A very peculiar problem - July edition

Another month, more subscribers! The increasing readership motivates me to deliver valuable content for your fault-finding endeavours. Before we explore this month's intriguing 'very peculiar problem' case study, let me share a call to action about AllMech.

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And now onto this months case study which, although simple, gave me an hour or two of serious thinking about how to apporoach and solve the issue since many things came into play that usually affect the DPF system on the M57 engine.

A background into this troublesome car - 2012 BMW X5 3,0D - M57 single turbo with Bosch EDC16 fuel injection system, DPF with oxidizing catalyser and a single pipe backpressure sensing system. The backpressure sensor utilises a an aluminium gel filled diaphragm with evaluation electronics built according to the nano-electronics principle. Here are a couple of images of a backpressure sensor that I opened up on another case study to show just how wonderful and intricate a sensor it truly is.

The problem on our X5 was that it would set a DTC for "too low back pressure". Now on an X5 with an M57 this is the opposite of what you would expect to see since the biggest issue on the M57 with a non-remapped engine and with the original DPF and oxidising catalyser is "too much back pressure". Too high a backpressure result when the DPf can no longer regenerate. Reasons for lack of regenerate include not exclusively but are the most common causes:-

• Failed coolant system thermostats in the block and the EGR cooler which cause regenerations to fail due to too cool an engine.
• Glowplugs and / or glowunit failures.
• BSD bus failures that lead to lack of communications to the glowplug control unit.
• Active fuel injector faults.
• Other serious active fault codes that prevent or delay the initiation of a DPF regeneration event.
• Driver abuse, negligence or indifference in ignoring problems. Continuing to drive the vehicle in this state leads to a complete blockage of the DPF monolith thus triggering the P480A and P481A fault codes.

The above scenario works becuase the DPF backpressure sensor is able to read that the accumulated backpressure is too high thus it triggers a warning event in the form of a stored fault code an an active check control message to warn the driver that all is not well with the exhaust emmissions system. Typical good, nominal backpressure readings on an M57 DPF system would be:-

1. Idle: 10-30 Mbar
2. 2500 RPM: 30-70 Mbar
3. 4000 RPM: 70-110 Mbar

From these values the DDE can initiate regeneration until the pressures fall to the lower end of the standard figures. But what about when the pressure is too low? Well on this engine the backpressure sensor when disconnected will force the DDE to display live data as 8192 Mbar which equates to 2.197V on the signal wire. Here are some examples of our problem car live data below:-

So - before we go further let me make it clear that this HUGE pressure amount is actually what we will see when the DDE cannot display plausible values. SO in these circumstances you will see this value:-

1. A broken backpressure sensor
2. A wiring fault on the ground, power or signal lines to the sensor
3. A disconnected sensor
4. A fault in the sensing tube supplying pressurised gas to the sensor hose
5. A cracked, removed or holed DPF
6. Modified or fremoved DPF that has not been chiptuned to remove the backpressure monitoring

So we had this weird value. Well I have seen this so many times and often it can be the sensor or the wiring chafing. I started to think about the easiest method in which to start checking. Removing the harness adaptor I immediagtely found some chafing. Sadly I thought I had found the fault easily and quickly but alas it was a false alarm. Still - I repaired the wiring in line with best practice.

Removing the back of the DPF proved great as it showed a DPF os perfectly good condition with no holes or cracks. Quickly popping back the exhaust pipe took all of 5 minutes so then it was on to the electrical side of things. On many occasions when wiring is suspected I have used the variable Voltage supply in place of an actual sensor. Doing this achieves a multitude of useful things:-

• Wiring can be checked for signal carrying capacity
• The DDE or engine control unit can be checked to see if it both receives and processes the transmitted signal
• The signal can be tracked linearly via live data to check its plausibilty

So as in the image above if 3.5V stimuli from the power supply produces the effect of an output value in live data of 441.88 Mbar clearly the DDE and wiring from the sensor is ok. The sensor harness was checked plugged in and unplugged with 5V, signal and ground present.

Being an expensive sensor we had none in stock, however my colleague had a spare. plugging this in displayed the same value - no sensor issues either. Just to make sure I decided to manually apply pressure the the sensor orifice with a Mityvac. The live data was just as plausible as with the voltage stimuli applied.

So that left only one plausible explanation - a severe restriction in the DPF back pressure sensing tube. I could feel plenty of exhaust gas coming from the steel exhaust tube in pulses but was it enough? As I had initially gone down the route of suspecting a wiring fault due to the poor state of the chafed wires I decided to take a back pressure reading. To my surprise it was very low thus confirming my suspicions of a blocked tube. However the endoscope would only go so far. The main take away of the borescope was a clean internal tube with no sign of soot at all. The pipe being at a 90 degree angle woulf not allow any implement to get in to poke about. I ended up using a plastic straw from an aerosol which cleared the restriction somewhat. A final result was 4 Mbar at idle - still a little low but we no longer had implausible fault codes set for backpressure.

The faults 4D03, 41BA, 4166, 4CF3, 4010 and 4011 successfully erased and did not return. A final test was to see how much volume in a few seconds of engine operation would inflate a small bag - the answer was actually a slow inflation. It took around 4 seconds to inflate a 200mm x 200mm plastic bag

The final thing to do with this job was to use JLM fluid to clean the DPF but now we turn our attention to the reason why this DPF was blocked so much that it created the opposite effect - too low a pressure value. To look into this will require examination of the glow system, cooling system etc. But for now she is up and running and displaying no fault codes. A very peculiar problem, maybe not for a Renault or others but on a BMW for me it was the first encounter of this phenomenon.

Until August let us keep fault finding.