2003 Caravan 3.8 has me puzzled. Sorry for the long explanation below but it's needed since I've done a lot without a solution yet.
Ongoing problem with intermittent miss/misfire, engine cold or at operating temp.
Sometimes runs fine/ sometimes frequent misses. No misfires are ever recorded by PCM real time nor in misfire history
(via Matco Maximus 3.0 scan tool)
Recently got P0171 (lean condition) due to bad fuel pump (I heard the old whining pump's pitch go lower right before setting code). Replaced old pump (44 psi) and inline filter - new pump is 55psi, within spec (58 +/-5).
No DTC's ever since pump replacement 2 weeks ago but miss issue persist.
LT Fuel trim 12-14 at idle, lowers at off-idle RPM sometimes quickly, usually gradually.
Short term fuel trim is responsive and follows 02 sensor signal well.
Primary O2 sensor switching above/below 450mv well.
*According to scan tool data stream, IAC valve remains at 255 steps (replaced IAC valve/motor with same result)?
Idle speed is within spec ~750 RPM.
TPS sensor voltage follows actual position smoothly.
MAP, IAT, Coolant temp are accurate in-spec.
Cleaned injectors today, ran good at first then intermittent miss returned.(may be coincidental?)
New spark plugs today (.050 gaps checked) = no change.
No vacuum leaks can be found via carb cleaner sprayed around possible leak sites (intake manifold, injector o-rings, vacuum lines, etc.
No vacuum leak found via clamping off vacuum supply lines from intake manifold to accessories (brake booster, EGR, EVAP, etc.)
No vacuum leak found using smoke machine other than smoke coming out of intake duct at air cleaner.
Any suggestions very welcome!
My first post on this forum = Hi everybody!

 

Ever get anywhere with this? I have very similar problem.

 

One problem was the fuel filter had never been replaced. Previous owner replaced the fuel pump (with a cheap part) but did not replace the fuel filter mounted on top of the fuel tank. He also replaced the injectors with some cheap Rock Auto injectors. But the fuel filter internal medium was disintegrating due to age, which soon caused fouling of the new injectors. I replaced the fuel pump with a Denso #953-3037 - (to restore in-spec pressure), replaced fuel filter and replaced injectors with the correct Siemens 04861238AB Fuel Injectors. It ran fine after that other than less than optimal fuel mileage due to slow response time on the O2 sensor circuit (lazy sensor). A new Mopar 4686938AB Oxygen Sensor solved this problem.
Since then I got an intermittent miss with no codes in the ECM. This was fixed with a replacement coil pack. I don't know why it didn't set a code nor record any misfires with the Matco scan tool, but it hasn't missed a bit since the coil pack replacement.
The only remaining problem I have is that the IAC likes position 255 and will not keep the idle speed at 750rpm when I turn into driveways etc. During low speed / high effort conditions the idle speed will drop to ~500rpm. This has caused some shuddering in my power steering circuit which I can remedy by lightly applying my foot on the throttle. But I need to find out why the IAC is not responding to keep the RPM @ 750 in this situation.

 

Thanks for your response, motivated me to throw a coil pack at it, and it fixed my misfire as well. I also didn't have any codes. After hearing your reply I figured well its worth a shot and BAM! Misfire gone. Thanks a bunch.