April 2021

APRIL 2021 AFTERMARKET 29 Now, please refer to Part two Fig.2 in the March issue, which is conveniently also Fig.2 here. This shows serial data during warm up, taken at a similar time as in the previous data from the faulty Audi A3. It is obvious the fuel pressure taken from the FR is dramatically higher than the AUDI A3, so what would cause the PCM to adopt a lower GDI pressure? Answer that in the privacy of your own mind. This is the moment that defines the essence of a diagnostic technician. Assess data, do not guess, measurements are essential, prediction is the mother of all mistakes. Pressure Back to the PICO scope, take a close look at the Lambda current at the point the fuel pressure is reduced (please refer to Fig.3). Red trace, lambda current, cursor set at zero = Lambda 1. Blue trace, rail pressure, cursor set at 45bar, which is too low? Green/ black = reference voltages, normal response to current change. Bank 1 sensor 1, red trace, suddenly outputs a negative current which theoretically represents a rich condition. The PCM obliges by reducing fuel pressure still further from 45bar to 38bar. This is the essence of the problem. The pressure was already too low. Looks like a faulty sensor. However, replacing the sensor had no effect on the fault condition. So, we went back to look at fuel trim characteristics, when 38 bar pressure was set, the pcm adopted between - 25-32% trim. With a reduction of mean injector quantity from 2.5m/s at 19mg/s to 1.6m/s at 12mg/s. remember the mean fuelling value is taken from two injection events per cycle. At this point, and based on the absence of any obvious sensor deviation or cross-reference variation, I suggested that cloning the PCM from the LEON FR would confirm or exclude any internal PCM error. My thoughts here were based on the PCM adopting a rich fuelling correction without any input request from a sensor, Lambda error accepted. Diagnostics can be defined by a series of negative results leading to the eventual successful conclusion. So long as it has discipline and a logical process, coding the donor PCM from the LEON FR did not solve the problem. That was a big positive for me. We now know for certain that the error is within the engine fuelling system or an obscure sensor input deviation. Endgame or Infinity War? Endgame, we hoped, arrived at ADS Preston, with David G, and me. Earlier interruptions did not help continuity of thought! Today David G and I were given uninterrupted time and access. I suggested we limited the scope observation to lambda current observation only, as this was the critical instant of the fault occurrence. Concentrating on focused blocks of serial data using VCDS. Fuel trim correction was selected with all the following group data: All temperature sensors App and throttle angle Camshaft angle and MAP values Mean fuel injection value High, low fuel pressure actual and corrected. There were no obvious changes with sensor inputs, except the puzzling rich Lambda response. However, we did note that bk1 Lambda did respond first, in other words appears Fig 3 to be driving the fault. There is a clue here. We then went back to Pico very carefully checking voltage and ground at the PCM, looking for any current surges, voltage spikes or background noise at the instant of Lambda reaction. No errors found. The fountain of knowledge was running dry, but my determination was growing stronger, although we could not see the wood for the trees. The vehicle must be lean? Yet the Lambda current indicates rich. It’s right in front of our eyes, yet we cannot see it. So, we cut the red Lambda signal wire at both ends, spliced a direct temporary circuit and started the vehicle. The Lambda current instantly indicated lean, i.e. positive current. This is good, as the PCM trim was previously in rich trim. The fuel trim very quickly corrected to 0% and the vehicle ran clean. Not daring to claim success David G went for lunch while I continued to monitor near perfect trim and perfect fuel pressures, 5bar priming, 160bar GDI, a figure not previously reached. My conclusion was that current appeared to be decaying between sensor and PCM, thus driving the fuel trim in an ever-spiralling rich correction. Yes, mistakes were made in the process and in my opinion due entirely to interruption and workload constraints. The real lesson here is an incredibly old and well understood one, you must separate diagnostics from the general workshop demands and environment, in both space and time. Remember, I am writing this in real time, and all of this happened yesterday. Since then, despite our efforts the fault has returned. David G has opted to remove the GDI injectors for the second time. If you want to guess how it ends, answers on a postcard please, and see you here next month for the exciting conclusion. We now know that the error is within the engine fueling system or an obscure sensor deviation ” Come back next issue for what is, Frank promises, the definitive A3 conclusion