Scanner Misfire PIDs question

I am working on a TVS S197 Mustang GT/Tremec with LTs. Car runs great except for one issue. On decel, the car will occasionally overshoot idle. Idle is set for 800 RPM and only occasionally it will drop to 400-500 RPM and then recover. Although I cannot hear a misfire, I logged the car with two misfire PIDS; Total misfires since key-on and Misfire Detected (no/yes). When the car does overshoot the idle, I see an increase in Total Misfires (at idle only, not at mid throttle or WOT), but what has me mistified is the Misfire Detected PID always returns NO. Never does the Misfire Detected PID return YES. I am wondering if anyone can provide some clarity on these PIDs and suggestions on what to search for in the tune. Car has new plugs gapped at .025 and fresh COPs. MAF transfer is great. No significant IPC errors either.

Anyone have any idea why one PID indicates NO Misfire, but the other PID shows Misfires? I am trying to finish this tune and this is the final step, to keep the car from overshooting the set idle RPM. The car increments the misfire count at the same time it overshoots idle. But at the same time, the Misfire YES/NO PID returns NO Misfire. Strange.

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Originally Posted by 15PSI

I am working on a TVS S197 Mustang GT/Tremec with LTs. Car runs great except for one issue. On decel, the car will occasionally overshoot idle. Idle is set for 800 RPM and only occasionally it will drop to 400-500 RPM and then recover. Although I cannot hear a misfire, I logged the car with two misfire PIDS; Total misfires since key-on and Misfire Detected (no/yes). When the car does overshoot the idle, I see an increase in Total Misfires (at idle only, not at mid throttle or WOT), but what has me mistified is the Misfire Detected PID always returns NO. Never does the Misfire Detected PID return YES. I am wondering if anyone can provide some clarity on these PIDs and suggestions on what to search for in the tune. Car has new plugs gapped at .025 and fresh COPs. MAF transfer is great. No significant IPC errors either.

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I also have misfire issues (P0300) on a S550 TVS, but under load and only in 5th gear between 4500 and 4800 rpm (also do not feel it). Tested all Hardware components, still no clue why and any help is appreciated.

During my investigation how ever I have also used the PID`s in the scanner. To my observation the Misfire Yes/No is set to yes only after a certain Misfire count in a certain time range, or if a P0300 or P030X is set of then the PID commits YES.

I never set a code. So I assume based on what you said, that the YES/NO PID will not set to 'YES' until a certain error quantity threshold (or errors per time value) is exceeded in the scanner. I would like Eric or one of the coders to verify that and possibly reveal what number the misfires must reach before the other PID returns YES. Either way, I can only assume at this time that it is either a timing or fuel issue that causes the overshoot and the incremental increased count on the Misfire PID. I am pretty sure it is not the BAP as it has plenty of fuel at 12 PSI with the ID1000s and shows no misfires at WOT or at part throttle, only at idle occassionally. I will keep looking.

So... After searching through the tune, I decided to look at basic mechanical issues to discover why the car was showing misfires occasionally at idle when the idle overshot the intended RPM. I idled the car from very cold to running temps while watching the scanner. There were no misfires or bad behavior when the car was below operating temps. Once the car completely warmed up, at a certain point, the idle lowered below the intended RPM and the misfire count increased. Then I discovered that it was dropping below the intended idle RPM when the radiator cooling fan powered up. Apparently with this car, there is quite a load on the charging system with the 2 11" Spal heat exchanger fans running constantly. Once the radiator cooling fan was enabled, on top of the load the Spals were placing on the charging system, the idle dropped, misfires incremented and then it would slowly recover. I raised the output voltage of the alternator in the tune and the problem disappeared. I wonder if anyone else has seen this issue?

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Originally Posted by 15PSI

I never set a code. So I assume based on what you said, that the YES/NO PID will not set to 'YES' until a certain error quantity threshold (or errors per time value) is exceeded in the scanner. I would like Eric or one of the coders to verify that and possibly reveal what number the misfires must reach before the other PID returns YES. Either way, I can only assume at this time that it is either a timing or fuel issue that causes the overshoot and the incremental increased count on the Misfire PID. I am pretty sure it is not the BAP as it has plenty of fuel at 12 PSI with the ID1000s and shows no misfires at WOT or at part throttle, only at idle occassionally. I will keep looking.

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I'm pretty curious about this myself.

I have never thrown a misfire code, but definitely accumulate misfires in that PID.

There is a table that seems to define a border where it doesn't care about them on one side. It's a little tricky to tease out where they are occuring in a chart, because the PID is a cumulative total. So, I recreated the [ECM] 39205 - Misfire Negative Torque Disable Line table, populated is with Total Misfires Since Key On, displayed cell count, and filtered like this:

[3136.slope(-50)]<-0 OR [3136.slope(50)]>0

This only gives me cell hits when the count is changing.

It definitely shows that most of my misfires are near this border where the PCM would be ignoring them, so I feel a little better about that.

On this particular log, I accumulated 40 misfires in a 30 min mixed driving log.

Attachment 94565

Misfires will definitely occur with low battery voltage. I was dynoing mine the other day, large IC pump running constantly, non stop, battery voltage was constantly dropping, (I didn't notice it until it started breaking blower belts) I charged battery back up, then no more misfires, and belts quit breaking. I wonder if there's any power in more voltage to the coils, and a larger plug gap? Like maybe BAP the coils.