Exhaust gas temperature (EGT) (and why it is important).
Tuning a diesel is fundamentally different to tuning a petrol. In a petrol you run excess fuel to reduce exhaust gas temperature and cool pistons and valves. In a diesel excess fuel has the opposite effect it increases these temperatures! You also get a visable warning that this is occurring in the form of smoke! So why is controlling EGT important, well it is important for reliability, you don’t want to melt anything. As EGT’s increase the turbo and pistons are both more likely to fail. Whilst the pistons on the L series are strong they aren’t indestructible.
What temperatures are and aren’t acceptable? Well it depends, the lower the better in general however with everything in life it is a compromise. If you are getting thick black smoke from the exhaust then chances are your EGT’s are too high, and you’ll not pass a UK MOT emissions test either.