All that has changed is the parts that are bolted to it and the way it is controlled.
All the way up to September 2009, the 3 cylinder, 800cc diesel engine was the
most reliable engine available in a fortwo. In October 2009, all of that changed.
Yep, unfortunately it's all the stuff bolted to it and the electronics that have let the side down.
From 2000 to 2009, the CDI engine conformed to Euro 4 specifications. The standards were set in January
2005 so the fortwo diesel had a 5 year head start and enjoyed 9 years without the need for any new
emissions regulations. However, in September 2009, Europe introduced the Euro 5 specifications.
The new regulations required that Nitrogen Oxide be reduced from 0.25g/km to 0.18g/km, HC+NOx
reduced 0.30g/km to 0.23g/km and Particulates from 0.025g/km to a very low 0.005g/km.
They added a DPF (Diesel Particulate Filter) which removes the particulates from the exhaust gas
and heats them up to to about 600 degress celcius and burns them off. Unfortunately, a standard
catalytic convertor only gets to about 500 degrees celcius.
The problem is how DPF's work. They spend most of their time not being hot enough to burn off
the particulates. They just sit there collecting the particulates until the DPF is blocked enough to
increase the back pressure to a set point. Once the DPF pressure sensor gets to a certain point,
the car increases the fuel use of the engine to make the DPF get hotter. The hot DPF burns
off the captured particulates and naturally unblocks itself. This is known as regeneration.
Well, take your pick! Firstly (and secondly) there are 2 temperature sensors.
The upstream one in the turbo...
...and the downstream one in the exhaust.
These measure the temperature of the exhaust gas and the DPF. They let the ECU (Engine Control Unit) know when the DPF is at the correct temperature. The ECU will then regulate the fueling to keep the DPF at its ideal temperature.
Unfortunately, these are prone to failure. As with lambda sensors, they expect to both see a very similar range.
So when 1 temperature sensor fails, the car doesn't know if the DPF is too cold to work or if the engine isn't
getting enough fuel. To save the engine and to attempt to keep within the emissions standards,
the ECU panics and puts the car into a reduced power mode.
One massive drawback is the price. At time of writing (2014), a replacement temperature sensor
from smart is nearly £100. They are already on the 4th revision of the exhaust temperature sensor
and the 3rd revision of the turbo temperature sensor.
Exhaust Temperature Sensor Revisions
A 008 153 25 28
A 000 905 08 32
A 001 905 15 00
A 000 905 17 32
Turbo Temperature Sensor Revisions
A 008 153 32 28
A 001 905 13 00
A 000 905 15 32
The third problem is with the DPF pressure sensor. This measures and compares the pressure from either
side of the DPF to tell when it is becoming blocked and tells the car to start regeneration to burn off the
particulates. Unfortunately this sensor is also prone to failure and is on revision 2.
If this fails then the ECU doesn't know how blocked the DPF is. If the DPF becomes totally blocked,
the car will not run and the DPF would need replacing (very very expensive). So again, the car panics
and goes into a reduced power mode. In this instance it is to get you to go to a garage and get it fixed.
The forth issue is a boost pressure sensor for the EGR (Exhaust Gas Recirculation) valve. However, for once,
the sensor doesn't fail. It just receives incorrect data. The fault was a strange one that took smart a while to
figure out. The sensor is piezo transducer which can measure pressure deflection and vibration. The problem
was that part of the wiring loom or brake vacuum line was running close to the sensor and eventually started
vibrating and tapping the back of the sensor. This caused a malfunction which created an excessive vacuum
which in turn opened the turbo wastegate and caused a massive loss of power and a stored fault code of P2359.
Smart actually released a fix for this issue which routed the wiring and pipes away from the sensor.
Finally, there are rumours of a wastegate actuator fault. The small amount of information I have doesn't confirm
or deny the problem exists. There are tales of wastegate actuators overheating and failing (internal gasket leak).
The result being an open wastegate so no power to the engine or a closed wastegate and the over boosting
putting the engine into safe mode (reduced power mode).
Unsubstantiated info suggests that smart have been replacing the wastegate actuator mounting bracket
with a revised version that sites the wastegate actuator slightly further away from the turbo to reduce the
radiated heat. This could be an issue with the increased temperature during DPF regeneration.
1 - Upstream (turbo) temperature sensor
2 - Downstream (exhaust) temperature sensor
3 - DPF pressure sensor
4 - Wires touching the EGR transducer sensor casing
5 - Over heating wastegate actuator