Social media is an amazing thing in my opinion. From a diagnostician’s perspective, having the ability to reach out and communicate (in an instant) with like-minded individuals across the globe, allows for some tremendous opportunities. Each one of us sees through a different perspective, has different experiences with different vehicles and can offer data that we might not encounter otherwise. With a group of like-minded individuals that both share a passion for the automotive industry and a desire to learn/share/educate equally, it’s a winning combination. Its these very traits that helped to make an important and otherwise expensive diagnostic decision, easy as pie.
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Same problems, different terminology
Earlier this month, I crossed paths with a fellow tech by the name of Ryan Colley. Ryan works in a shop called Elite Automotive Diagnostics, located in a small village called Bishops Hull, in Taunton, United Kingdom! Ryan reached out to me because both of us network commonly with other techs through a few Facebook automotive groups. Each one of us in the groups has a particular arena that they are comfortable in. I happen to be comfortable analyzing pressure waveforms acquired from different points on the vehicle. This is the reason Ryan reached out to me. Ryan is faced with a 2006 Audi S4, housing a 4.2L DOHC-V8 engine under its hood (or should I say “bonnet?") as seen in Figure 1. The engine performs very poorly and was brought to his workshop for analysis. Ryan quickly recognized the symptoms the vehicle — with 77,564 miles and an automatic transmission — was exhibiting, as the cranking cadence of the engine indicated something mechanical is “going to pot.” The vehicle’s PCM was scanned for DTCs. Looking at the DTCs, we can see that misfires are being flagged for cylinders 5,6,7 and 8. The DTC pertaining to the bank 2 camshaft position is the “cream on the plum pudding.” All of the supportive evidence thus far indicates a shift in camshaft timing on bank 2 of the engine.
Ryan had the sense to employ some testing techniques that were both easy to perform and delivered an abundance of diagnostic information. The resulting data from the DTC scan also guided him to the next logical test that was more involved, but would lead him to a more pinpointed answer, regarding the root-cause of the fault concerning this vehicle. Ryan doesn’t shoot from the hip with his diagnostic approach. He lets the easy tests justify the need for more involved tests (no “guess-work” — just logical, solid testing techniques).
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Logic told Ryan that the results of a relative compression test would further back-up his theory. Ryan performed the test using an amp probe and a lab scope. The current flowing through the starter supply circuitry is measured and plotted over time on the lab scope. After the engine is disabled from starting and is cranked over (for a few cycles), the resulting current draw is plotted as a trace and presents as a series of “peaks.” If this Audi exhibited no mechanical fault, and because all eight cylinders are engineered the same, they should place the same load on the starter (as they approach top dead-center of their respective compression strokes). Ryan’s theory (due to the supporting evidence) is a mistimed camshaft on bank 2. Ryan anticipates a relative compression capture displaying a variation in “peak amplitude” comparing cylinders from one bank to the other bank. Figure 2 is the result of the test and confirms Ryan’s hypothesis. Ryan sees the variation in peaks but it doesn’t exactly represent the waveform he anticipated seeing. It appears to have a few back to back peaks of low amplitude and the fear is “engine damage,” sustained from the loss of camshaft timing. Ryan proceeds with yet the next logical test procedure, drawing him closer to a diagnosis and recommended course of action.
What would your next test be?
It’s quite clear that the engine is out of time. The questions then become:
- Is the valve train damaged?
- Is there damage to the pistons/lower end?
The questions are logical but they hold a bit more significance then they first appear to. The configuration of this engine places the timing cover on the rear of the powerplant. To even visually inspect the timing components requires removal of the front-end of the vehicle, the engine/transmission assembly and they then must be separated from one another, as the timing components are sandwiched between the two units. To replace the timing components requires the better part of a 30-hour job! Completing the repair is no easy task, to say the least. Consider the situation if the timing components were replaced, but the engine sustained damage, unknowingly!