Anatomy of a Waveform - Part IV

"The energy produced by the primary and used by the secondary is the same with every firing event. The only variable, usually measured in milliseconds, is the amount of time it took to use the energy known as the spark line." This is a favorite quote from a friend of mine, Joel Burrows of Precision Tune’s corporate training headquarters, and was conveyed to me several years ago. The truth never changes in that we still use this wisdom whenever diagnosing a primary and secondary waveform.

If you recall, we stated in earlier articles that the spark line portion of a secondary waveform is the point that yields the most diagnostic information and is actually what we call our window inside the combustion chamber. Keeping that critical point in mind, let’s look at some waveforms.

Note the parade pattern in Figure 1. Do you notice a 2-4 kV difference in the firing kVs? You already know that it is perfectly normal for firing kVs to float to

2-4 kV. In addition, this pattern is from a DIS system, whereas half of the firing events are positive while the other half are negative. Recalling our previous article, the kV probe is before the plug gap on negative firing events and after the plug gap on positive firing events. Do you notice a kV demand as half of the cylinders are higher than the other half? If you said that #4, #5 and #6 were negatively fired cylinders, you were exactly right.

Now, let’s focus on spark duration: at 5 ms per division, you can see that we have sufficient spark duration during idle-no load conditions. Take a look at Figure 2. Do you see any problems within the firing lines? I don’t. Now focus on the spark duration period. Do you see a problem on cylinders #4 and #3? Notice the shorter-than-normal spark duration period, caused by an open plug wire on cylinder #4? Remember, if an electrical problem exists on one side of a DIS system, it also will effect the spark duration of the companion cylinder. In this case, it is the #3 cylinder. Even though this is a parade pattern, we will have to focus on the spark line to properly diagnose the problem. Figure 2 was captured during idle-no load conditions. Notice that the raster pattern in Figure 2.1 (captured off the same vehicle) allows us to focus more on the spark line. Do you see the shorter-than-normal spark duration periods on cylinders #4 and #3?

Now, let’s turn our attention to a different problem. Let’s focus on the raster pattern, which will allow us to focus on the all-important spark duration period. Note Figure 3, which was captured during idle-no load conditions. Do you see the 1.8 ms spark duration periods? This tells us that the secondary loading effect is good and that the coil’s energy is being used wisely.

Take a closer look at this car in Figure 4 where we actually had a lean density misfire at idle. Flowing propane actually smoothed the idle quality so we suspected a clogged injector. Remember, propane usually will not smooth out a rough idle unless you can flow the propane at the exact point of the air leak. In this example, we would be hard pressed to make a diagnostic decision as to what cylinder was suffering from the lean density misfire. We would however, be able to note that cylinders #4 and #3’s spark durations seemed to favor the short side (as compared to the other cylinders). This is what I call the "3 M Rule" - Milliseconds Matter Most. Do you recall that we made an earlier statement: that it is necessary, at times, to scope secondary under specific dynamic conditions?

Next, focus on Figure 5, which was captured during an off idle-power brake condition. Notice the increase in spark line voltage and turbulence and the shorter spark duration on cylinder #4. Do you see a change in the secondary loading effect? The increase in spark line turbulence and the increase in spark line voltages results in a shorter spark duration period. Technicians would find it difficult to find this problem on a parade pattern due to the fact that the spark line is actually too compressed to view. Notice how much of a closer look we can get out of the spark line with the time base shifted to 1 ms per division.

Sixty percent of all misfires are directly related to an air/fuel ratio problem. Since we are on the subject of lean density misfires, a recent survey (Summer 2001) by an independent testing firm has concluded that 85% of the gasoline on the market at that time varied significantly in the content and make-up of detergent additives. These additives prevent the olefin and diolefint build-up at the injector nozzle areas. Furthermore, the survey said that the minimum amount of detergent additives (to be effective in preventing injector restrictions and internal engine carbon deposits) should be 100 parts per 1 million parts gasoline. The testing firm stated that only 15% of today’s gasoline meets this requirement while a whopping 85% uses a deficient 1 part per million detergent additive to gasoline ratio!

It is important at this point to also mention another one of my favorite patterns, known as the super-imposed pattern. The super-imposed pattern in Figure 6 on page 31 was captured during idle-no load conditions. Notice the uniform spark duration periods at idle indicating no problems. Keep in mind that this is a DIS system, half of the firing events spark kV points will be higher than the other half due to the relationship of the kV probe position (before the gap on negatively fired cylinders and after the gap on positively fired cylinders). At any rate, you can see that we have good spark duration periods in Figure 6.

Notice now under specific dynamic conditions, such as an off idle-power brake condition, the problem cylinder now comes out, as illustrated in Figure 7 on page 31. At this point in a super-imposed pattern we have no idea which one is the problem cylinder. However, as you recall, the raster pattern from Figure 5 pinpointed the actual problem cylinder. Remember, as we open the throttle plates, the cylinders breathe more air, thus increasing the ionization demand. Whenever the cylinder is lean (not enough conductive fuel molecules) the spark line voltage demand increases.

Now, notice the same vehicle after replacement of the bad injector, as shown on the raster pattern in Figure 8 (captured during power brake conditions). Do you see the difference? You will notice some distinct characteristics of coil oscillations between the negative and positive firing events. Do you see them in Figure 8? The positive firing event coil oscillations go negative while the negative coil oscillations go positive - a perfectly normal phenomenon.

Let’s now take a look at an electrical insulation problem. These types of problems actually make up about 20% of all secondary misfires. Do you see any spark duration discrepancies in Figure 9 on page 33 (captured at idle-no load conditions)? Obviously not. You may remember earlier when we mentioned that kV demand will increase as cylinder pressure increases. The law of ionization tells us that the secondary insulation also would be stressed as the kV demand increases during a power brake of this engine.

Keeping this in mind, watch what happens to our spark line characteristics in Figure 10 on page 34 (during a power brake condition). As the cylinder pressure increases, the resulting increase in kV demand will stress the secondary insulation (whether its poor secondary insulation or external/internal coil carbon tracking).

Focus on cylinder #5, which indicates longer spark duration and no turbulence. This is actually the result of spark occurring outside the combustion chamber. Again, we used the specific, dynamic rule to see this problem.

In the example as illustrated in Figure 11, what is the waveform telling us? Let’s focus on the spark line voltage and/or the changing angle. Do you notice the high spark kV point? Or worse yet, the sudden drop in spark line voltage indicating a partially fouled plug? Depending on the severity, you may detect this at idle or only during power brake conditions, as in this example.

Suppose that you do not have a specific scope that will give you a raster pattern or even a super-imposed pattern.

Note: I placed the secondary kV probe directly on the plug wire (on a DI system). Since we are downstream of the voltage drop from the rotor air gap, our signal may be a little weak. Nonetheless, what we actually care about is our spark duration period and spark line characteristics.

In Figure 12, notice our 1.2 ms spark duration period from a good firing event (captured at idle during no-load conditions).

In Figure 13, we purposely created a lean condition and captured it at just off idle. Do you see a suspicious short duration and a rising angle? If you said yes, you are on the path of freeing yourself from the unreliable OEM test procedures of looking at the pressure drops across the injectors (on many systems you cannot find them, feel them or even see the injectors). At this point, you may be asking yourself, "What about COP systems where there is no access to secondary?" Please stay tuned for our next article. Hint: it involves spark duration measurements and spark line characteristics.

Review the two waveforms in Figure 14. Channel #1 is hooked to the coil tower. The #1 trigger of channel #2 was hooked to a specific plug wire on a good cylinder (blue trace). Notice our good 1.2 ms spark duration period captured during an idle-no load condition (yellow trace).

At idle, during a no-load condition, are you starting to see shortened spark duration as indicated in Figure 15?

As shown in Figure 16, our #1 trigger is now on the plug wire of the lean cylinder. Do you see an increase in spark line turbulence, a shorter-than-normal spark duration and an increase in spark line voltage, all indicative of a lean cylinder? In addition, a good reliable tip off here is the high spark line blow-out point. This example (Figure 16) is just another reason you have to look at secondary under specific dynamic conditions.

Figure 17 shows the secondary after the fix was implemented for the example as shown in Figure 16 (same loaded conditions applied). Do you see the difference in spark line voltages? We hope so! Until the next time, "Scope it Out"! You may be surprised on what you find or what you may have missed!