Low RPM stumble/miss

[ForcFed93]

So I've been chasing a low RPM miss for a while and I'm stumped, so I'm turning to the experts.

1997 Cobra with 69k miles. All stock engine/exhaust with the exception of a Spectre air filter.

When I got the car several months ago, I did a bunch of maintenance. New PCV valve, new plugs (autolite XP104), new plug wires (FRPP), fuel filter, air filter. Ran great. Ran great before I did all of the maintenance too, but it was all factory original, so I figured it was due.

Recently, over the past few weeks, I've been getting a rough idle with an audible miss. It doesn't miss at idle when cold. Also, I get a low RPM stumble/miss under load. It will still stumble/miss when cold, though not as bad as when warm. Cruising is fine. It will stumble in gears 2-5 between 1500-3000RPM. Once the IMRC's open it screams clear to redline.

Since this has happened, I've pulled and regapped the plugs smaller (from the original .054 to .047) thinking it wasn't sparking. I've tried to isolate the miss by pulling plug wires one by one and I can't seem to pin down any specific cylinder. I've listened to the injectors to make sure they're clicking steady and they all are. I've cleaned the MAF, IAC, EGR valve, IAT sensor. I did the seafoam through the PCV and let it sit.

I did get a code initially when it started this which was a code for the passenger side o2 sensor. I replaced both upstream O2's with motorcraft sensors. That seemed to take it away temporarily (could've also been in my head), but it came back quickly.

No CEL since. It didn't seem to coincide with any of the aforementioned maintenance.

Also, it does seem to have a slight smell of a clogged cat and it does have a rattle at low RPM (like just off idle when backing it out). All of the cat heat shields seem to be tight.

I'm left thinking it might be fuel pump (though I would think fuel starvation would be more prominent at higher RPM than lower)
MAF (maybe the sensor itself is bad and cleaning won't help)
Clogged cats (I would also think this would be more prominent at higher RPM)

Thoughts everyone? I've done hours of research and it seems like the people with similar symptoms never respond back with a resolution.

 

[Shake&Bake]

Sounds like a bad coil pack to me.

 

[ForcFed93]

Sounds like a bad coil pack to me.

Any idea how to test them with a multimeter?

 

[ForcFed93]

Also, if I decide to upgrade coils, should I go MSD or screamin demon? Or is stock replacement best?

 

[ZeroDCX]

Also, if I decide to upgrade coils, should I go MSD or screamin demon? Or is stock replacement best?

Stock

 

[jgt58]

Coil packs

1996 mustang cobra... many mods...

 

[all snake]

Start it up have someone help & put it under a load at nite hood up in the dark see if you see something arcing. NEW parts can be bad rite off the shelf.
Good luck sound like plug wires ?

 

[ForcFed93]

Start it up have someone help & put it under a load at nite hood up in the dark see if you see something arcing. NEW parts can be bad rite off the shelf.
Good luck sound like plug wires ?

It does have a miss at idle and I've turned the lights off to look for arcing. Nothing. Unless it's down in the valve cover tubes, they aren't arcing.

 

[bumsoil]

i have been running screamin daemon coils for 4+ years with no problems. 10/10 would buy again.
Plus the livewires look really good.

 

[ForcFed93]

ORX will be here tonight, coils come in Monday. Hopefully one of those two will solve this issue.

 

[ZeroDCX]

Did you try checking/testing primary and secondary resistance of the coil packs? Have you scanned for stored ECM codes?

http://troubleshootmyvehicle.com/ford/4.6L-5.4L/how-to-test-the-coil-packs-1

 

[ForcFed93]

Did you try checking/testing primary and secondary resistance of the coil packs? Have you scanned for stored ECM codes?

http://troubleshootmyvehicle.com/ford/4.6L-5.4L/how-to-test-the-coil-packs-1

Negative and negative. Last time I checked for codes was before I replaced the o2 sensors and the only code was a P1131 and replacing the o2 sensor cleared it. I'm not going to bother checking the coil packs since I have a pair on the way. If that doesn't resolve it, then I'll do some more digging. Thanks for the link, I'll check back on it if needed.

 

[jgt58]

Negative and negative. Last time I checked for codes was before I replaced the o2 sensors and the only code was a P1131 and replacing the o2 sensor cleared it. I'm not going to bother checking the coil packs since I have a pair on the way. If that doesn't resolve it, then I'll do some more digging. Thanks for the link, I'll check back on it if needed.

Disconnect the battery for 10 min

1996 mustang cobra... many mods...

 

[ZeroDCX]

If its already been resolved and the CEL turned off, then there is no need to disconnect the battery.

I would recommend data-logging, this would be the best way to trace the source of your issue before throwing parts at it.

Logging STFTs (B1 + B2), LTFTs (B1 + B2), MAF (counts, lb/min, g/min or g/sec), O2 Voltage B1S1, B2S1 among others can tell you a lot about what's going on and possibly why its occurring or at least point you in the right direction.

 

[ForcFed93]

Disconnect the battery for 10 min

1996 mustang cobra... many mods...

I had it unhooked most of this morning. Oh, ORX did not clear it, so cats must not have been clogged to the point it affected it. But, it does run much better. And sounds good to boot.

If its already been resolved and the CEL turned off, then there is no need to disconnect the battery.

I would recommend data-logging, this would be the best way to trace the source of your issue before throwing parts at it.

Logging STFTs (B1 + B2), LTFTs (B1 + B2), MAF (counts, lb/min, g/min or g/sec), O2 Voltage B1S1, B2S1 among others can tell you a lot about what's going on and possibly why its occurring or at least point you in the right direction.

I'm going to do coil packs tomorrow. If that doesn't work, I'll look into data logging.

 

[ZeroDCX]

The code you're posted P1131 is due to a detected lean condition on bank 1 during closed loop operation. This is pre-catalytic converters, so installing an OR-X will have no effect on alleviating the code versus the stock H-Pipe. In all actuality, installing an OR-X w/o a tune or MIL eliminators would result in the following codes P0420 and P0430, nothing more. Signifying that the downstream O2 sensors are reading high values (rich), since the catalytic converters are not there to burn off post combustion fuel. Upstream O2 sensors are not effected either way stock H-Pipe w/ cats or catless OR-X.

Strictly regarding the posted code: The system either has a faulty MAF, vacuum leak, exhaust leak (pre mid pipe), fuel delivery (faulty injector) or faulty O2 sensor issue affecting bank 1. If you were to datalog this, you would see that system's STFT's on bank one relatively high (more than 0% and closer to 25% - meaning ECM is commanding more fuel as its detecting a lean condition on bank 1), it should try to correct the lean issue by bringing LTFT's higher in value (adding more fuel), to bring STFT's back to zero. Both STFT's and LTFT's should be "near" 0% in engine burning fuel stoichiometrically (14.7:1).

More than likely, I would be looking towards an exhaust leak. If the mid-pipe is not tightly secured, it could pull fresh air between the manifold and mid-pipe, bypassing the engine and read by the upstream O2 sensor. This would result in a lean condition detected by the ECM. Also make sure your O2 sensors are tightened properly and all connections are secured. A faulty MAF "should" read lean on both banks 1 & 2, but things can happen to prevent such an occurence. Under the right conditions a vacuum leak could present it self on only one bank or be worse on one bank versus the other.

If the coil packs were an issue, then there would be a rich condition from fuel failing to be ignited. The ECM "should" throw a misfire or rich code. You could also data log to verify this by watching STFTs/LTFTs. A rich condition would result in the ECM commanding negative fuel trim values (ECM pulling fuel) to compensate.

Either way, good luck and let us know how it goes.

 

[ZeroDCX]

^ Regarding the STFTs and LTFTs, this is strictly concerning a naturally aspirated car. Force induction cars are a little different.

 

[ForcFed93]

The code you're posted P1131 is due to a detected lean condition on bank 1 during closed loop operation. This is pre-catalytic converters, so installing an OR-X will have no effect on alleviating the code versus the stock H-Pipe. In all actuality, installing an OR-X w/o a tune or MIL eliminators would result in the following codes P0420 and P0430, nothing more. Signifying that the downstream O2 sensors are reading high values (rich), since the catalytic converters are not there to burn off post combustion fuel. Upstream O2 sensors are not effected either way stock H-Pipe w/ cats or catless OR-X.

All of my research pointed me towards a faulty O2 sensor. I replaced both upstream O2 sensors with OEM Motorcraft prior to installing the ORX. The CEL resolved,but the misfire problem persisted. I've been wanting an ORX as well and the car was giving off a heavy smell that reminded me of clogged cats. I didn't think it would solve it, but I figured, why not?

Strictly regarding the posted code: The system either has a faulty MAF, vacuum leak, exhaust leak (pre mid pipe), fuel delivery (faulty injector) or faulty O2 sensor issue affecting bank 1. If you were to datalog this, you would see that system's STFT's on bank one relatively high (more than 0% and closer to 25% - meaning ECM is commanding more fuel as its detecting a lean condition on bank 1), it should try to correct the lean issue by bringing LTFT's higher in value (adding more fuel), to bring STFT's back to zero. Both STFT's and LTFT's should be "near" 0% in engine burning fuel stoichiometrically (14.7:1).

More than likely, I would be looking towards an exhaust leak. If the mid-pipe is not tightly secured, it could pull fresh air between the manifold and mid-pipe, bypassing the engine and read by the upstream O2 sensor. This would result in a lean condition detected by the ECM. Also make sure your O2 sensors are tightened properly and all connections are secured. A faulty MAF "should" read lean on both banks 1 & 2, but things can happen to prevent such an occurence. Under the right conditions a vacuum leak could present it self on only one bank or be worse on one bank versus the other.

The problem has persisted with both the factory H-pipe that has been on there since the car was born and after I installed the ORX. I highly doubt it's an exhaust leak, especially considering that the P1131 resolved with a new O2 sensor. I've sprayed every vacuum line I could see to try to detect a leak with no luck. I'm not ruling it out as a possibility, but I haven't been able to find one.

If the coil packs were an issue, then there would be a rich condition from fuel failing to be ignited. The ECM "should" throw a misfire or rich code. You could also data log to verify this by watching STFTs/LTFTs. A rich condition would result in the ECM commanding negative fuel trim values (ECM pulling fuel) to compensate.

Either way, good luck and let us know how it goes.

I'm hoping it's the coil packs since I have a pair arriving today. I'll try to get them installed this evening to determine if they have an effect. It just feels like a spark issue to me. If the coil packs don't resolve it, I'll either pull the plugs and replace with cheapo autolite 764 coppers, or try to find a way to datalog it.

 

[ForcFed93]

I'm trying my best to get this log readable, but between diablosports software and Windows crap, it's not cooperating.

I borrowed an intune to datalog.

Most LTFT's were within -10 to +10 the whole time.

STFT's were usually between -10 and +10 but under full throttle I did notice up to about +33. They seemed pretty even bank to bank. And I didn't notice anything out of the ordinary, but I was alone so I couldn't really do much observing whole driving.

I did notice that fuel pump duty cycle thing was at 100% the whole time.

Thoughts?

 

[ForcFed93]

Ok, I've got the minimums and maximums for LTFT and STFT

LTFT Bank 1 min -10.94 max 3.125
LTFT Bank 2 min -10.16 max 5.469
STFT Bank 1 min -16.41 max 32.81
STFT Bank 2 min -14.84 max 32.81

And I did notice it was missing under WOT, just not as noticeable as part throttle.

I switched out to a used set of MSD coils and if anything, it made it worse.

I unplugged the MAF and it died. Then I started it unplugged and it ran but still had the miss at idle so I didn't attempt to drive it unplugged.

Also noteworthy, I haven't had a CEL through any of this. Not even from the ORX (with no tune or MIL eliminators).