Low RPM stumble/miss under load

ForcFed93 · May 29, 2017

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.

g36 monkey · May 29, 2017

First I was going to tell you to replace the O2 sensors but I see you've done that. What state are you in. If you don't have emissions, you can pull the midpipe and knock the cats out. Slight performance gain, slight increase in sound, and you would know for sure they aren't clogged up. When cats are clogged it is not necessarily more apparent at higher RPM.

ForcFed93 · May 29, 2017

I went ahead and ordered a Bassani ORX. I wanted one eventually and have been looking so I might as well do it now. Along with MIL eliminators.

I may just go ahead and do new coil packs too, but I hate to throw money on something that doesn't give me any benefit if they're not the problem.

g36 monkey · May 29, 2017

I absolutely understand that.

ForcFed93 · May 31, 2017

I have a set of used MSD coils on the way. Got them cheap enough to try and not lose sleep over. And the ORX will be here tomorrow.

ForcFed93 · Jun 5, 2017

Swapped out coils with a used set of MSD coils, if anything it got worse. Swapped back to stock. Does anyone know much about datalogging and STFT's and LTFT's?

I did some datalogging on the car tonight.

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

Fuel pump controller duty cycle was listed at 100% the whole time. Not sure what to make of that, if it means anything at all.

Timing advance was in the 14-18 range under cruise, up to around 30 or so under hard acceleration.

07GtS197 · Jun 6, 2017

Im surprised its not throwing rich codes as your fuel trims are so messed up. Were these numbers taken when it was warmed up and what conditions? (Cruise, wot, acceleration, etc). I dont think you can datalog fpdc because you have a return style fuel system but I could be wrong. Have you pulled the vac line off the fuel pressure regulator to smell for gas?

ForcFed93 · Jun 6, 2017

07GtS197 said:
Im surprised its not throwing rich codes as your fuel trims are so messed up. Were these numbers taken when it was warmed up and what conditions? (Cruise, wot, acceleration, etc). I dont think you can datalog fpdc because you have a return style fuel system but I could be wrong. Have you pulled the vac line off the fuel pressure regulator to smell for gas?

Most of the time the STFT's were between -5 and +5, standard driving/cruising and idle. WOT is where it spiked +30.

Most of the time LTFT's were also between -5 and +5. I did take one picture while idling.

I changed out the iridium plugs back to the factory platinums (with 68k miles) and now it's worse. Cruising, part throttle, WOT and idle all have a stumble.

I have not pulled the FPR vacuum line. Good call, I'll try that tonight. I have a fuel pressure tester, but I don't have the adapter to fit to the smaller schrader valve. I think between trying coils, plugs and having new FRPP wires with no visible arcing, I'm pretty close to ruling out spark. I'm still curious about the MAF though. I unplugged it last night and it died, then I started it back up and still had the miss at idle so I didn't even bother trying to drive with it unplugged to see the difference. I'm major stumped.

Also, no codes, anywhere, not even for the ORX. Except the one P1131 I got a long time ago.

Here are the plugs I pulled last night, in order from left to right is 1-8. In case you all see something I don't.

ForcFed93 · Jun 6, 2017

07GtS197 said:
Im surprised its not throwing rich codes as your fuel trims are so messed up.

Also, I was told on another forum that when it's high positive, it's trying to add more fuel because it detects a lean condition. You're basically saying the opposite. Which is true?

I'm leaning now towards a fuel pump since the issue seems to be getting worse and with the car mostly stock, if it's going lean and can't add enough fuel to correct it, it's gotta be fuel supply.

07GtS197 · Jun 6, 2017

Oh ok I misread your fuel trims. Yes if they are positive then its running lean and trying to compensate. As far as unplugging the maf and it dying, that shouldnt be the case. It should still be able to calculate airflow via o2 sensor feedback and throttle position. Ive unplugged the maf on both my 02 gt and 98 crown vic and neither have made it stall. Itll run but quick changes in throttle position will cause it to stall or come close to it. I think there is more going on, perhaps even the pcm. I would explore other avenues before going that far though. Maybe check the crank and cam position sensors first. Also check manifold vacuum and get that adaptor for the fuel pressure tester.

ForcFed93 · Jun 6, 2017

07GtS197 said:
Oh ok I misread your fuel trims. Yes if they are positive then its running lean and trying to compensate. As far as unplugging the maf and it dying, that shouldnt be the case. It should still be able to calculate airflow via o2 sensor feedback and throttle position. Ive unplugged the maf on both my 02 gt and 98 crown vic and neither have made it stall. Itll run but quick changes in throttle position will cause it to stall or come close to it. I think there is more going on, perhaps even the pcm. I would explore other avenues before going that far though. Maybe check the crank and cam position sensors first. Also check manifold vacuum and get that adaptor for the fuel pressure tester.

I think it's a stretch to think it's the PCM considering the tuner I'm using will read the PCM just fine and datalog all of the parameters. I'm not familiar with the cam and crank sensors, but from what I've read, those aren't likely culprits. I'm intrigued by the MAF though. The symptoms seem to fit the MAF, but they don't usually go bad, just get dirty and mine has been cleaned with the CRC MAF specific cleaner multiple times with no change. Also, I'll be doing my best to find an adapter for the pressure tester or perhaps see if I can rent one from the parts store if I can't find one.

ForcFed93 · Jun 6, 2017

Update. So I switched out the FRPP wires for the original 1997 wires and she runs like a champ. I'd like to say this issue is resolved, but I really don't want to jinx it. So I'm tentatively hopeful.

Also, I hope Jegs will give me a refund on those since I've only had them for a few months.

g36 monkey · Jun 6, 2017

Hopefully!

DKblue98GT · Jun 7, 2017

ForcFed93 said:
Update. So I switched out the FRPP wires for the original 1997 wires and she runs like a champ. I'd like to say this issue is resolved, but I really don't want to jinx it. So I'm tentatively hopeful.

Also, I hope Jegs will give me a refund on those since I've only had them for a few months.

I wondered if that was going to be the case. I recently installed a new set of Ford racing wires and developed a slight stumble at low rpm. Unfortunately I threw the old wires away. I found a used set and will change them out this weekend.

Sucks when a brand new part doesn't work!

SnakeBit! · Jun 7, 2017

was going to chime in on the wires. I have had that issue with my car twice and a friend's once...both with seemingly good wires and wires that were not that old. no arcing is seen or heard, I guess they internally are breaking down. last set on mine that went bad were Ford racing and they are only a few years old and looked brand new. it was the last thing I tried as well as I had discounted the possibility.

ForcFed93 · Jun 7, 2017

Jegs is issuing a refund. Sounds like FRPP which I had read were highly regarded, must not be anymore. Anyone have any suggestions? I mean, I don't mind running mine until they deteriorate, but they are 20 years old now.

OEM Motorcraft 8mm?

SnakeBit! · Jun 7, 2017

I actually had a set of Wagner wires I got off e-bay on the car for several years with no issues of any kind. I replaced them only to have the blue Ford racing wires to make the engine bay look nicer. my friends 96, same thing..his wires went bad, he got the Wagners and they were fine. he replaced Wagners with red Taylor 10mm that he still has on his car for now. when my blue ones went, I used his Wagners he had kept and car ran like it was supposed to again. should have kept mine. wound up with some off brand blue ones for now. my friend's car is going to the coil on plug conversion so he will have two sets that I will probably buy off of him to keep for my car.

DKblue98GT · Jun 8, 2017

I had Ford Racing wires on my GT and they were fine but apparently not these. I also cleaned the MAF tried new plugs and swapped the MSD coils back to stock. Not sure which wires to go with now. I am thinking Motorcraft but am open to other options.

ForcFed93 · Jun 8, 2017

DKblue98GT said:
I had Ford Racing wires on my GT and they were fine but apparently not these. I also cleaned the MAF tried new plugs and swapped the MSD coils back to stock. Not sure which wires to go with now. I am thinking Motorcraft but am open to other options.

I'm tempted to buy a set of MSD. FRPP has no warranty, so I'm lucky Jegs is going to refund. MSD has a 1 year warranty. Plus, I also have a set of MSD coils now sitting on a shelf. Should I use them? They made the misfire with the FRPP wires worse, but I'm assuming the additional spark might have made it arc easier.