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Maintaining engine spark

If a picture is worth a thousand words, an ignition pattern is more like a new e-reader.
Tuesday, December 27, 2011 - 00:00

If a picture is worth a thousand words, an ignition pattern is more like a new e-reader.

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Coil output is determined by the mass of the iron core being converted into a magnetic field. Creating this magnetic field takes time, called the current ramp. Once the iron core is saturated, increasing the current flow does not add to that stored magnetic energy. More current only speeds up the time it takes to come to full saturation. Once all the north poles and south poles are in line, the coil is fully saturated and the collapsing magnetic field is independent of engine speed or load.

Modern ignition systems are designed to maintain a spark for as long as there is fuel left in the combustion chamber. Therefore, the duration of the spark coincides with the amount of hydrocarbon, which is the conductor for electrons to travel. Fuel distribution is determined mostly by the amount of air inhaled, which is based on the cubic displacement of the engine.

A mass air flow (MAF) sensor precisely measures air intake. This data is passed on to the computer, which in turn responds with dictating the duration of the injector on time. When all that is perfectly monitored, we can observe this on the scope pattern, as seen in Figure 1. The nose is the left-over coil energy, which perfectly matched the hydrocarbon with very little residual energy left. Since hydrocarbon is a conductor, we can see a smooth firing line, providing fuel up to the last part of coil energy.

If the nose is absent, it might indicate a leaky injector with fuel supply not controlled by the computer. When the opposite happens and the scope shows a very high nose, there is a shortage of fuel. Therefore we use the coil energy as a yardstick to compare with each cylinder.

Inductive Kick

The inductive kick has value only as a comparison. By itself, it has no diagnostic significance of what occurs in the combustion chamber (Figure 2). It is the result of a collapsing magnetic field and the amplitude depends on how close the winding is to the iron core. If the secondary winding is closer, there will be a greater coil output but a reduced primary inductive kick.

The amplitude is insignificant for diagnosing engine performance. If the amplitude is lower and the firing time shorter as compared to the other coils of this engine, use a current probe to test the coil.

WARNING: The primary inductive kick (up to 400 volts) will damage any lab scope unless it is protected with at least 10 to 1 attenuation. If you are using the DIA-4 adapter, all A and B scope input channels are protected.

Turbulence on the Firing Line

Look at Figure 3 and note that the spark line is jagged due to turbulence in the combustion chamber at higher speed and under load. If all cylinders are fairly much equal and there is not a lot of difference in comparison, then accept this as a likely designed feature of that engine.

Some pistons are designed with a hump-like top, thereby creating turbulence to move the flame-front for faster and total combustion. Imagine a string lying straight on the table. Now create a zigzag pattern and note that the ends are closer together but the string is still the same length. So it is with the firing line: it is jagged but not broken. For diagnostic purposes, look where the firing line starts.

Note the red arrows in Figure 3. The key is to compare with the idle capture. If all captures are at the same level, it tells you HC is present at the start of the firing line.

Multiple Coil Ignition

Every late-model vehicle will have multiple individual ignition systems, either distributorless or coil-on-plug. This makes it easy to compare good and bad systems. Comparing waveforms of all cylinders at varied speed and load is ideal to pinpoint the one that stands out from the rest. Comparing all cylinders under the same driving condition has the great advantage of an easy pick of any failure.

Where, when, how many and under what condition it acts up narrows down possible failures as a first step in logical analysis. Looking up an archive without when-where-what is a meaningless exercise. I am using a four-position switch to make testing from the driver's seat simple. Because the objective is comparing the firing line, I feel that more than two cylinders on one screen makes it too crowded. With the adapter, all scope inputs are attenuated.

Analyzing the Nose

When we analyze a scope capture, we have to think coil energy. It takes energy to drive the voltage up to a potential high enough to jump the gap. We call that KV demand. When that gap is under compression, it takes a lot more energy. Therefore, at increased RPM the timing advance makes the plugs fire at a lower compression and we see KV demand dropping.

Worn plugs increase the spark gap, making it harder to ionize the plug and using more energy. That can be seen in the secondary pattern, see Figure 10. But all this valuable information is hidden behind the inductive kick. Later, we will discuss a method how we can tell there is a problem at the start of the firing line. For now, consider HC as the conductor, providing the electrons an easy path to flow.

When that conductance prematurely stops, the voltage suddenly rises, peeking out as a high nose. This indicates an absence of HC possibly due to a faulty injector. But when the computer is trying to compensate for a lean injector, to prevent misfire, the nose may look like sloping up (Figure 6). If the volume of hydrocarbon is equal to the end of the coil energy, a small nose and a low number of oscillations appear after the nose (Figure 1). If there is no nose and no oscillation, we likely encounter a fouled or wet spark plug possibly caused by a dripping injector.

Analyzing the Project

Let us go on a tour! The customer's complaint is misfire when trying to pass on the freeway. The coils were replaced twice. After repairs, the problem disappeared for a short while and then it happened again. The vehicle is a 2010 4-cylinder Ford. The scan tool did not record any problems. At idle (multi-strike) it did not show anything unusual, so we proceeded to 2,000 rpm and scoped first cylinders Nos. 1 and 2 in FC (firing order), then cylinders Nos. 3 and 4 and finally back to cylinders Nos. 1 and 2.

It was obvious No. 1 had a higher nose and shorter firing line. The No. 2 cylinder is a typical example of a perfect match between coil energy remaining and HC conductivity. A small nose confirmed that. The other cylinders showed perfect, just like No. 2, so we will concentrate on cylinder No. 1.

When we increased the load while maintaining 2,000 rpm, it reduced the spark duration some more. The slope up indicates an increase of resistance that points at not only less volume of HC looking at duration, but also a leaner condition observing a progressive slope up. We are looking here at a still picture, but when you have a live scope pattern it is a good idea to watch for consistency or a change in this pattern. If it varies and occasionally corrects itself, it likely will be caused by erratic pintle movement in the injector, a condition a good cleaning might correct.

The Effect of Resistance

The expression analyzing means gathering information. Diagnosing means coming to a conclusion based on the information gathered, and then we verify with adding fuel or snap-test or disconnecting or connecting something. When we are observing something under load or of greater demand, we have to think: resistance, obstruction or restriction. All of those have the same meaning when we get involved in diagnosis or analysis under load.

Resistance has a greater effect at high demand. First, a corroded battery post has little effect on headlights, but that same corrosion has a drastic effect on the cranking circuit. An 80 percent restricted fuel filter has no effect at idle, but you will not make it uphill. This holds true for all drivability functions, such as intake airflow, exhaust restriction, restricted injectors, etc.

We noticed a slight restriction in cylinder No. 1 at 2,000 rpm with a shorter spark duration, but that became worse under load. Look where the firing line starts. The slope at cylinder No. 1 indicates rapidly diluted fuel, but it was still under computer control. It was the quick acceleration that drove the demand for more fuel. This, in turn, caused a leaner condition, increasing the KV demand resulting in the crossfire. Once that breach in the insulation was created, it became an easy path to follow even at slight acceleration. The scan tool did not find a code and all coils were replaced twice.

This became a nightmare, going back and forth with temporary fixes. A simple scope check solved the mystery.

What is a Crossfire

A crossfire is a condition where insulation breaks down when the secondary discharge (KV) finds a path of conductance outside the combustion chamber.

Crossfire is not an uncommon occurrence with coil-on-plug ignition systems for several reasons. When plugs are not easily accessible, replacement often is neglected, resulting in higher KV demand in the combustion chamber, enticing the spark to jump. When plugs are located in a deep well, surrounded by ground potential, it is an easy target for crossfire.

In this case, it was the lean condition that drove the KV demand high enough to jump somewhere else. Figure 8 shows the crossfire. It shows higher with a shorter firing time because there is no HC to create a path.

Progressive Analysis

The feature of comparing with other cylinders to identify a failure cannot be over emphasized. We could not ask for a better sample of comparing good vs. fail than looking at identical engine displacement at the same speed and the same load using the same injectors and the same coil. This is the easiest way to locate the one or two cylinders or bank standing out as abnormal.

Something to consider is knowing the conditions when it happens. If it only happens at idle but not at a higher rpm, this provides important data to draw a conclusion. The same holds true when it only occurs under load or only during acceleration. The more is known of the condition and frequency and number of cylinders, the easier it is to make logical accurate conclusive diagnosis. After hook-up, it only takes three minutes for a total analysis.

The Effect of KV in Primary

While KV demand is not visible in primary, the effect is noticeable in a shorter than normal firing line when there is no reason for it (Figure 10). Make this comparison at idle, when there is no turbulence. When there is no higher than normal nose and the duration is shorter than normal, then we can suspect energy loss at the beginning of the firing line. That can be found only between coil and plug and that is under the boot. Take the boot off and inspect. If you find a discolored spring, there had to be some arcing.

Here's a hint: Any preconceived notion or judgment based on past experience is detrimental to logical thinking and closes the mind from objective analysis. Make it a habit to "think ICE" (Internal Combustion Engine), not make or model. Experience is important and should be a help, not a hindrance to confirm the diagnosis. Remember, analyze, don't memorize.

Mac VandenBrink enjoyed the activity at Allen Test Products for more than 30 years, from tech writer to project engineer. He is considered the father of the Smart-Scope. He owns and operates Dynamic Auto Test Engineering Corp. (DATEC) and still is active doing training seminars.

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