Anatomy of a Waveform, Part III

In previous training, you may have learned about the OBD II Misfire Monitors only to learn of the severe limitations of this monitor in the real world. For example, most OBD II misfire codes require two consecutive failures before lighting the MIL. In addition, some OBD II systems will allow as much as a 40% misfire at idle only to be ignored by the PCM. In cases where an actual misfire is flagged and the MIL illuminated, we are now faced with a diagnostic decision as to what actually caused the misfire. While it is always important to note the valuable freeze frame information as to what conditions the engine was under at the time of the misfire, the actual cause remains unknown. It also should be noted that there are many cases of false misfire codes on OBD II systems before model year 1998, when the manufacturers were granted misfire relief.

So whether it is an OBD II system or not, this article will cover the five types of misfires using the five types of detection strategies. You will see that this article will revolve around reading a secondary waveform under specific dynamic conditions. It is very important here to remind our fellow technicians that we will reapply what we have covered in our two previous articles on the "Anatomy of a Waveform." See March and June issues of Underhood Service, or Search Back Issues using the key word "Fulton" at www.underhoodservice.com.

Figure 2
Specific Secondary Testing

Idle KVs Cruise KVs WOT Snap Max/Min KVs Single Cylinder Spark Line Characteristics (Load/No Load) Cranking KVs

The chart in Figure 1 lists the five types of misfires in the left-hand column. Notice that in the right-hand column, the specific part of the secondary waveform that you would need to focus on for a specific type of misfire is listed. For example, for a lean density misfire, it would be highly unlikely that you would ever see it on the firing line. As you can see by the chart, one would have to focus on the important spark line to see any density type misfire. Looking at the right-hand column, does it appear to you that the spark line yields the most diagnostic information? We hope so.

There are five specific dynamic secondary tests indicated in Figure 2. Any of the five types of misfires can be detected in one or more of these dynamic tests.

Let’s first address the electrical misfire problem. This is usually caused by a faulty secondary component, such as plugs, wires, cap, rotor, or terminal connections. These components make up what we call the secondary loading effect. In other words, it is their responsibility to distribute the coil’s energy wisely and uniformly. The forces that attack the secondary circuit are still with us. Below is a list of secondary component problems that cause electrical type misfires, which are easily detected by focusing on the firing line.

• Weak Secondary Insulation
• Carbon Tracking
• Center Electrode Doming
• Unauthorized Air Gaps
• Secondary Terminal Corrosion (if excessive)

It is always important to keep in mind that misfires caused by carbon tracking problems or weak insulation problems, will typically never show up at idle or even at 2,000 rpm under no-load conditions. The key here is that you must create specific dynamic conditions for this type of misfire to occur. What we mean here is to create an engine condition that requires a sustained peak kV demand.

The Cranking kV Test
The bar graph in Figure 3 depicts the max kV demand during a WOT clear flood cranking mode. Do you see the 29-31 kV demand? What is this test telling us? Let’s say at idle we were looking at an 8-12 kV demand. But during this WOT cranking kV test, the kV demand soared to 30 kV. Why? Remember that the throttle plates are wide open and the PCM has cut off the injectors, thus eliminating any conductive fuel molecules inside the combustion chamber. Since each intake stroke takes advantage of peak volumetric efficiency (big gulps of air), we now should have peak compression resulting in more non-conductive air molecules compressed inside the combustion chamber. This effectively raises the ionization demand. Remember the law of ionization that was discussed in our previous article? This test has effectively done three tests for us:

1. Stressed The Secondary Insulation;
2. Verified Good Compression;
3. Stressed The Coil or Coil Packs.

By the way, you cannot possibly duplicate this kV demand with a test drive. It is always nice to detect any electrical misfire by doing this test in your bay, instead of performing a test drive.

Now let’s look at Figure 4. The secondary waveform has the label of being the second-most complex automotive waveform. We will discuss the most complex automotive waveform in a later article. Viewing Figure 4, it is important to separate the dwell section from the actual firing section before you start making diagnostic decisions on possible electrical misfires. The dwell section represents the coil charge period, which must properly occur first before we could ever have a good firing event. Keep in mind, that during this test, we are inductively coupled to the secondary circuit with our kV probe. Detecting coil charge problems can be done much better by viewing a primary waveform (whenever possible). This also will be covered in a later article. Since we are viewing secondary, let’s use the dwell section to help detect a primary miss trigger problem. In our single cylinder good example in Figure 4, notice that our primary turn on occurred. Note the initial voltage oscillations at the point of primary turn on. These oscillations are the result of the two opposing primary and secondary magnetic fields. A shorted secondary lead or a shorted secondary coil winding would result in the loss of these oscillations. This would also include a totally fouled spark plug.

Now, what if the point of primary turn on did not occur? We would obviously not have a good firing event since we did not charge the coil, resulting in the loss of the firing line.

Go back to the Figure 1 chart. Let’s discuss the #5 type of misfire known as a "Primary Missed-Trigger." Have you ever noted a parade pattern where an electrical misfire randomly shifts from cylinder to cylinder? Now note in Figure 5 the extended parade pattern. The trigger icon depicts the #1 firing event. Counting left to right for a firing order of 1-6-5-4-3-2, do you see that we lost the #3 and #2 firing events? Now since we have extended our screen time to view multiple firing events, do you see all six cylinder firing events starting over again with no resulting misfires? Why? Look closely in the area of the two missing firing events. What is missing? If you said the point of primary turn on is missing, you are exactly right. Did the secondary waveform alert us to a primary missed-trigger problem? Yes. (We will discuss primary in more depth in a later article.)

Now, since we are on the subject of parade patterns, let’s take a look at Figure 6, known as the Idle kV Test. If you remember in our previous article, we stressed the importance of making measurements. Your first measurement should be the firing kV demand at idle during no-load conditions. Figure 6 indicates an 8-10 kV demand, which is good for 0.045 plug gap with a little spark advance at idle. Keep in mind that these values will vary 2-4 kV due to the changing cylinder pressures.

Now note Figure 7, which was captured from the same vehicle at 2,900 rpm under no-load conditions, known as a Cruise kV Test. Did you notice the increase in kV demand to 15-18 kV? Why did the kV demand increase? Remember as the throttle plates are opened, the cylinder pressure increases, which creates a higher kV demand. This will only occur during the absence of spark advance. Most car line PCMs will advance spark timing as the rpm increases. The resulting spark advance will actually drive the kV demand lower at 3,000 rpm compared to what was actually required at idle. If we use the manufacturer’s procedure to set base timing by disallowing PCM spark advance, we also can drive the kV demand higher at 3,000 rpm. This also can be a good dynamic test to isolate a carbon tracking or secondary insulation problem. As you can see, the kV value at 3,000 rpm is nearly double the kV demand at idle. In addition, this also helps verify uniform compression.

Now let’s take a look at Figure 8. This waveform was captured at idle from an OBD II system that stored a P0300 random cylinder misfire code. You obviously do not see a missing firing event. But remember your first measurement. Do the kV values seem high for a 0.045 plug gap? Notice the 18-22 kV demand captured at idle with no load. The high kV demand was due to an open condition inside the distributor cap.

Keep in mind that less than 30% of the diagnostic information available from a secondary waveform can be seen by viewing a parade pattern. Why? You will note that due to the time per division, the spark line is too compressed to view properly. Most of our diagnostic information while viewing a secondary waveform comes from the spark line. One scope manufacturer that we know, offers what we call an expanded secondary parade pattern. This pattern allows us to view the firing line and the spark line together in a parade pattern. See Figure 9.

Now let’s discuss the WOT Snap Test, which reflects the max and min kV demand. Figure 10 indicates a parade pattern from a DI system. As we snapped the throttle open, the kV demands peak at 20 kV due to the brief increase in cylinder pressure and the PCM retarding the timing. As we slammed the throttle shut, we essentially cut off most of the incoming air. The min kV values are noted and are basically indicative of the rotor air gap kV demand. Note the 6-8 kV demand. How can we determine this? As the manifold vacuum increases, the fuel molecules are drawn into the combustion chamber, which creates what we call a hydrocarbon spike across the spark plug air gap. For a split second, the spark plug air gap is bridged with conductive fuel molecules. The only remaining authorized air gap in this system is obviously the rotor air gap. Most, but not all rotor air gap kV demands, will fall in the 3-5 or 6-8 kV range. You may ask, can this test be done on an EI system? Yes. The min kV demand represents the kV needed to conduct the waste firing event. The max kV demands are a result of the increase in cylinder pressure, just like DI systems.

There are a couple of good points to make here concerning this specific dynamic test. Number 1: If all the kV values appear good and uniform at idle, but one or more firing events go low during the WOT snap period, the problem is usually outside the combustion chamber, such as weak secondary insulation. Number 2: If one or more cylinders appear low at idle and fail to significantly increase during the initial WOT snap test, the problem is usually inside the combustion chamber. Although these two statements are not always true, they are a good rule of thumb to go by when performing this dynamic test.

Finally, keep in mind that the most valuable diagnostic information from a secondary waveform comes from the spark line. In fact, at least 70% of the diagnostic information from secondary is available from the spark line alone. It is still surprising to me that most technicians view a parade pattern most often and never focus on the spark line. In the next installment of this series, we will focus on the spark line under specific dynamic conditions.

Until next time, scope it out! You may be surprised on what you find or what you may have missed.