Rocker on Tommy?
Vehicle diagnosis is not dissimilar to navigating a minefield; Tread carefully. How’s that for an opening line? Not exactly an encouraging start I know, but bear with me. Within this topic I am going to explain Audi variable valve lift operation, design, servicing, and diagnosis. In order to underscore the risks – I did just say it’s a minefield – I have an actual fault from our workshop.
Principles of operation are remarkably similar across all engine groups. They are some differences though, operating on either inlet, exhaust notwithstanding, or cylinder select variants. The camshaft responsible for shift and lift control has a spline. On this is a sleeve containing the lobes for operating two valves independently, which slide laterally. Two helical grooves provide for the actuator’s engagement and disengagement. The lateral movement of approximately 7mm is locked by a spring-loaded ident ball, like that controlling gearbox selection rods. Please refer to Fig.1, which shows a variable lift control sleeve.
Different engine types vary in the approach, but this should broadly explain how the system works. Inlet valve operation is asymmetrical, i.e., each pair of valves operating from a common sleeve share different lift and closure profiles, but they open together. The closure event is offset however. This improves cylinder charge and creates a tumbling motion with the fresh air intake. Intake swirl flaps are no longer required complimented by Audi TFSI piston crown profile.
Due to physical limitations, the valve rocker arms on the variable lift camshaft are required to be narrower than their fixed-opening counterparts. This has been complemented by larger roller bearings. The lift control actuator is not a simple solenoid, controlled by the engine PCM. A permanent power supply is switched to ground via PCM command. This is achieved by a saturated control pulse, identical to early generation 15ohm injectors. The actuator pin is extended by a 3amp current flow with 100 G acceleration, requiring a permanent magnet and percussion damper to prevent the pin bouncing out of the helical groove. The back EMF pulse is used by the PCM to confirm actuation.
The actuation pin return is provided by the helix ramp, 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.
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.
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.