Using Oscilloscopes on Vehicles

WisdomAugust

WisdomAugust

Lean injector: In following picture, we noticed that cylinder B has a shorter firing time.

The absence of hydrocarbon is particularly visible at the end of the firing time, where it

terminates with a high nose. It is as if the coil is saying, "It's not my fault, there is plenty

of energy left." However, there is something else to observe in cylinder B. The evidence

of a lean condition already starts at the beginning of the firing time with a higher kilovolt

demand and higher firing line starting point in comparison.

WisdomAugust

Exploring Firing Time

The coil output is designed to maintain the firing event for as long as there is controlled fuel in the combustion chamber.

Because a collapsing magnetic field determines coil energy, engine RPM or load doesn't affect it. The firing time indicates

how well this energy is used in burning the fuel mixture. After establishing the validity of the electrical components at

idle, firing time is our ruler to measure conductance.

Because all cylinders and injectors on that engine are created equal, comparing the firing pattern is the easiest method

to seek out the cylinder that is different.

WisdomAugust

WisdomAugust

Leaky Injector

At low RPM, it might look like a fouled plug, but at higher RPM the dripper might start firing

because there are fewer drips between firing events. But when this cylinder fires, the fuel

mixture is still rich and the scope shows a lower kilovolt plus a longer firing time with hardly

any nose. The computer looks at that dripper as too rich and subtracts fuel, driving the other

cylinders extremely lean.

There is one exception: the cylinder next in firing order, which benefits from the overflow of

the dripper. With computer controlled engines, total engine analysis of all cylinders tells us

the complete story and verifies diagnosis, and that is important to save valuable time.

WisdomAugust

What About Fouled Plugs?

Scope analysis has a lot to do with common sense. Let us imagine what a fouled plug looks like on the scope. No picture illustration to memorize, just use your imagination. There is no spark gap and therefore no nose or residual energy left. Because carbon resistance replaces the spark gap, the coil energy is drained off, as indicated by a curved slope from the kilovolt spike to the dwell line. Resistance of carbon foul could be as high as 2 Megaohms demanding more kilovolts than, for instance, a wet plug caused by a dripping injector. How do we know the difference? Very simple!

WisdomAugust

How Do We Know It Is a Plug Wire?

Let us assume a cylinder at idle showed abnormally high kilovolt, and the firing time seemed

to be shorter compared to the other cylinders. We need to find out where that high kilovolt is

created, internally or externally. Play with the throttle, accelerate and decelerate and watch

the response. If the kilovolt stays high, this indicates that the greatest gap was external.

How should the scope respond to rpm increase?

At 2,000 rpm, the timing advance is between 30 and 40 degrees BTDC. At that time, the

piston barely starts the compression stroke resulting in less compression when the plug

fires. Therefore the reduced kilovolt tells us that the timing advance is functional and there

was compression. We have verified the greatest gap is indeed in the combustion chamber

and not external.

WisdomAugust

Replacing a part to see if the misfire goes away without knowing the cause is a

fruitless exercise. Crossfire occurs when kilovolt is driven high beyond the capacity

of the insulation. Then the spark is enticed to find a path to ground outside the

combustion chamber. When that happens, there is no HC present outside the

combustion chamber to sustain conductivity resulting in high resistance creating

shorter firing time at a higher kilovolt level.

For practical purposes, there is no current flow until the plug fires as indicated by

the firing time. When the coil output runs out of energy, the residual energy dissipates

into oscillations. Because hydrocarbons (fuel) are conductors, they not only assist in

the ionization process, but also affect the conductivity during firing time.

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The Secondary Ignition Waveform-4

Low Compression

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The Secondary Ignition Waveform-3

A cylinder lost compression and we see less kilovolt demand

increasing the firing time. The opposite is true when the kilovolt

is high. We learn that kilovolt is a necessary parasite, robbing

from the burn time in the combustion chamber. Kilovolt does

not burn fuel, firing time does! A lean fuel mixture, (reduced HC)

causes the kilovolt demand to increase even more in proportion.

Therefore, an insulation breakdown causing a misfire is most

often the result of unusual high kilovolt demand created by any

combination of worn spark plugs and lean mixture.