1999 Mustang 3.8L V6 – Rough Idle/Power Loss

This explanation might be a little long winded, and I apologize for that, just want to be as thorough as I can. I also want to preface this by saying I’m not really much of a mechanic; I know enough of the basics to get by with (usually), but that’s about it, so please bare with me if I ask any stupid or “obvious” questions; I promise I’m trying.

The issue I’m having started about a week ago. I went out to run errands (total drive time of about 30 minutes), and right as I was parking at my last stop, the car began to idle very rough and sounded like it was going to stall (even though it never did). No check engine light. After a minute of listening to it, I shut the car off and went inside to complete my errand. About 30 minutes later, I start the car back up and the idle is fine again; no issues driving back home (<5 minutes away).

The next day, I go out for more errands (again, about 30 minutes of drive time) and the rough idle returns after idling in park for ~10 mins; the check engine light also comes on. The car does not want to rev passed ~2,000 RPM and would barely reach 25 MPH while nearly flooring the pedal (thankfully my next destination was close by). Again, after I shut off the car and wait a little over 30 minutes, I start the car and the idle is normal (check engine light still on); but it only lasts for about 5 minutes before the symptoms return. I have the car towed back to my house.

With limited diagnostic skills and several google searches, I decide to replace the throttle position sensor (I also clean the throttle body with carb cleaner); this does not fix the problem. I purchased a cheap OBD2 reader and pull codes; 1 code is present, P0353. After researching this, I start looking at diagrams to find cylinder 3 and I notice my plug wire routing doesn’t match the diagrams I’m seeing. I thought this might have something to do with it, so I remove the plug wires from the coil pack and plug them in according to the diagram. I clear the codes, pull the negative cable from the battery to reset the computer (not sure if this was necessary?), and start the car.

No more P0353! But the rough idle is still present (arguably worse), and now there is a fuel smell and a kind of “putt-putt” sound at the exhaust (only 1 “putt” at a time, about 10 seconds or so between “putts”). But there are no codes being thrown? The symptoms sound like a misfire, so I go back to double check my wiring with another diagram; I now discover the coil pack is actually installed upside down (wire harness connector is on the bottom, where all of the pictures I’ve been seeing have the connector on the top). I curse my lack of observation and re-do the plug wire layout on the coil pack in “upside down” configuration (hope that makes sense).

All symptoms remain. I purchased a new coil pack and a set of plugs and wires. I replaced the coil pack last night and the symptoms remain; still no check engine light or codes. I am going to try to replace the plugs and wires today after work, but I wanted to get this out here now in case anyone has any ideas or suggestions or tips that I’m missing. If I left out any needed info please let me know.

Again, I apologize for the small novel, but hopefully this presents a clear picture of what’s happening and the events leading up to the current state. I’m happy to answer any questions, take pictures/video, or perform any diagnostic task (assuming I can find a tutorial for it; the OBD2 reader I have does have an option for “Live Data”, but I’m not sure if any of that would be relevant here). Thanks in advance for any help or advice!