Engine Random Misfire at idle.

[Gulftown980x]

I have a 2002 Mustang GT, I've owned it for about a year and a half and since i bought it it has has a idle misfire with no engine code. I cannot tell if its a specific cylinder but I believe it is a random misfire. I have a list of data I've collected to the best of my ability and things I've tried to resolve this issue.
I'm new to these cars and Looking through forums hasn't helped and most people have told me "they just do that" or they have a similar issue that was never resolved. I am yet to take it to a shop for a full diagnostic, but it is on my list to do if I cannot figure it out on my own

Here is what I got so far, unsure if it will help any but i figured the more data I can give the better, other than just me saying "misses, idk why"

Vehicle
2002 Mustang GT 4.6L 2V
160k+ miles
Stock engine internally
SR Performance CAI
Custom 2.5" X-pipe
Original exhaust before modifications also had the issue
Original upstream O2 sensors (B1S1/B2S1)

Main complaint
Random idle misfire W
Has done it since I bought the car
No check engine light
No stored codes
Seems mostly at idle
Improves when throttle is opened

When returning to idle:
Sometimes pops
Then settles back down
Previous repairs/tests
Replaced coils with NGK coils → no change
Replaced IAC -> no change
New IAC caused ~3000 RPM startup flare
MAF replaced previously → no change
MAF unplug test -> still misses

Spark plugs removed:
All looked clean (maybe too clean I'm unsure)
No obvious abnormal cylinder

Fuel trims

Warm engine:

Idle:
LTFT Bank 1: +25%
LTFT Bank 2: +25%
Cruise:
LTFT around:
B1: +18%
B2: +21%
2500 RPM neutral:
B1 LTFT: +15.3%
B2 LTFT: +21.6%
Light acceleration:
B1 LTFT dropped to about +4%
B2 LTFT stayed around +24–25%

Short-term trims

At idle with A/C on:

SHRTFT B2S1:
Around 0.0, +1.6, +3, +4.7
Occasionally -0.8 / -1.6
B1 tended to go negative more often
O₂ sensor readings

Upstream sensors:

B1S1:
Normally around 0.250–0.820V
During miss saw it drop to about 0.010V
B2S1:
Normally around 0.150–0.750V
During miss drops low and moves the most

Both sensors:

Periodically dip very low during the miss


MAF
Warm idle:
3.8–5.2 g/s

Engine temperature
208°F while warmed up

Fuel pressure PID

From scan tool (not mechanical gauge):
Around 60 psi idle with A/C on
Goes to about 63 psi when revved
Not confirmed with physical gauge

History
Previous owner drove it with a lean code for a long time
Unknown code (old phone lost)
Previously had a collapsed passenger side PCV hose that caused a lean code and was replaced

Exhaust/fuel related notes
X-pipe crossover makes it hard to identify which bank sounds worse
CATS still installed currently
Considering replacing upstream O2 sensors because they are original
Things ruled down somewhat
Not a simple MAF unplug fix
Not obviously one bad plug
Not obviously one dead coil
No obvious visible vacuum line damage (tested with carb clean aswell)
Battery and Alternator voltage are consistent
IAC Valve
PCV valve


Things still suspected
Vacuum leak (especially PCV tube/elbow)
EGR leaking/sticking
EVAP purge valve
Upstream O2 sensor accuracy (especially B2S1)
Misfire causing false lean O2 readings
Exhaust leak before upstream O2's
Injector/fuel issue

I appreciate any and all input, I'm honestly at a loss and I think I am overlooking something obvious. As far as I am aware the car has never been tuned and still runs standard fuel.

A short Clip I have, its not as noticeable in the video but in person its rather loud.


View: https://youtube.com/shorts/lb7mjACh9kk?feature=share

 

[Noobz347]

I [hear] the popping but I also hear the engine and it sounds steady and stable to me.

Does anybody else hear something I'm not hearing? My sound system for the PC is theater quality.

@Gulftown980x
What does it sound like to you from the engine bay (versus the exhaust)?

 

[Gulftown980x]

I [hear] the popping but I also hear the engine and it sounds steady and stable to me.

Does anybody else hear something I'm not hearing? My sound system for the PC is theater quality.

@Gulftown980x
What does it sound like to you from the engine bay (versus the exhaust)?

I think it is just my phone audio quality sadly, with headphones I can make it out but in person its a very distinct PUFF that happens randomly. Inside the car I can hear and feel it vibrate the car slightly. sometimes the RPM will dip slightly if it misses a couple times close together or is a particularly bad one usually on cold starts its the worst. From the outside its pretty loud and from the side of the car where you cant hear the idle you can very clearly hear it missing. On another note if i start my car without the AC on to let it warm up i can hear the misfire in my living room more than the cars idle. In the engine bay I don't notice much other than the motor shaking slightly when it misses and still being able to hear it from the exhaust pipes.

I can try to take better videos but that's best I have right now, it just stumbles a lot constantly, misses less with the AC on but still does it,

 

[Noobz347]

I also see that you replaced the coils but not the wires. Is that correct? Have you idled with the hood open when it's pitch dark to see if there's any arching around the wires or coils?

 

[Gulftown980x]

I also see that you replaced the coils but not the wires. Is that correct? Have you idled with the hood open when it's pitch dark to see if there's any arching around the wires or coils?

Correct, new coils but same harness and connectors. I had checked them the night I took the video and even unplugged and reseated all the coil pack connectors. One of the reasons I was out that night actually. I've even reseated all of the coil packs themselves in case they weren't making good contact with spark plug. I'll check it again tonight though in case I somehow missed that though. I also plan to pull my EGR valve to see if its sealing correctly and possibly clean any carbon deposits. its the last thing I wanna check for tonight. Thanks for the idea to check for that specifically, I don't believe it is but i could've 100% have missed it if since i wasn't looking for that specifically.

 

[Noobz347]

In the meantime, it might also be worth looking on Amazon for an El-Cheapo smoke machine.

A few couplers and some tape will let you test in the intake tract as well as exhaust.

Look carefully tonight for any arching. Look each wire over (they can leak) and look each coil pack over, they can leak too.

A trouble light can help with this when it's really dark. Lightly touch everything once it's fully grounded.

Did you [ever] have O2 sensor codes? I kinda got the feeling from your post that you might have seen some before.

 

[Gulftown980x]

In the meantime, it might also be worth looking on Amazon for an El-Cheapo smoke machine.

A few couplers and some tape will let you test in the intake tract as well as exhaust.

Look carefully tonight for any arching. Look each wire over (they can leak) and look each coil pack over, they can leak too.

A trouble light can help with this when it's really dark. Lightly touch everything once it's fully grounded.

Did you [ever] have O2 sensor codes? I kinda got the feeling from your post that you might have seen some before.

Ill do some research, its worth buying to add to my toolkit anyways. As for o2 codes, nothing, this car hasn't thrown a code since the day i bought it, it just ran weird but not bad enough to throw a code. Only ever threw a lean code when I bought it because a PCV hose collapsed and that was just super simple. Just has the odd idle and would stumble a bit when starting sometimes. Nothing bad enough for me to be extremely concerned but I want to fix it before doing fun stuff with the car like 3.73 gears. Maintenance before fun. I also have a set of nice mufflers and an O/R X pipe waiting for when I solve this problem.

 

[Noobz347]

Yeah, I'd go the smoke machine route. There may be some crack or something somewhere if you can't find an ignition problem somewhere.

 

[squeak93]

Sounds like you're looking at all the normal culprits. I can't hear anything on my phone, will try to listen to desktop speakers tomorrow if I remember for some better audio.

For the IAC - try buying a ford/motorcraft one if its not. IF you didn't do a hard idle relearn -discconnect battery for 30+ min to kill KAM. Then start, don't hit throttle or anything, let idle for at least 10 min to find base, then introduct radio, lights, electrical draw. Idle another 5-7, then keeping all that one introduce ac. That will help with the flaring. It will still high idle a few drive cycles but then should settle to 1200/1000, then 775ish as you come to a stop depending on an auto or manual.

For the misfire, any loss of mpg, have you checked/swapped/cleaned injectors? Stock coils (new ones) would be better but there's a good debate on that depending on who you are. Of course finish your vac line checks and make sure all that is functional etc.