SCOPE & SCAN: Divide and Conquer

Jan. 1, 2020
When using an ignition scope to view information for a misfire diagnosis, it helps to separate the testing areas into two categories ...
Divide and ConquerWhen using an ignition scope to view information for a misfire diagnosis, it helps to separate the testing areas into two categories. These categories are determined not only by what the ignition scope sees, but more importantly, by where the event seen occurs. The first viewing category we’ll call “Inside” the combustion chamber, and the second viewing category we’ll call “Outside” the combustion chamber. Let us define what we mean by the terms Inside and Outside.
FIGURE #1 Various problems that can be seen in secondary ignition scope pictures.
(Photo: J. Garrido)

Inside the combustion chamber are the cylinders, piston assemblies, intake and exhaust valves. Also included are the spark plug electrodes and the gap, and last, the air/fuel charge that ends up in the cylinders during combustion. Of course, the intake and exhaust manifolds exist Outside the combustion chamber area.

Yet, these manifold systems do affect the conditions being viewed Inside the combustion chamber. It is only Inside the combustion chamber that the ignition scope would be able to see defects in those manifold systems.
Fuel delivery and supply systems also exist Outside the combustion chamber, but again the effects are seen by the ignition scope only as they exist (or not) Inside the combustion chamber.

Outside the combustion chamber are the ignition system parts: the Engine Control Unit (ECU), ignition modules, ignition wires and harness assemblies. The portion of the spark plugs that extend outside the combustion chamber also are included in this category.

With all these various parts in mind that are contained in both the categories of Inside and Outside, take a look at some waveforms taken from different misfiring vehicles. See Figure 1.
FIGURE #2 If it can jump this big gap with a strong blue spark, the ignition system is healthy.
(Photo: J. Garrido)

Waveforms No. 2, 3 and 4 look very similar don’t they? All three waveforms are from vehicles that are experiencing misfire at idle. All three show an abnormally excessive resistance in the horizontal spark line sections of the waveforms. This excessive resistance is evidenced by all the upward vertical spikes in what should be a relatively flat spark line like the one shown in waveform No. 1.

These waveforms can be caused by parts in either the Inside or Outside categories. An effective technique can be used to reduce testing time. Divide the parts and systems into the categories of Inside and Outside first, then only test the indicated half.

In order to use this divide-and-conquer approach with your ignition scope, you will need one of my favorite tools, a high energy ignition (HEI) spark tester. See Figure 2.

The HEI spark tester is a fixed-gap test spark plug. It takes about 25KV to jump the fixed gap of the tester. As this tester gap is breached by the spark, you can both easily hear and see the arc at the end of the tester.
The HEI tester is the dividing gate between the Inside or Outside categories. If you install the HEI spark tester on the end of an ignition wire and the wave shape now looks like waveform No. 1, the cause of the defect was Inside the combustion chamber. If the waveform still exhibits the defect, the cause is Outside the combustion chamber.

Let’s use the three defect waveforms shown here to give some examples of testing once the Inside or Outside question has been answered. If we have waveform No. 2 displayed before installation of our HEI tester, pull the spark plug wire for that cylinder off of the spark plug. Inspect the plug boot for signs of arcing down the inside of the boot. Also check for corrosion of the metal end of the plug wire.
FIGURE #3 This homemade double-male connector makes it possible to connect a spark tester to a coil-on-plug system.
(Photo: J. Garrido)

If both look OK, attach the HEI tester to the wire. If the pattern now looks like waveform No. 1, there is only one more consideration before concluding that the defect is exhibiting itself Inside rather than Outside. The last step is to measure the resistance of the offending cylinder’s spark plug (between the connector end and center electrode) and compare it to a non-misfiring cylinder. A spark plug’s resistor is considered Outside the combustion chamber.

Now that you have determined the pattern to be caused by something Inside the combustion chamber, you may have a sticking, burned or bent valve like the example in waveform No. 3. Use a vacuum gauge or cylinder leak detector to further diagnose the cause.

If the defective pattern persists with the HEI tester connected to the plug wire, you likely have an arcing ignition system shown in waveform No. 4. Use a known-good plug wire to rule out the installed plug wire, then check for arcing at the coil.

Having trouble attaching your HEI tester to coil-on-plug (COP) systems or hard plastic boots? Use this homemade double-male connector tool and a test sparkplug wire to hook your scope and HEI tester up to the system. See Figure 3.

The connector tool is made from two broken-out spark plug center electrode shafts joined by a butt connector with yellow shrink-wrap.  

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