Hot under the collar: Audi Rs6 Case Study
AUTOTECHNICIAN CONTRIBUTOR GARETH DAVIES, WHO RUNS A GERMAN CAR SPECIALIST WORKSHOP IN SOUTH WALES, IS REMINDED OF THE IMPORTANCE OF VALIDATION IN THE DIAGNOSTIC PROCESS. As vehicle technicians/business owners, I am sure we have all at some point in our careers been subjected to the curse of “Ever since you did/changed…”. It’s never a pleasant feeling, and even the coolest of operators can struggle to make peace with the explanation to the customer of why/how it’s unfortunate but unconnected. A few summers back, I found myself potentially facing the same scenario but stopped as soon as the warning shot was heard. My case study touches on the humanistic element of this particular job, and also the technicality.
CASE STUDY
The job in question was an Audi RS6 ‘plus’ 2009 5.0 v10 TFSI. I checked the job sheet and saw that the customer requests were: Investigate rumble/vibration under load, AC not cold, and rear-view external mirrors not demisting. The customer was also a new, first timer.
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I walked out to the car park to get the vehicle prepared for a road test to check the rumble/vibration before going into the workshop for more tests. Unfortunately, that’s when the warning shot went off. I attempted to start the car and it simply cranked with no engine start. Trying again, I got the same result, non-start. Now at this point, a conscious decision was made before any investigation to advise the customer of the fault and ask if they’d like us to look into it. They were surprised but agreed. I was also surprised as it was driven to us earlier that morning under its own steam. An initial scan showed multiple faults in various controllers, some of interest to perceived complaints, some academic or possibly historic. The faults of interest were stored in fuel pump electronics and engine ‘Fuel pump control unit defective – Static’ and ‘Fuel rail pressure too low – Static’. I left the car for
a few hours and returned to the car to push it in, armed with a brace of technicians, and to my amazement, the car started and would restart on multiple occasions. Moving on with my process, I now had the task of understanding the plausibility of the faults now that the car had returned to a running state, with no intervention other than some pixies that may have happened upon it without me noticing.
let's start with the basics Looking at the system layout loosely, it was evident that the fuel system was made of a pre-supply pump in-tank, a return regulator and pipe system to feed returned high pressure fuel to the left side of the saddle fuel tank over to the right side where the delivery unit was. Moving from low pressure to high pressure, there were two high pressure fuel pumps in the engine bay, both with integrated regulators (one feeding each bank of five cylinders) being supplied by a single common feed from the tank. The delivery unit is regulated by a fuel pump control unit and pressure is measured at two locations, low pressure feed at Y-Piece for split to high pressure pumps, and in the rail on one bank of cylinders, using a dumb logic that rail pressure will be equal in both rails, even though monitored in only one. Beginning with the ‘Fuel pump control unit defective’, which by now had gone sporadic, I wanted to ascertain some basic checks. Most modern high pressure injection systems, both petrol and diesel, are now likely to have a fuel pump control unit to control pre-supply pressure as it offers greater control over the delivery unit by means of a Pulse Width Modulated (PWM) signal, rather than a straight 12v on/off state controlled by a relay, as it was for many years before. The rationale being greater control over the unit’s operation can be achieved by pulsing voltage (like a dimmer switch) versus on/off. Before
