I was also kind of also wondering if underfuelled might produce smoke. Thought engine would be more likely to not run if that bad though. Also the glow plugs output voltage would all be lower than the normal range. Far less combustion heat. But as I stated since he observed larger than normal bubble in prefilter the lines should not be ignored. I have found on too many occasions what seems logical does not turn out to be the case many times once the problem is located. That is either my lack of experience or luck of the draw perhaps. We at least have the advantage of the answer to these questions being posted by the thread originator pretty soon. A good check also might be to disconnect the return line and make sure fuel is flowing out of it if fuel starvation was suspected. Again the milli volt readings might reflect that quicker and easier. Plus have a look for too many bubbles in the fuel line. Hopefully just a bad injector as he has some spares. Ultimatly when people post their milli volt readings with description of faults they will assist other people in diagnosing probable actual causes and eliminate things. Or they will know themselves what is wrong before posting from their readings. Time will tell. We have never had a diagnostic system for these diesel non computor cars with so much possible potential in my opinion.