Aftermarket December/January 2023

DECEMBER/JANUARY 2023 AFTERMARKET 25 which also generates a small voltage spike which the PCM uses to confirm end of lift operation. If high lift operation fails, the PCM will limit the RPM to 4,000, with all cylinders in low lift with reduced power output. If high lift return to low lift fails, the PCM will hold all cylinders in high lift with full power and RPM, however idle will be less smooth. I mentioned alternative variants; Take as an example the 2-litre 888 engine. Here, the exhaust valves share lift modification and cylinder select variants, whereby the PCM selects a zero-lift lobe preventing cylinder charge. This provides a pneumatic damping for the reduction in cylinder operation. Example The vehicle in our workshop, an Audi SQ5, was presented with a complaint of intermittent rough running. I deliberately avoided the phrase misfire as incomplete combustion may be caused by ignition, fuelling or mechanical faults. Tiptoeing through the minefield, David Gore our diagnostic tech at ADS initially appeared to rule out all three possibilities. Please refer to Fig.2, which shows the minefield…sorry…engine in question. Via a scan using his Pico scope no ignition anomalies were found. This included monitoring spark line profile and primary current ramping profile. Fuelling errors were carefully ruled out with observation of fuel trim and oxygen sensor data. Broad band sensors are highly effective at responding to excess oxygen content in the exhaust stream. While fuel trim may mask subtle fuelling errors, the rear catalyst zirconia sensor will always display an incorrect voltage. Nominal voltage from a stable load condition should be 0.7volts. The fuel injectors had not been removed at this point due to the balance of fault probability against cost. Do not let this consideration deviate your need to conduct further potentially costly testing though. David also conducted cranking current analysis, confirming uniform current draw on compression across all cylinders. This is a simple, accurate, means of comparing the physical load expressed in amps in overcoming the work done during the compression stroke. Initial results normally conducted over 10-15 seconds showed no deviations across all cylinders. In a throwback to the old engine tuner days, cranking tests were extended to 30 seconds at which point he discovered a single cylinder discrepancy with current draw. This must be a mechanical consideration only. I was not excluding an injector fuel delivery problem at this time, which could cause bore wash, affecting compression. Please refer to Fig.3, a Pico image showing compression loss. Synchronizing the event with number 1 ignition coil 5- volt PCM control signal, he quickly established the faulty cylinder. Upon dismantling the engine top end, a faulty rocker bearing was discovered, the effect of which allowed the rocker to hold the valve slightly open. Please refer to Fig.4, which shows a faulty rocker. Assessment Further investigation via ETCAS confirmed a modified version of the rocker was available. The vehicle is currently awaiting the necessary modified parts. This brings me back to my earlier comment about servicing requirements. As specialists in Audi and VAG in general, we see extensive premature engine mechanical failures. In my assessment, this is due to long-life servicing strategies, often recommended by dealerships or adopted by owners as a cost-saving measure, with extended intervals between services. This is generally a bad idea. We have several clients operating similar engine variants with trouble-free mileage approaching the 200,000 mark. This we believe is due entirely to oil replacement intervals not exceeding 10,000 miles or 12 months. Fig. 2 Fig. 3 Fig. 4