friction drag .. .. particularly before the engine and oil have reached optimum temps.
( my choice of helper for that .. :
ZX1 Home )
dirty crankcase vent system .. resulting in excessive crankcase pressure ( more 'work' for the engine )
inneficient fuel combustion :
dirty injector nozzles .. ( or faulty injectors )
fuel contaminants such as water
( to combat those .. i use ..
http://www.landyzone.co.uk/lz/2409712-post6.html )
dirty air filter
faulty MAF that might cause ovefuelling ..
sticking egr valve .. ( where fitted )
faulty air intake temp sensor causing a default of -5c to be used by the ecu ..
faulty coolant temp sensor .. that would Probably effect injector timing in theory
faulty glow plug system ( inneficient combustion on cold start up )
faulty turbo actuating system .. although by how much that would effect mpg i not be sure
i.e. for an 'overboost' the ecu would cut the fuel supply to a safe level ..
for an 'underboost' .. in theory the ecu would see not enough manifold pressure / air-mass to supply desired fuel ..
although would have thought anything that detracts from efficiency could potentialy waste fuel
a faulty coolant thermostat that caused the engine to run too cold would effect mpg
low cylinder compression
~~~~~~~~~~~~~~~
the ecu needs accurate readings of air temp. / coolant temp / air mass / manifold pressure
to compute fuel delivery / injector timing for a particular go-pedal 'demand' ..
if the sensors were to give the wrong readings .. the ecu could supply/inject more fuel than needed for optimum combustion .. therefore wasting fuel ..
and / or .. provide the wrong injector timing .. again wasting combustion energy ..
disclaimer: i don't know the particulars of the td4 engine mapping .. or 'defaults'
but on investigation .. many common rail diesel engines with computerised management systems
have certain things in common to provide best fuel combustion for a given power demand
~~~
obviously many of the above .. if a 'fault' .. would .. or might .. show other symptoms ..
rough idling .. lack of power .. etc etc
~~~~
use of the PTI heater causes the idle rpm to rise .. adding to fuel consumption ..
same for .. if the ecu sees high electric consumption for some other reason ..
i.e. idle rpm raised to compensate for load on the alternator ..
~~~~~~~~~~~~~~
overall condition of the drive train .. clutch / gears / vcu / diffs ..
tyre pressure ..
aerodynamics ..
driving habits ..
what 'ianjones' said regarding the vcu / tyres .. post no. 21 .. makes sense ..