Hello, welcome to the Forum and apologies for no earlier response.
Congratulations on advancing your education in diesel technology! I trust you will be looking for a position with a fleet operator or government.
To be clear, you have an intermittent P0203 and slight haze at idle, which clears up on-throttle, But no other performance issues, correct?
Sounds like you have already spent time researching the MIL and sorry, but I have no knowledge of that particular situation.
Regarding haze at idle, what is most likely happening is the VCO (Valve Covered Orifice) injectors in your LB7 are aging and no longer shutting off completely. As you may know, VCO’s were only used in the LB7 and were replaced by the much more reliable SAC type injectors in all later Duramax engines. Until not long ago, only higher flow SAC’s were available for LB7’s, which required a tuner for best performance, but those injector types are now available in OEM flow rates and are an excellent option for R&R, with good reports from owners who have installed them.
And yes, that’s what you are looking at
What happens is fuel dripping from an “off” injector begins to carbonize on the injector nozzle and not only disrupts the flow pattern, but also causes a small amount of excess fuel to constantly enter the combustion chamber and at idle, even that tiny amount of fuel is more than the engine can burn, resulting in a smelly haze of raw diesel coming out the tailpipe.
While a variety of chemical treatments are available (Stanadyne, endorsed by the GAO for government vehicles comes to mind) and may mitigate the problem temporarily, once hazing presents, it’s all downhill and injector R&R needs to happen. Around 60k seems a typical life expectancy.
BTW, haze disappears off idle because the small amount of dripping fuel is inconsequential compared to normal fueling and other than some loss of MPG, may not be an issue. The engine will continue to perform adequately but a major concern is fuel leakage will begin to wash cylinder walls, with obvious consequences (not to be confused with “making oil”, which is generally a failure in the return circuit under the valve covers). At night, you may see haze in the headlights of following cars and will also likely blow a cloud following periods of idle. (Not to be confused with vaping
Lots of 411 here on injector R&R, so unless someone has a better idea on a chemical solution (fairy tears?
), you know what you gotta do.