Engine Misfire on Cold Start or Idle

[John89]

Hey Guys, my 2023 ram 1500 had its engine replaced due to flooding. Recently, I had the check engine light (CEL) come on with a code for cylinder #7 misfire. Went to the dealer and they “fixed” it. I get home and put on the live data module using an OBD-II scanner and all cylinders had a misfire count of 0 (zero). A week later, I scan the engine again and looking at live data, I can see nearly all cylinders misfiring yet no trouble code was thrown as it seems there’s a threshold that needs to be exceeded for a trouble code to be thrown.

My question is, is engine misfire normal for cold-starts or when the engine is idling? I don’t see any misfires or at least the misfire counts slow down while driving. I’ve attached some pictures showing the misfire counts for reference.

 

[Rick3478]

I'd think a couple misfires would be expected on startup, but don't run around with an OBD scanner, so don't know what mine looks like. The question got me thinking, though. How does the system even detect a "misfire"? I wouldn't think it possible to analyze the exhaust gases from each and every cylinder in real time, so what signals can be brought to bear? Vibration? Rotational speed? And what did they do to "fix" it? Sorry more questions than answers, but sometimes I just wonder about things.

 

[John89]

I'd think a couple misfires would be expected on startup, but don't run around with an OBD scanner, so don't know what mine looks like. The question got me thinking, though. How does the system even detect a "misfire"? I wouldn't think it possible to analyze the exhaust gases from each and every cylinder in real time, so what signals can be brought to bear? Vibration? Rotational speed? And what did they do to "fix" it? Sorry more questions than answers, but sometimes I just wonder about things.

Yeah, good questions. So the “fix” involved replacing a spark plug that was improperly gapped according to the tech at the dealer. From what I understand, the OBD2 systems detect cylinder misfires by monitoring variations in the speed of the crankshaft through the crankshaft position sensor.