OK - Misfires?

Hi,

Not sure how to describe what I'm currently encountering, but it is about misfires. So, I'll just shoot.

On two cars I'm working on, which aren't reporting any misfire DTCs or any sort of DTCs, I notice low count misfires at idle.

For example, on a 2013 GT500 (3500 miles), when I snap throttle it, it will usually increment the Mode 6 current misfire count for current run on one cylinder at a time. In addition, I hear a slight backfire. There are no DTCs, the fuel trims are great, and the car drives like a Cobra. But, the mode 6 "history" and "last drive cycle" do NOT show any misfires, even when I just ran the test in a previous run. So, it is like I'm receiving misfires, but the PCM thinks it is ok. One last thing, if I just let the car idle by itself, there are no misfires whatsoever.

Another car, a GM 1500 (200K miles) clearly has an audible valve tick is periodically receiving a low number of misfires at idle and snap throttles. Yet, there is no misfire DTC. I used a vacuum gauge, expecting it to bounce like crazy at idle (due to the valve tick), but it didn't! In addition, both long term and short term fuel trims are below 10% for each bank and usually around 0 throughout the RPM range. This one has me scratching my head. But this isn't what I'm trying to ask.

Here is what I'm asking. Is it 'normal' for some cars to have periodic misfires at idle or snap throttles and be ok? Or is it always an indication of an issue if mode 6 or misfire data shows a misfire?

Any input is appreciated on this.

Thanks

Misfire PID's are never 100% reliable, due to the way misfires are detected by measuring tiny variations in crankshaft speed and the fact that no two combustion events are identical make it inevitable that the numbers will never be 100% reliable. The misfire counters work by counting the number of misfires in a given time frame after which they zero and the count re-starts.
Only if if the number of misfires exceed a pre-set limit in that time frame is a fault code set.
Another thing to be aware about with misfire data is it can very often flag the wrong cylinder.

Andy is spot on here.

I know in vehicles I have fooled around with in the past, I was able to get misfire counters to flag misfires, even when a misfire never occurred. The engine computer definitely has a threshold it's looking to in it's monitoring, and uses this to account for subtle miscalculations like these. Accessory engagement, transmission shifting, harsh road conditions, and all sorts of things can slow crankshaft rotation in a manner that can be easily perceived as a misfire by the engine computer, and that is why these maximum allowable thresholds are used.

Yeah, the misfire counts can definitely be deceiving. :lol: Like Andy and SniperDan said, it's very possible to see miss counts on normal running engines.

This is a shot of some Mode $06 data I took off an '07 F-150. This cylinder wasn't misfiring, but had low counts anyway.