A custumer brought in a BMW 325 Sport Diesel, complaining of poor performance, bad fuel economy and a history of DPF problems. After an initial consultation with the customer about service history, type of fuel and drive style, a clear picture was painted and I was happy to proceed with the diagnostic process, confident we could fix the problems at a much lower cost than had been quoted by the main dealer.

It took a little while to convince the customer, having already spent several hundred pounds with attempted repairs by local BMW 'specialists,' which just simply attempted forced regeneration without even driving the vehicle! Reluctantly, he approached the franchised BMW agent who quoted for a new turbo, EGR and replacement DPF, estimated to cost over £3,000. Now, obviously I wasn't at the dealer so I don't know what had been discussed but I struggle to see how they could have arrived at that figure without first investigating the fault.

I first interrogated DDE fault memory and recovered several DTC's. With so many DTC's present I decided to contact the customer and explain the potential cost of repair and the plan I had in mind. Although there was plenty of evidence pointing towards a DPF issue, I was more concerned on the initial cause of the potential blockage. I needed to check if it was indeed blocked and the first back pressure reading showed exhaust gas back pressure at 940m/b at 2,000 rpm. Next, we simply tested sensor plausibility which showed a reading of 0 m/b with ignition on engine off.

There are several possible reasons why a DPF could be showing a reading like this. Either the sensor was faulty (or the EGR was stuck) and was giving a false reading or the filter really was completely blocked. Possibly both scenarios were true.

In any case, it would be no use performing a forced regeneration on it - if the sensor was faulty then that would do nothing to cure the problem. Time and time again, we have witnessed other garages attempting to regenerate badly blocked DPF's statically. It's potentially dangerous to force regeneration on a DPF filter showing that much back pressure and that's why most dealer tool platforms have removed the ability to do so, due to excessive temperatures causing fires and further damage. During the forced regeneration process the Exhaust Gas Temperature (EGT) can rise to 650ºC.

Clearly more investigation was needed. I wanted to make sure our customer knew exactly what was going on. After all, he had left one garage with a bad taste in his mouth, so I wanted to make sure he was being looked after to the very best of our ability and was aware of all of the options and likely costs. This is what we call 'managing customer expectations'.

I explained to the customer that we could simply replace the DPF. Given that the part alone costs more than £800, plus labour, this would not be the cheapest option. Secondly, we could remove the filter and clean it off the car. This is a real possibility, though doing this would still incur the time and costs associated with removal and refitting. Finally, we could clean it with some chemical additives. I've had good results using a trade product for cleaning these filters in the past and after explaining the process, this is the option the customer chose.

At this point, it is worth noting that we also recommended the car had an oil change and an engine flush at the same time. Although I was satisfied that the blockage in the DPF had been due to constant stop-start urban driving, there was every possibility that the wrong grade of oil could have been used. Any motor oil that isn't a 'low-ash' type can very quickly play havoc with a DPF - meaning that if you choose not to change the oil for a grade you know to be correct, it could well mean a repeat visit from an unhappy customer.

We investigated the cause of the blockage by looking at the EGR valve which we found was badly contaminated with carbon build-up and was stuck wide open, causing much lower EGT's - making passive regeneration under normal driving conditions impossible. Note that passive regeneration will occur without intervention from the DDE control unit around 250-350ºC on motorway driving.

I decided to remove the EGR, clean, refit and carry out full electronic re-adaptation. I then carefully examined live data looking at the air mass at idle, to ensure no more gas was being allowed past the EGR. I was completely satisfied and confident the EGR was OK. The turbo variable vane control arm had become dislodged due to a missing clip, this was undoubtedly the culprit causing the turbo variable vane control DTC. After a quick root through my turbo spares drawer, I came up with a replacement clip which was put back in place, I then carried out a full actuation test through the Autologic handset, testing the full range of movement. Again, I was completely satisfied with the turbo actuation. The EGR and turbo play major roles in DPF regeneration and any defect found in either will prevent both passive and forced regeneration from taking place.

I cleared down the remaining DTCs and was only left with DPF blockage. Due to the heavy carbon soot deposit, we fed a BG diesel air induction system cleaning chemical through a special device that I have which resembles a hospital 'drip'. This also acts as an effective DPF cleaner. Note that it is important to monitor the pressures whilst doing this. During the flush process, I noted EGT rising above 550ºC at 1,750 rpm. Make sure you don't increase RPM for more than 10 mins at a time to allow EGT to return to normal. The next pressure reading I took showed a much improved reading of 333mb, a third of its original pressure reading. I was now confident of having fixed the original cause of the DPF blockage and I could carry out forced regeneration in a 30-40 mile test drive cycle whilst monitoring EGT and backpressure.

During the forced regeneration cycle, I noted exhaust gas temperature rise to 650 ºC for ten to fifteen minutes. Note that the DDE unit will not allow forced regeneration to take part longer than 15 minutes to avoid damage to the DPF. A final reading was taken back at the workshop which showed a nominal pressure reading at 2,000 rpm of just 25 mb.

I was confident we'd made a good job of it and presented the customer with all the photographic evidence and serial data I had gathered. I had saved him so much money I managed to persuade him into a performance tune! The total bill including a full engine de-coke, oil change, repaired EGR, DPF and performance tune came to approximately half what he had been quoted by the main dealer. Long may it continue! To say he was pleased with the bill and the result is an understatement.