Ignition Waveforms Before and After Spark Plug Gap Adjustment on my 17 KIA

Since day 1 I have been fighting with hesitation on my 2017 KIA Forte5 with 32K miles. Some of you saw my post about the valve cleaning I did which seems to have helped the cold start misfires but once in a great while it still misfires a few times and has a bunch of hesitation points throughout the power band. After adjusting the spark gap, not only did the engine vibration smooth out and the spark line duration waveform extend, but the power band is much smoother. The hesitation were not misfires but only way I can explain it is incomplete combustion.

I stumbled upon this by accident while testing different inductive DIY probes and I was surprised at what I found and decided to investigate it some more.

All waveforms captured @ idle, Operating Temp, Alternator loaded up with high beams, blower on high and rear defroster to add some load to the engine.

Now immediately I interpreted the waveforms as "Secondary Resistance to HIGH for the potential KV output of the coil". The engine never misfired during all the events you will see in the videos below. The engine had a pretty good constant shutter to it which I thought was a bit odd as well. This being a new car, I knew the odds of all 4 coils being weak was very unlikely and having 4 bad spark plugs was also very unlikely. I tested the KV ouput on cylinder 1 coil and it jumps 1" consistently which i'm pretty sure is over 35,000 volts. So what else could it be? If the conditions in the cylinder were effecting it, than the spark plug gap may be to much. These were gapped to KIA spec.

KIA SPEC: 0.0394 - 0.0433
Original Plug Gap .040 - .043
New Plug Gap .030-.031

Cylinder 1 - BEFORE
drive.google.com/file/d/15mKeoonlP5l2E6E...kLR/view?usp=sharing

Cylinder 1 - AFTER
drive.google.com/file/d/1Iak_TzJMH5JAQgz...5df/view?usp=sharing

Cylinder 2 - BEFORE
drive.google.com/file/d/1ynsYXb9X4IgqWBD...mQI/view?usp=sharing

Cylinder 2 - AFTER
drive.google.com/file/d/1l7F7SKLZjZHMvqP...30V/view?usp=sharing

Cylinder 3 - BEFORE
drive.google.com/file/d/1-vMZV12Cs4VM_YV...7Wj/view?usp=sharing

Cylinder 3 - AFTER
drive.google.com/file/d/1eZkr2jpCyqtL8TC...A8v/view?usp=sharing

Cylinder 4 - BEFORE
drive.google.com/file/d/1nMOKsvEgbZ2wSdF...dl6/view?usp=sharing

Cylinder 4 - AFTER
drive.google.com/file/d/1aDra-JfTNNbHVN2...3t2/view?usp=sharing

I learned a lot from this experiment.

One thing I don't know, is how small of a gap is too small? I am thinking of the surface area around the spark to ignite the fuel/air mixture. Anyone know more about spark plug gaping?

According to this page inked below, you can calculate the voltage required to jump a gap.

You can calculate the voltage needed for electricity to jump across a spark gap with a simple formula: voltage equals the air gap length in centimeters times 30,000.

sciencing.com/calculate-voltage-spark-gaps-8776030.html

So 1" is 2.54 centimeters * 30,000 = 76,200?

I had trouble playing your videos on my Notebook but got them to play on my Fire Tablet.

Here is the little I know about spark plug gaps.
Before we went over to modern ignition systems plug gaps were a lot narrower particularly with Euro manufacturers.

Military vehicles Rolls-Royce B series engines --- 0.015" -- 0.375mm --- this may have been for other reasons radio suppression or deep water fording.

British Lucas & German Bosch -- 0.025" -- 0.625mm

European GM and European Ford used wider gaps in the range 0.032" to 0.035" --- 0.8 to 0.9mm

With the coming of modern high energy ignition systems and longlife platinum spark plugs wider gaps were specified by every manufacturer sometimes crazy wide it is now rare to find a specified gap outside the range 0.040" to 0.060" --- 1.0 to 1.5mm

It has been noticeable that vehicles that use wider spark plug gaps tend to suffer more issues with the secondary side of the ignition.

Now that is Awesome!!!

Kia's spec is out to lunch, the wider the gap the shorter the burn time.
1.0-2.0 MS is considered 'normal'

1.5MS is puuurfect at idle, good job!

You cyclinder 1 BEFORE event shows your spark intermittanly arcing before the spark plug btw. at exactly 13sec in your cyl 1 before.

It would be good to check snap throttles as well.

Nice Work.

Edit Content Below, This is a copy and paste, but I believe to be true.

"With the engine at normal idle, observe the Burn Time duration and note the readings. Then increase the engine speed to about 2000 RPM and note the readings. The spark Burn Time durations for each cylinder should be about equal.
Spark Burn Time Duration changes during acceleration and deceleration, that is why we need to know and compare them under all three conditions. Under acceleration the Spark Burn Time increases to it's Maximum to accommodate for the richer mixture. This is known as Spark Burn Time Maximum. Under deceleration it decreases since there is not enough fuel to burn. This is known as Spark Burn Time Minimum. The Minimum burn times and the Maximum burn times are just as important as the "normal" Spark Burn Time Duration at idle.
     A commonly used method to simulate acceleration and deceleration conditions is the to Snap Throttle Test. With the engine idling, simply snap momentarily the accelerator pedal right down to the floor and take you foot off immediately. This will rev up the engine momentarily, loading the cylinders (and the ignition system) with fuel. Now, there is more time needed to burn all this fuel so the spark will run for longer, and hence produce the highest burn times it can. Vice versa, when you take you foot off sharply you'll "choke" the engine leaving it with no fuel or air to burn. It's like going downhill in gear with no throttle depression. This will shorten the spark burn times duration as there isn't much to burn."


Its nice to see you took the time to perform this testing and make corrective measures, and not believe some 'engineer' that obviously did not take the time lol!

Super cool captures, sir.

TBH, I've been burned a couple times on plug gap issues. It's been beaten into me ever since community college that platinum/iridium plugs don't get gapped (or even checked). It took a couple comesbacks for me to get over that one. :blush: Now I check gap on every plug I install. Ford EcoBoost engines come to mind:

www.scannerdanner.com/forum/post-your-re...nder-load.html#17320

Since day 1 I have been fighting with hesitation on my 2017 KIA Forte5 with 32K miles.

How many visits to the dealer do you think it would have taken to figure that one out? :silly:

Tyler wrote: Super cool captures, sir.

TBH, I've been burned a couple times on plug gap issues. It's been beaten into me ever since community college that platinum/iridium plugs don't get gapped (or even checked). It took a couple comesbacks for me to get over that one. :blush: Now I check gap on every plug I install.

I honestly have not been checking gaps lately. I did on one vehicle and my own (when I cleaned the valves). This makes me want to check them more often. Here is the kicker, should we believe dealer specs considering the trouble I had? I am going to guess that most dealer specs are OK but not perfect.

Tyler wrote:

Since day 1 I have been fighting with hesitation on my 2017 KIA Forte5 with 32K miles.

How many visits to the dealer do you think it would have taken to figure that one out? :silly:

Ha! Well I know for a fact, the dealer I bought my car at, does not own any oscilloscopes, I worked there for 4 days and walked out haha. I was so skeptical they would find anything I had called a dyno shop asking how much they would charge to do 3-4 pulls to map out AFR and power/torque curves so I can visually see the hesitation points. I was going to hookup my oscilloscope as well to try and map stuff out even more....some how

Weycraze wrote: Now that is Awesome!!!

I THOUGHT SO TOO! Not every day you see something, do something, and notice something has changed enough to report something with proof haha.

Weycraze wrote:
You cyclinder 1 BEFORE event shows your spark intermittanly arcing before the spark plug btw. at exactly 13sec in your cyl 1 before.

During all those events at idle the engine never misfired. I would imagine if it arced before the plug I would have heard the engine skip a beat, or at least would have felt it. You can here the engine in the background idling fine. Now usually when I see spark lines like that I would also think the spark is going some where else but since the engine never skipped a beat, this is what also told me that there is a spark occurring at the plug for a very short amount of time.

Weycraze" wrote: It would be good to check snap throttles as well.

I am in total agreement here. Though on this car, computer control throttle, very hard to get a quick, harsh snap throttle. Its more of, a quick gradual throttle opening with a .5 second delay in throttle response haha. Kia sucks for throttle response. It might be enough though.

I did capture all 4 coil primary circuits while at WOT if you want to see those. I'm new at analyzing WOT snap shots, to me the engine looks to be running lean as there is a spark line but tons of resistance and very short spark durations with very high energy reserve at the end of the spark line.

On another note, I was doing some thinking while working on the car and thought about spark duration. I am always told its controlled by the computer but that can't be 100% correct since the PCM is only turning the primary coil on and off. Unless the amount of time (Dwell Time?) the pcm is turning on the coil can determine the charged capacity of the coil which can then determine the approximate spark duration under assumed conditions? I think the coils charged capacity (The amount of energy stored/created at the moment the transistor is turned off), conditions in the cylinder and overall resistance in the cylinder, determine the spark duration along with other factors. So if I shorten the gap more, would I increase the spark duration during WOT runs due to less resistance between the electrodes.

The next problem with shortening the plug gap on these plugs, is the ground electrode is no longer "straight", its slightly angled downward which might have a negative impact as well since the area of the electrode that is designed for the spark to jump to, now has a varied gap. 0.30 being the closest area.

US based manufacturers tend to use longer spark duration than those from other parts of the world. Normal spark duration at idle varies between manufacturers.
Anything 0.8ms over is a normal secondary spark duration for a lot of the cars I work on.

Dwell time controls the amount of energy that is stored in the coil. Dwell time is measured in ms and is the length of time the coil primary is switched for before the spark is triggered. Dwell used to be a major issue for the technician on engines with old style contact breaker points ignition but now with engine computers it rarely causes issues.
The dwell time the coil is switched on for is limited for three reasons (1) A coil can only store a fixed amount of energy before it becomes magnetically saturated at which point it can start to overheat (2) A high RPM the time available to charge the coil is limited. (3) At high RPM there is less time available to re-charge the coil between firing events.

Because less time is available at high RPM for dwell time manufacturers adopt very different strategies with regard to dwell time and how it is mapped to RPM. Some work on a constant short dwell time period while others use longer dwell times at lower rpm but shorten them at high RPM.

Because it controls the energy stored in the coil dwell time has a direct effect on the available secondary voltage, spark duration and coil "ringing" after the secondary firing line ends. Closing the spark plug gap won't always increase the spark duration as it is also affected by the conditions inside the combustion chamber.

If the spark plug gap is too close there will be excessive "ringing" oscillations at the end of the spark line , if it is just right there will be some ringing but it will die out within in one or two oscillations.

Thanks that makes things more clear.

If the spark plug gap is too close there will be excessive "ringing" oscillations at the end of the spark line , if it is just right there will be some ringing but it will die out within in one or two oscillations.

I was doing some thinking. When I analyze an ignition waveform, I also look at the ringing but I interpret the ringing as reserved energy that can no longer sustain a spark. I figure if the gap is closer with a good coil, more energy will be used for a spark since there is less resistance which in turn makes less reserve energy available equaling less ringing??

I like getting technical. Someone with your knowledge is good to have on this forum.

Andy.MacFadyen wrote: If the spark plug gap is too close there will be excessive "ringing" oscillations at the end of the spark line , if it is just right there will be some ringing but it will die out within in one or two oscillations.

:lol: Welp, now I gotta go test this out. THANKS, ANDY.