Steve Carter of Train4Auto Consultancy, explains what caused unusual backpressure readings on the Citroen C8 2.2 HDI.
“Citroen vehicles were some of the first in the UK to be fitted with a DPF (often referred to as a ‘FAP’). Blocked DPF systems have plagued Citroen owners for years.”
The car in question had been bought at auction and was being prepared for resale. Just as it was about to ready itself for a new home, an anti-pollution fault flagged up on the vehicle’s dash. Unfortunately, the EOBD reader wouldn’t reveal sufficient information about this fault, or where to go next in fixing the vehicle. With the appropriate level of diagnostics, however, it was possible to view the fault codes in more detail.
The first fault related to the overloading of the particulate filter, while the second identified that the pressure differential sensor was too high on engine start-up.
FAULT 1: A SIMPLE FIX
The first fault code is fairly simple – the vehicle had covered 58,000 miles and had a counter reading of 98g of soot residue within the DPF, meaning it had reached its design limits and needed replacing. As a consequence of this, the Eolys tank must be nearly empty so this required refilling with the appropriate Eolys (remember, there are two types).
FAULT 2: DIFFICULT DIAGNOSIS
The second fault was more interesting, as it indicated excessive backpressure within the now clogged particulate trap before the engine was even running. Viewing the serial live data, it showed a backpressure differential of 993mb with the engine off.
Plausibility prognosis
Naturally, the engine’s Electronic Control Module (ECM) has to reduce engine performance in order not to generate high levels of backpressure, which would damage the turbo.
However, its plausibility routine had determined we had excessive backpressure without the engine running.
