The colour of smoke tells you a lot about its source
Burning oil. Blue smoke is never a good thing and is caused by burning oil! The usual suspect is a worn turbo that is leaking oil into the air intake. Don’t worry a little oil in the turbo pipework is perfectly normal and actually comes from the engine oil breathers so don’t get paranoid if you see some oil when removing some of the turbo pipework. However beware as a diesel engine burns oil to make it run so a bad oil leak can cause the engine to rev uncontrollably until all the oil is used up which is usually catastrophic for the engine. If the engine starts to rev on its own above 1300rpm then try to stall the engine. Engage 5th gear and sharply release the clutch whilst keepiong your foot on the brakes. If this fails you can try to remove/cut open the intercooler hose (as this will remove the oil supply to the air intake), however being next to an engine that is trying to explode may be hazardous if something does let go. If you have a halon or CO2 fire extinguisher you can fire this into the air intake to try to stall the engine. Don’t use water as this can also destroy the engine.
Incompletely burnt fuel. This can occur for many reasons, and some black smoke is normal on hard acceleration, even more so on tuned vehicles. If you are getting excessive black smoke under normal driving then there are a couple of common causes:
EGR valve stuck open – remove valve, clean and make sure valve is closed. On replacing leave the vaccum pipe to it disconnected and plugged. (This may illuminate the engine management light on late model 25/45/ZR/ZS’s)
Blocked air filter
Split intercooler hose – common problem on all L series – replace
Defective MAF sensor – Clean sensor with alcohol/Replace sensor/Fit MAFAM (MAF amplifier box).
Incorrect ignition timing
Also never a good thing! If you have coolant getting into the exhaust gases then you have headgasket failure and severe gasket failure at that. Some condensation and steam in the exhaust is normal so don’t worry about this. If you have lots of steam coming from the exhaust it’ll be obvious that something isn’t right.