CASE STUDY: FORD FOCUS TDCI
With the car owner reporting a loss of power and this Ford Focus 1.6 TDCI'S engine management light being illuminated, the first step was to perform a full scan to check for fault codes from all of the car’s ECU modules.
The scan revealed a wealth of communication issues from other modules – but are they the cause or the result of an engine management fault? Never place all of your trust in fault codes.
Sure enough, an unexpected DPF pressure differential sensor signal has been detected, but further investigation is needed to establish if it is a faulty sensor, duff wiring, or even corrupt software.
As cars experience problems in service, updates are applied to the various ECUS. It is worth checking that all software is up to date – for this, the battery will need a back-up supply. An unexpected power cut could ruin one, or more, ECUS. A live data reading confirms that the DPF pressure differential sensor’s output of almost 2 KPA is too high, when the engine speed is varied and the readings from the MAF sensor and EGR valves are monitored in real time. To find out if the pressure differential sensor is faulty, or if there is an issue within the DPF, the DPF inlet pipe is removed at the sensor end. A mechanical pressure gauge is connected to it and the engine started.
If the DPF were blocked, the pressure created could damage the mechanical gauge, which is why the more sensitive and costly digital machine is used afterwards, to give a more accurate reading.
The sensor can be tested by applying a pressure to it and noting if a voltage difference is recorded. Alternatively, connect it to an oscilloscope and compare the sensor voltage readings with the pressure measured across the DPF, as in Step 7.
The actual pressure reading (pictured blue) does not correlate with the signal from the DPF pressure differential sensor (pictured red), which is at the top of the scale, indicating that either the sensor, or its wiring, is faulty.
It is worth noting that the pressure pulses, measured across this DPF during the firing stokes, are not identical. This might indicate a further running issue that could have consequences for the particulate filter.
In this engine’s case, a fuel injector possesses a slight compression leak – a common problem with the DV6 engine range. This causes the affected cylinder to run with insufficient air, increasing the DPF soot loading.
Apart from an obvious visual check for corrosion (pictured inset – this connector is in good order), you can back-probe the rear of each wire, provided that you have suitable data available to identify each wire function and the expected output.
Pressure differential sensors are expensive, which is why it is worth testing them before investing in a replacement. Be wary that aftermarket component quality can be variable and consider that identical-looking parts can be calibrated differently. Blocked DPFS can generate pressures high enough to damage sensors. Replacement sensors are easy to fit, however, tending to be retained with only a few screws and the pipes/wiring being the pull-off and push-on variety. With a new sensor fitted, the system has to be recalibrated. Some vehicles perform this automatically; others require diagnostic intervention – this applies even to parts sourced from main dealers. Comparing the diagnostic readings after the sensor is fitted with that pictured in Step 5 reveals that the sensor reading is now at 0 KPA (meaning between 1-9 millibars – the pressure will never be zero), proving the DPF is unblocked. A 30-minute road-test allows the car to complete its own ‘active regeneration’ cycle, satisfying the engine management. Once the fault codes were re-read, the spurious issues noted in Step 2 did not reappear.