2014 Challenger 3.6 with a very peculiar misfire that is driving me insane!

Ok this car belongs to a friend of mine, and she first brought it to me last July. At that time, the car had 94k miles, and the only code was P0305. The idle with the transmission in Park was somewhat rough, and you could feel it shaking the car significantly, sitting in the driver’s seat. However, in Drive, the idle smoothed up to the point where you could barely feel anything. The mode 6 misfire counters showed high counts for cylinder #5 at idle in Park, but low counts in Drive, consistent with the seat of the pants observations. Since this cylinder is on bank 1, which has easy access to the coils, I swapped the coil with the adjacent cylinder, but the misfire remained on #5.

I asked her if the spark plugs had ever been replaced, and she said that she didn’t think so. I checked the plugs on bank 1, and they were all severely worn, all indications being that they were the original plugs. The worst plugs were #2 and #4, which had gaps of .063 and .057 in., but all the others were not too far off from those. I replaced them with the same OEM spec Champions, and verified that each had the same .041 in. gap. I saw no evidence of arcing or boot damage on any of the coils, they all looked good.

The plug replacement did not solve the problem, as I fully expected it to. The same code returned, again, being the lone code present. The misfire was still present to the same extent, confirmed by seat of the pants and misfire counters. I then went down a logical, step by step path, methodically checking all the common things that could cause a misfire. I smoke tested the intake manifold, found no leaks. Manifold vacuum was within specs. I did a compression and leak down test on bank 1, with the engine warm, and all three cylinders were 135 psi, and 5% leakdown, double checked each cylinder to verify. Again, well within specs. I checked running and static fuel pressure, and they were both steady at 58 psi, and leakdown was 53 psi in 5 mins, all within specs. The LTFT on bank 1 was approximately +10%, bank 2 about +5%. I then ran an injector cleaner with a tool connected to the fuel rail, suspecting a clogged injector. It made no difference.

I finally scoped the #5 coil primary, as well as the adjacent cylinders in the firing order, #4 and #6. They all looked identical, and I saw no dropouts or major differences. I current ramped all the coils, saw nothing abnormal there, all identical. I scoped the injector for #5 and the adjacents, both voltage and current, and again noticed no abnormalities, they again looked identical. All these checks were done under the conditions where the misfire was prominent, at idle in Park, with all accessories off. Crank sensor waveform looked good, with no dropouts or abnormalities either.

What makes this misfire so peculiar, is that it is most prominent only at an unloaded idle, meaning in Park, with all accessories off. In Drive, the misfire counter still increases, but much slower than in Park, and seat of the pants it is almost imperceptible. Turning the AC on at idle in Park, has the same effect of greatly reducing the misfire. So does increasing the RPM to approximately 1500, under the same idle Park condition, with AC off.

I have gone over this in my head endlessly, trying to rethink every possibility, and I am totally stumped. I keep telling myself that I MUST be overlooking something simple, but I cannot figure out what at this point. I haven’t had a car stump me this bad in a LONG time. It’s literally driving me insane. I would greatly appreciate someone telling me what I am missing here.

If you have a pulse sensor hook it to the intake and see if it has even intake pulls. Then repeat with exhaust pulses. Or if you have an extra ac pressure sensor hook it to your compression tester and check the waveform. This sound like a valve timing/adjustment issue or a cam lobe was eaten if all else is good.

I appreciate your input. Very recently, I have actually been thinking about something similar to what you suggested, which is scoping the MAP sensor. I do not have a pressure transducer, but that AC pressure sensor is a very interesting idea, I’ve never heard of that before. I had also considered potential cam/valvetrain issues, but I could not understand how that would make sense based on the symptoms, since the problem gets better with either load or engine speed. If a cam lobe was worn for example, I would expect the problem to get worse at higher speed or load, due to reduction of airflow for that cylinder. I will definitely scope the MAP though next time I get to work on it, and see what that reveals.

Is your scope cape able of graphing frequency? If so, then scope the crank sensor. Anything that can slow down the crank signal will be interpersonal as a misfire by the computer.

When I see + 10 trims I think of fuel related. When I see anything under that I think ignition or mechanical.
How did the spray pattern look on the injectors?

Does it feel like a complete miss or partial combustion?

Yes my scope can display frequency. That is something that I never even thought about, because it’s something I’ve never used before. However, I can tell you that I have analyzed the crank sensor waveform to a high degree, and the pulses are all consistent in voltage and number per revolution. I am well aware that any crank sensor signal abnormality, or anything that could slow the crank, could be interpreted as a false misfire. After the injector cleaning failed to fix it, that was my next line of thought. But after spending countless hours with the scope on the car, and on my own time analyzing saved data, I never once saw any indication of that. I’m not quite sure how I would go about using frequency to identify a potential abnormality in the crank signal, can you elaborate a little?

The misfire feels like a complete miss, which is intermittent, but periodic. I can hear it from the right tail pipe. One time I actually attempted to listen to the exhaust and count the number of misses in 60 seconds. I don’t remember exactly how many there were, but I believe it was somewhere between 5-10.

I have no idea what the injector spray pattern looks like, since I never removed the injectors.