level should slowly increase with no air bubbles coming out. This proves or disproves any air issue. you also might see a stream of bubbles flowing through the pre filter. Have a look.

I assume you did check the valve clearances.

Does the engine have engine shocks? Apparently they are required at idle for some engines.

Now I am going to introduce a wild one. The north american engines have a cigar hose. Your pump type does not usually I believe. I do not know what enables this difference to exist but it does. I would temporarily replace the cigar hose with a piece of fuel line temporarily. Not personally expecting any difference but not sure either.

Next I might check for equal power pulses across all cylinders. You could do what I consider a crude test now of backing off each injector nut fitting it turn. Listening for equal revolution drop.

Or employ the milli volt method. This is more likely to identify the cylinder if any and it could be a marginal injector. This is easily verified if found. Just switch suspect injector locations. Check again. I too like all my engines to be as smooth as possible at idle.

First of all though make sure the engine is not ideling below factory recomendations.