Secondary KV Isn't Everything!!

Often times when looking for a small or erratic misfire, a technician may only take a quick glance at secondary ignition with an ignition scope. If it is a DIS system he may even use a single cylinder setting and quickly view each cylinder individually. A common mistake, however, is only taking the time to notice the initial firing kV of each cylinder. There are many things other than firing kV that can be learned from a secondary ignition pattern, but in this article I will only discuss burn time.

With a standard distributor ignition system the temptation is to glance at the parade pattern and see if any one cylinder is using more voltage than the others. This is a little more difficult with a DIS system as most scopes will only view one cylinder at a time, so the technician must remember the firing kV value from one cylinder to the next. Taking the time to switch to a raster view in the case of the standard ignition system, or also jot down the burn time on a DIS system can help find those small, barely noticeable misfires.

The burn time is the length of time that the spark continues to travel through the spark plug gap. It takes a significant amount of energy to overcome the initial resistance of the spark plug gap and ignition cables, which causes the initial secondary voltage to jump up to around 10,000 volts on a cylinder that is performing properly. Once the initial resistance is overcome and the spark makes its initial jump, it usually only takes 400 to 1000 volts to maintain the spark until the coil has dissipated most of its energy (there will be some residual energy left in the coil, but not enough to maintain the spark).

As seen above, the burn time begins immediately after the firing line and continues to the beginning of the first coil oscillation. The burn time in the capture above is about 1.4 milliseconds, which is typical of most ignition systems at idle, although it can vary from about 1 millisecond to around 2 milliseconds and still be normal. The best way to find normal for a particular vehicle is to scope a few cylinders at idle with a smooth running engine. Keep in mind that the burn time WILL decrease as the engine is put under a load. This is normal.

Take a look at the two secondary ignition captures below of a 2000 Chevrolet Malibu. This vehicle had a very slight and intermittent misfire present.

The burn time on each of these two captures is around 1.4 to 1.5 milliseconds. Now look at the next two captures from the same vehicle.

Note that both of these captures show a burn time of 1 millisecond or less. One of these two cylinders was misfiring and the other is the companion cylinder that fires from the same coil. By only looking at the initial firing kV of each cylinder it would be very easy to miss what could be a quick diagnosis. This is especially true when looking at one cylinder at a time as opposed to looking at all cylinders on a standard ignition system. (Some ignition scopes will connect to all cylinders at once, even if it is a DIS system, but most technicians do not have access to one that will.)

The problem with this particular vehicle turned out to be an easy find. One of the ignition wires had a poor connection at the coil causing the intermittent misfire. (See below). This caused significant corrosion/rust build up on the ignition coil, which had to be replaced as well.

Now, take a look at this raster pattern from a Dodge van with a 3.9l V6. The customer complained of a slight intermittent misfire on acceleration and a check engine light with a P0303. Granted this was an easier diagnosis since the computer did flag a misfire code, but it is still a good example of how burn time can be affected and initial firing kv look normal.

By using the raster view it is easy to see that there is a problem with cylinder #3. If only the parade pattern had been viewed the problem would have been much harder to notice as with the example below.

While it is possible to see the burn time difference in the parade pattern above, it is much easier to spot with the raster view. Also notice that the firing kv shows no problem with cylinder #3, but it does show some slight variations with cylinders 5 and 2.

This next example is from a 2000 Chevrolet 4.3 liter C1500. If you look closely, the problem cylinders can be seen in the parade view, but the problem stands out much more clearly in the raster view.

Just remember that a secondary ignition misfire does not necessarily have a high firing kV or an exceptionally low firing kV; burn time is also very important!!