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Monday, April 1, 2019 - 06:00
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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.

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.

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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