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diesel emissions systems: DPF, DEF and what they need

modern diesels are far cleaner than the ones from twenty years ago, and the equipment that makes them clean is the part that needs understanding.

the diesel particulate filter

the DPF traps soot from the exhaust. it fills up, and it clears itself by burning the soot off at high temperature, which is called regeneration.

regeneration needs the exhaust hot, and the exhaust gets hot on a sustained drive. short trips are the enemy, because the vehicle keeps starting a regeneration and keeps having it interrupted when you switch off.

do that repeatedly and the filter blocks. a blocked DPF means limp mode, and a forced regeneration at a shop or, at worst, a filter that has to be replaced.

if your diesel does mostly short journeys, a longer run at highway speed every few weeks genuinely matters. it is not folklore.

diesel exhaust fluid

DEF, sold as AdBlue, is injected into the exhaust to convert nitrogen oxides into nitrogen and water. most diesels since about 2011 use it.

things worth knowing:

  • the vehicle will refuse to start when it runs out. not limp mode: it will not start. this is required by regulation and it is not a fault.
  • it has a shelf life, roughly a year, and it degrades faster in heat.
  • it freezes at about minus eleven degrees. the vehicle heats the tank, so this is normal and not a problem, but a jug in your garage in January will be solid.
  • only DEF goes in the DEF tank. filling it with diesel, or diesel with DEF, is a very expensive mistake and it happens.

the EGR valve

exhaust gas recirculation reduces combustion temperature by feeding some exhaust back in. it also, over time, cakes the valve and the intake with soot, which is why short-trip diesels develop rough running and poor economy.

the honest summary

a diesel doing long runs is an excellent thing. a diesel doing five kilometres to work and back is a maintenance problem waiting to happen, and if that is your driving, the gas engine is the better vehicle.

we would rather tell you that before you buy one than after.