April 2021

BY Frank Massey B efore I begin part three I have somewhat of an important admission, right up to the closing paragraph of this instalment; I still don’t know the actual cause of the incorrect fuel pressure during warm up. I hope that part two showed a methodical approach to data acquisition to determine how the fault occurred and a clear path towards further evaluation. I also need you to accept that a great deal more testing behind the scenes had been carried out, but for the flow and purity of the topic I have cherry-picked the more interesting elements within the logic timeline. In other words, I have not cheated you with the facts, as presented to me. So, what do we know? GDI fuel pressure is reducing in a predictable, non-random manner under PCM control. We have not yet discussed Lambda feedback. We did monitor this much earlier in our evaluation, but I decided to introduce it within the topic, in a way that is logical, allowing me to explain fully, and in detail, the diagnostic process with component functionality. Understanding It is not possible to accurately diagnose any fault without fully understanding how the system responds to data input. With any fuelling fault evaluation, you must observe request and corrected data in order to understand if the PCM is responding in closed loop or attempting to correct a fault condition. Our PCM is in closed loop but appears to be causing not correcting the fault. Most sensors in Europe tend to be 5-wire Bosch, the remainder fall into the 4-wire DENSO type. The 5-wire ID is as follows: Grey, NBV; White, pulsed heater ground; Yellow, reference low 2.2v; Black ref high, 2.8v; Red signal milli/amp, voltage. 28 AFTERMARKET APRIL 2021 TECHNICAL www.aftermarketonline.net SETTING THE BAR HIGH Frank’s tale of the Audi S3 goes on, with more suprises along the way The early Bosch variant carries a zero current on the signal wire there will also be a voltage transition between 2.2v and 2.8v, if AFR = Lambda 1. An excessive oxygen condition will cause current to go high of zero and oxygen deficiency would cause current to go low of zero (+/- 5ma). Voltage response on red is similar, lean above 2.8v, rich below 2.2v. The two reference voltages, black/yellow do not change. I am mindful to avoid the rich/lean description as it can lead to incorrect diagnosis especially without noting fuel trim characteristics, air leaks and dribbly injectors for example, as our problem vehicle clearly demonstrates. Pressure Now look in your pocket. I previously mentioned the GDI system storing pressure, unlike common rail diesel. If you rev the engine hard and cut the ignition at peak RPM, you should reach approximately 180 bars. Cycle the ignition back on and observe for any pressure decay. Pressure will hold semi-indefinitely over time. The later Bosch broadband sensor as fitted to our 1.8 engine is somewhat different in circuit response. Both high and low reference voltages are a little higher, the red signal wire does respond to current in a similar way, however both reference voltages do change in the opposite direction. Sorry if this is confusing, I did warn you. We need to confirm what the Lambda current is doing on the Audi A3 at the instant of the fuelling anomaly, i.e., when the fuel pressure drops, and more to the point what the PCM is doing about it. To be sure of our findings, we obtained an identical engine management system fitted to a SEAT LEON FR. Please refer to Fig.1, our first Pico image, which shows current dropping below zero red trace, with both reference voltages rising symmetrically in response to low exhaust oxygen content {rich} From left to right, red trace, initial current at zero, open throttle, load enrichment, overrun fuel off, high exhaust oxygen, repeat test. All normal responses. PART THREE Fig 1 Fig 2