I'm new to the diagnostics world. I have been trying to diag two different vehicles. Both are higher mileage with no DTC's. One in particular has a rough idle.
While looking at the engine datastream for both, I am seeing around 100 "cycles of misfire data" on my scan tool as a data point. When I look at the stream for missfire data on each individual cylinder, the scanner shows "0".
I think the data pulled for the "cycles of misfire data" comes from the crank position sensor. But how are individual cylinders monitored by the scanner? Same sensor? If so why is a misfire showing generally but not individual cylinders?
Engine management computers employ different/more than one routines when watching out for misfires. They may distinguish between cat damaging misfires and no damaging ones, there may be weighted monitors averaging x number of engine cycles, there may be a different submonitor which does real time data sampling based on a different number of engine cycles and so on.
The data pid "cycles of misfires" indicates the actual number of completed engine cycles within the applicable algorithm or surveillance strategy. Look at it like a live counter showing the running progress of the routine.
THIS DATA PID IS NOT TO BE TAKEN AS MISFIRE COUNTER, IT DOES NOT SHOW MISFIRES.
The misfire data for individual cylinder actually DOES SHOW misfire events as perceived by the pcm. The (still) most common way to calculate and relate these events to individual cylinders is to look at the crank speed deviations and crank position through the eyes of the crank sensor.
When the pcm detects a misfire but is not able to attribute the event to a specific cylinder then a"unspecific" code will be flagged, the famous P0300.
Newer systems do apply much more sofisticated strategies....
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The following user(s) said Thank You: Tyler, rpd14893
Juergen is right on. For context, what are the year/make/model of the vehicles you're working with?
I usually find "Cycles of Misfire Data" to be a GM thing. If the 'cycles' PID is counting, then I generally figure the misfire monitor is active and paying attention. If the 'cycles' PID isn't moving at all, then I generally figure the misfire monitor has been suspended for --reasons--.
Tyler the vehicle with the rough idle issue is a 2004 Jeep Grand Cherokee Laredo. It has some significant negative fuel trim numbers idle that go away during normal driving. This got me looking at the cycles of misfire numbers but also found an injector issue.
Was also working on a 09 Chevy Impala and noticed the same cycle of misfire numbers when looking at something unrelated and got me thinking if I was chasing a ghost on the first vehicle. So yes a GM vehicle was involved
For context, what are the year/make/model of the vehicles you're working with?