Hey folks. Owner of a 3.8l 2001 mustang here.
A bit of background... I hate for this to be my first post but I've lurked many a mustang forum and have yet to find any concrete help regarding misfires. It seems that everywhere a few get started each week and noone ever fixes it, or if they do, they don't elaborate.

My problems started a year ago (I know, a year of misfiring? How do I even still have an engine?) and has continued off and on after numerous attempts at fixing them.

To start, two things that MAY have caused them. First off, I worked at firestone for about two years. A buddy of mine changed the fuel pump and filter and everything was fine and dandy. Only issue was, he took the plastic jacket off of the filler neck and poked a hole in it, probably by accident. Went on for about 6 months, no issues. Then I lost my keys... And here comes the second possible thing, which is really just a combination. My stang sat un-used for roughly 3 months, with a hole in the filler neck.

After starting it, no problems. That summer the misfires started. It was just an occasional hesitation/stutter during low acceleration. Replaced plugs. Fine for a bit, then came back. Replaced wires. No effect. Replaced coil... Ran great for a while, then started to come back, seemingly after an oil change/degreasing of the engine. Since then, it's gone from slightly noticeable to barely being able to pull out in traffic due to my abysmal acceleration. Anything under 2k rpm was rough. At this point, I patched the hole in the filler neck, ran seafoam through my tank, and tried a couple of other "cleaning" methods found on other people's misfire threads. Cylinder two was popping codes the entire time, although it started with a different one BEFORE changing coil/plugs/wires. It was driveable, but acceleration remained an issue for quite some time.

Fast forward to two days ago. Left my lights on, completely draining the battery (by accident :P) and driving home was terrible. After getting jumped, I figured it was just struggling to keep up with the power demand, since it felt a little more bogged down than usual. It started idling VERY rough at stoplights, and then it was running rough at anything under 2krpm. Acceleration even at high rpm's was almost non-existant. Got home, and opened the hood. Saw smoke rising slowly from under the car, it seemed. Smelled like burnt plastic. I shut the hood and went inside to research. During this whole time I didn't see the engine light come back on. (had been on for the past year or so)

I found nothing of help, so I decided to do two things. First, I changed the plug in cylinder 2. I really examine it until I got home after work, but it looks like one sideof the insulator is brand new with a couple smudges, and the other has some signs of wear, although it does smell slightly of varnish. No oil, doesn't look wet (even when it first came out), and in general looks pretty good for a year of use.

After changing that plug, which didn't help, I went to auto-zone to get my codes checked. No misfire codes at all, just p0445 and p1401. The dpfe sensor voltage high code (1401) had been there for a while, as had the p0445. I bought a new dpfe sensor, and put it on later that night. Again, no help. Interestingly enough, I found what kinda smelled like gas in/on one of the tubes coming out of the old sensor. Makes sense if a cylinder isn't firing, I suppose. Anyway, all of this leads me to believe that there MUST be something wrong with my fuel system causing all these problems. I may wind up changing all the plugs again anyway... NGK's are cheap.

Anywho, on to my final and hopefully answerable question. Do either one of these codes (being p1401, dpfe sensor circuit higher than expected; and p0445 which reads as EVAP purge/cent solenoid circuit condition) potentially lead to misfires or other problems that wouldn't pop a code but still contribute to misfiring? If so, where should I start? I can pretty much do all of the maintenance myself, I am apparently just no good at diags.

 

I'd try to see what burnt up. Also my car runs shitty on any plugs other than copper plugs. If it's platinum my car is terrible.

 

Well after doing what I should've done years ago (repair manual) I now have a handful of tests to potentially see if the evap or egr codes are causing the issues. Something has to be wrong with the systems, since codes have been up for so long. I have used nothing but copper plugs. Once I find the issues with the other two systems I'll be completely changing each plug to ngk coppers again, just to make sure they aren't causing any other issues.

 

Ok. At this point it seems like I'm just making a journal of what I've done, but maybe itll help someone. Changed the EGR Valve, DPFE sensor, and fuel filter. I tested most of said components and they all tested fine, but figured they were cheap enough I'd throw em on.

After my latest bout of tests and replacing the fuel filter I got discouraged and tried disconnecting plug wires from the coil whilst running. Found out that two cylinders are just not firing for some reason. They have been the culprits for back-firing I'm sure, but tommorow I'm going to change the plugs and coil (under two years so coil should be free under warranty). I'd suspect the fuel system again but since the only real link between 3 and 4 is being shared on the coil, it's probably the culprit. I'll update as I find out more.

Edit: Now that I think about it, I doubt it'd be the fuel system just because if I'm getting back-fires, either something else is seriously wrong or it's those two cylinders just pumping un-ignited gas into the exhaust.