So, I'm just posting this because I find it quite interesting. I have noticed that I can drive days with zero misfires, If my Air Conditioning is on. When I turn it off, I will have one to as many as 11 in the same 14 mile distance. Now, for the important information.
Plugs, cops, boots, springs have all been replaced. Phasers, chains, tensioners, guides, oil pump have also all been replaced. The misfires are completely random. This morning I had 3, one at cylinder 8, one at 7 and one at 5.
With the weather here in Utah becoming more fall like, it is not going to be warm enough to run the AC all the time. 5.4, 156,000 Miles.

 

Run your A/C with the temperature run up to 80 or 85 in the winter. Mixer box should blend enough warm air to keep you comfee. LOL.

Seriously, what is the Idle RPM under both conditions? And are they occurring at or near startup or during the drive cycle?

 

This is weird I've had a similar experience with my 06 truck I have Torque Pro running all the time at random I can have up to 4 misfires on a 50 mile run stop and go traffic all different cylinders but most of the time its 0. For the heck of it I have replaced all the fuel injectors (motorcraft) all the COP (motorcraft) the spark plugs are the sp546 and they only have 15,000 miles on them. The truck runs strong no hesitation or shakes its smooth at idle but I did notice recently that the motor leans to the left a little and looking underneath I can see that the motor mount has failed the fluid from it leaked out and it left a trail seems like its been like that for a while. I have heard that a very hard bump can trigger a misfire on this engine since the sensors are very sensitive to being out of sync.

 

Normally with the AC on it is around 550 to 600. AC off sometimes it dips to 480's. I have also replaced my TPS. The only other thing I can think is the Throttle body.

Sometimes I get them at Startup, and when that happens I normally have 5 or more in the drive.

 

Hey @Jojodapyro your idle RPM appears a little lower than mine. I don't think I ever see mine dip that low, and normally will run around 590 to 610 with A/C on and in gear. I would think very low idle RPM would make one much more susceptible to misfires. I have often wondered about some of the variation I see in posts.

Being the curious 'ole fool' that I am, I've been pooking around. I've never seen any discussions about adjusting Idle speed. However I notice four OBDII parameters indicating Trim values and one for idle air flow trim and IACMODE. I don't know if they are 'adjusted' by the PCM or if they are HARD CODED constants. (If so, maybe ForScan could adjust - if we figured them out). Mine have values - some small negative binary numbers and some small positive numbers). If you want to GOOF with it here they are. I may try to put together a Torque Pro dashboard showing them.

Here is a snipit from my database if you are interested.

..PID.....................Bytes...Acronym......... .......Description................................ .............Units
0976 ...................... 2 .... IACTRIM0 ......... IAC airflow trim 0 in KAM .................... LB/MIN
0977...................... 2 ..... IACTRIM1 ......... IAC airflow trim 1 in KAM ................... ... LB/MIN
0978...................... 2 ..... IACTRIM2 .......... IAC airflow trim 2 in KAM ........................LB/MIN
0979...................... 2 ..... IACTRIM3 ........... IAC airflow trim 3 in KAM ...................... LB/MIN
097A...................... 2 ..... IACSTRIM .......... IAC Short Term airflow trim ................... LB/MIN
097B...................... 1 ..... IACMODE ........... Idle Speed Control Operating Mode....... INT