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.
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.
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| Figure 1 |
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).
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.
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| Figure 2 |
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!
Ryan consulted with his coworkers and most all agreed the best course of action was to recommend replacement of the engine. He knew that acquiring the resulting pressure waveforms (from the intake manifold and within the cylinders) may offer a bit of insight as to the true condition of engine overall. It may also tell him if the resulting drivability fault was simply due to incorrect cam timing or not. This of course, would allow him to offer the proper solution to the customer, and do so with confidence.
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| Figure 3 |
Figure 3 is a capture representing pressure changes inside the intake manifold. To acquire this data, Ryan affixed his pressure transducer to the intake manifold and coupled it to his PC-based lab scope. The engine is once again disabled from starting and the engine is cranked over for multiple engine cycles. What’s great about these tools and process is Ryan can acquire this data as an active file and share the resulting captures via email or through chat groups like Facebook offers, directly. First, Ryan’s concern was of the seemingly similar random-looking pressure changes in the intake manifold. He was interested in tying the results of the capture to a loss of cam timing on one bank. This is where I come in to play. Let’s analyze the waveform.
First, using a point of reference (from a known ignition event) I was able to determine when an entire engine cycle began and ended. This allows me to capture the data reflecting each of the engine’s pistons contributing to the intake manifold. Researching the firing order is necessary to determine how the activity in the intake manifold correlates with each of the cylinders. A piston chart was added to the capture to aid in analysis (and in explanation to Ryan) as to what I see occurring in the data. We have to first understand that as each cylinder enters the induction-stroke portion of the engine cycle, the intake valve for that cylinder is open. This piston will descend and inhale the fresh air from the intake manifold. Since we are viewing data from the perspective of the intake manifold, each one of these induction events results in a momentary increase in intake manifold vacuum. This causes the trace from the pressure transducer to decrease in amplitude (or “head south”). We can call these events “pulls.” We are interested in seeing which cylinder created which pull.
Cursors are spanned the entire engine cycle which offer the ability to partition the capture in to eight equally-spaced areas. These areas represent the eight cylinders contributing to the intake manifold vacuum trace. Using the cursors, I’m interested in seeing when these pulls occur, relative to the vertical cursors. A late pull will occur further from the cursor than a pull which occurred on-time. Indicated by the gray circles, these represent the pulls from bank 1. Take notice to the cursor, just left of the circle. Indicated by the yellow circles, are the pulls from bank 2. Take notice to the cursor, just to the left of the circle. If you compare the proximity of the circles to the cursors, it’s clear to see that bank 2 pulls are occurring later than bank 1 pulls. This also explains the random-looking pattern of the cranking intake vacuum trace.
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| Figure 4 |
Using a piston chart to aid in analysis and explanation, and now referencing the yellow dots superimposed upon it, it too makes it clear to see that all of the bank 2 intake pulls are late, relative to the bank 1 intake pulls. Logic supports a bank to bank timing issue, and we would anticipate every other pull to be late. That would be true in many cases, but not in this Audi’s case. Take a moment to view the engine configuration in Figure 4. The firing order on this engine’s configuration supports a firing event that alternates between banks…. except when cylinders 6 and 8 fire. They are two cylinders that fire consecutively ON THE SAME BANK! This is what you see occurring in the intake trace for pulls 1 and 2 (as indicated by the red numbers, at the top of the capture. These numbers are calling out the cylinder responsible for the pull below it. If you then reference the piston chart, you will see that I have encapsulated an area with a yellow box. This area contains two black stars that indicate when cylinders 6 and 8 approach TDC/compression consecutively. Because the bank 2 intake cam is late, the intake valves for bank 2 cylinders will open late but close late as well, leaving more volume to be shed back to the intake manifold, as the pistons approach TDC/compression. This is why the intake trace rides “hi” for so long at this point (two consecutive cylinders pumping extra volume back to the intake manifold). This is why it has that random-look to it. The appearance of the trace is due to engine configuration.
Figured that out! On to the next step
This data will tell us why the cranking intake vacuum waveform appears as it does, but more significantly, drives us further to the next logical test. A running in-cylinder pressure waveform was acquired form an easy-to-access cylinder on bank 2 (the suspect bank). The pressure transducer is used in place of a mechanical gauge and reveals a tremendous amount of data, and not just peak compression. Because the data is captured on the lab scope, with cursors we can effectively associate time with the 720-degree engine cycle. In other words, we can confirm camshaft timing (to within a few degrees of accuracy) as well as monitor the breathing characteristics of the cylinder, dynamically.
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| Figure 5 |
As can be seen in Figure 5, the running in-cylinder compression waveform (for one from the suspect-bank) is captured and annotated. The cursors denote the characteristics supporting the late intake cam timing, as suspected. The red annotation indicates low running compression for this engine. The orange annotation demonstrates the point where the intake valve opened. The cause of the deep in-cylinder vacuum (at almost 24” hg) is due to the piston descending down the cylinder with the valves closed. Only when the intake valve opening finally occurs is the vacuum relieved. Finally, the intake valve is seen to be closing at about 127 degrees ABDC of the induction stroke. These are all key indication of a late intake camshaft. The key is that the captures display what the symptoms offer as suspect. The real prize is the fact that we can see a suspect bank’s cylinder draw a vacuum and create compression with no leakage…all without disassembly and from the other side of the great Atlantic Ocean! Ryan now has the evidence to prove he may begin this saga of a repair. More importantly, he has the confidence to do so. This now perpetuated by his newfound knowledge, his understanding of the pressure waveform analysis!
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| Figure 6 |
The better part of a week goes by and I receive another Facebook video capture. Ryan completes the disassembly and confirmed bank 2 intake cam was indeed “late”. The cause of the retarded camshaft was due to a damaged VVT actuator, where the locating pin interfaces to lock the actuator in place (Figure 6). Ryan replaced the timing components and reassembled the vehicle. The engine starts, runs smoothly but the best part was Ryan’s excitement. He was dead chuffed! I only wish I could’ve been there to see the look on his fellow employees' faces!
Shortly thereafter, I received the resulting post-fix captures. The in-cylinder trace reflected strong running compression, an intake valve that now opened on-time at about 16 degrees ATDC. This allowed the piston to inhale freely and prevented the deep in-cylinder vacuum we witnessed earlier. The compression began to increase way earlier as well. The intake valve now seats at about 60 degrees ABDC. Much better than previously (Figure 7). The cranking intake vacuum waveform now exhibits pulls whose proximities are all very close to one another, regarding where they fall, relative to the vertical cursors (Figure 8). This is indicated by displaying both the YELLOW dots (representing bank 2 pulls) and the GRAY dots (representing bank 1 pulls).
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| Figure 7 |
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| Figure 8 |
We can all now see how difficult gambles can be avoided by moving forward with technology. Learning, by sharing information with peers all over the world and employing newer testing techniques that allow you to make tough calls with the utmost confidence.