Low RPM stumble/miss

Plan of action for this weekend is to get a set of copper plugs and try it without the iridiums.

Went ahead and swapped out plugs for the original plugs I still had. It's actually worse now. Cold has a miss, warm, idle, WOT, part throttle, all of it. Still no code.

I'm going to pull the FPR vacuum line and check for a fuel smell, as suggested by another forum.

Here's an image of the plugs I pulled last night. Probably less than 1k miles. See anything I don't? Left to right is 1-8.

ForcFed93 said:

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).

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Two things. You will not be able to drive with the MAF unplugged. The reason why is in the name of the sensor.

Just because you don't have a light does not mean the ECU hasn't logged codes. I would bet the one for the missing rear o2s are already there, without a light.

Don't bother with MIL elinimators, turn the rear ones off.

If this were me, I would continue to datalog and review all the basics (again). Replacing random parts is expensive and frustrating. ZeroDCX had good advice.

Do you plan on tuning the car yourself? If so, investing in a Quarterhorse and Binary Editor will allow you to diagnose now and tune later.

Thanks. I checked for codes. None are there. Perhaps the diablosport intune I'm using can't check for stored codes, but it doesn't show any codes.

I won't tune it myself. I'm pretty close to just biting the bullet and taking it in to Ford.

So, given what ZeroDCX has said about the fuel trims, it would appear to me that STFT's in the 30's under hard accel and mostly positive values even at idle and part throttle cruise (the negative values were mostly under coasting/decel conditions) means the car is trying to add fuel to correct a lean condition. The higher the values, the more fuel it's trying to give (and probably can't give enough).

I'm guessing it's a fuel pump that's going bad. Not to mention it appears to be getting progressively worse. I pulled the vacuum line at the FPR with not a trace of fuel smell at all.

Anyone else have any thoughts?

What you posted concerning min and max STFT/LTFT data isn't sufficient to make an informed diagnosis. You would need to evaluate a multitude of factors (PIDs) in real-time or via logs to determine what's going on.

Also, during WOT, the system is in open loop. During such, O2 sensors are not used to determine stoichiometric AFR. The PCM refers to preset tables. So you can ignore STFT/LTFT data during WOT. Also I would not recommend hard accelerations until your problem is resolved. Plus its easier to diagnose in closed loop w/ O2 sensor feedback.

With these cars, any time I've had a misfire, stumbling or hesitation issue its always been the wireset as the root cause. They can arc where you cannot visually see them, where the boot in inserted into the head. Have you attempted verifying that your wires are not arching. Pull them off one by one and look for anything abnormal. Take your time in doing so as the slightest hole can be easily over-looked, but lead to the very problem you're experiencing.

Another problem source would be the the fuel filter, but you're already replaced this.

Have you attempted cleaning your MAF?

Also you mentioned it was running fine before all the maintenance was performed. Did you put a Duralast wirelset on it? I've went through three "brand new" sets all with arching issues which presented as hesitation/misfiring before switching back to a FRPP set to resolve my issues. Would you happen to have the old wireset you could try?

You mentioned the fuel pump duty cycle was at 100%, I'm not positive we can log this on our cars. Maybe some else can chime in.

You can try a fuel system pressure test to see where you're at. You should be around ~32 psi with the FPR connected and ~42 psi with the FPR disconnected at idle, IIRC.

Regarding the possible leaking FPR, you would see this when datalogging. Abnormal STFT/LTFT values when it starts leaking and PCM tries to adjust for this. Also when it leaks O2B1S1 and/or O2B2S1 would read constantly high in voltage ( closer to 1.0V with 0.0V indicating lean and 1.0V indicating rich ). Normally O2B1S1 and O2B2S1 should be switching between 0.0V and 1.0V. This is also another way to determine if an O2 sensor is faulty.

For future reference, if you have a O2 sensor code reading lean. Look to these PIDs and see what they're doing. If they're not switching, then you've found a problem. You can also try to spray a shot of carb cleaner or something in the intake and watch those voltages, when you do they should rise closer to 1.0V. If the target sensor does not rise in voltage it is most likely faulty. Switching O2 sensors across bank 1 and datalogging while performing the above is also another effective tactic to verify your findings.

ZeroDCX said:

What you posted concerning min and max STFT/LTFT data isn't sufficient to make an informed diagnosis. You would need to evaluate a multitude of factors (PIDs) in real-time or via logs to determine what's going on.

Also, during WOT, the system is in open loop. During such, O2 sensors are not used to determine stoichiometric AFR. The PCM refers to preset tables. So you can ignore STFT/LTFT data during WOT. Also I would not recommend hard accelerations until your problem is resolved. Plus its easier to diagnose in closed loop w/ O2 sensor feedback.

With these cars, any time I've had a misfire, stumbling or hesitation issue its always been the wireset as the root cause. They can arc where you cannot visually see them, where the boot in inserted into the head. Have you attempted verifying that your wires are not arching. Pull them off one by one and look for anything abnormal. Take your time in doing so as the slightest hole can be easily over-looked, but lead to the very problem you're experiencing.

Another problem source would be the the fuel filter, but you're already replaced this.

Have you attempted cleaning your MAF?

Also you mentioned it was running fine before all the maintenance was performed. Did you put a Duralast wirelset on it? I've went through three "brand new" sets all with arching issues which presented as hesitation/misfiring before switching back to a FRPP set to resolve my issues. Would you happen to have the old wireset you could try?

Click to expand...

I have the original 1997 date coded wires I could try. I have a set of FRPP 9mm wires on it now that have maybe 1k miles on them. There is absolutely no visual indication of arcing that I can see. One thing that I did do was pull individual plug wires off of the coil packs while it was running to try to isolate the misfire, but it seems like all that would happen would be a steady misfire with a secondary miss on all cylinders. I couldn't isolate a single cylinder that didn't have a secondary miss with the plug wire pulled.

Perhaps I'll try swapping wires back to OEM and see if that helps.

Also, to answer the other questions, I've also cleaned the MAF multiple times and the fuel filter also only has about 1k miles. The thing that's odd is that it ran good before I did all of the maintenance and after for quite a while (I say that, but realistically, it was probably 500 miles or so over 2-3 months).

ZeroDCX said:

You mentioned the fuel pump duty cycle was at 100%, I'm not positive we can log this on our cars. Maybe some else can chime in.

You can try a fuel system pressure test to see where you're at. You should be around ~32 psi with the FPR connected and ~42 psi with the FPR disconnected at idle, IIRC.

Click to expand...

I have a fuel pressure tester, but I don't have the adapter for the Ford rail. Any idea where I could pick one up? There is quite a lot of pressure at the valve just by pressing it and watching it gush, but I know that is not sufficient to tell me anything. Nothing abnormal about the priming cycle it all. Sounds 100% normal.

ZeroDCX said:

Regarding the possible leaking FPR, you would see this when datalogging. Abnormal STFT/LTFT values when it starts leaking and PCM tries to adjust for this. Also when it leaks O2B1S1 and/or O2B2S1 would read constantly high in voltage ( closer to 1.0V with 0.0V indicating lean and 1.0V indicating rich ). Normally O2B1S1 and O2B2S1 should be switching between 0.0V and 1.0V. This is also another way to determine if an O2 sensor is faulty.

For future reference, if you have a O2 sensor code reading lean. Look to these PIDs and see what they're doing. If they're not switching, then you've found a problem. You can also try to spray a shot of carb cleaner or something in the intake and watch those voltages, when you do they should rise closer to 1.0V. If the target sensor does not rise in voltage it is most likely faulty. Switching O2 sensors across bank 1 and datalogging while performing the above is also another effective tactic to verify your findings.

Click to expand...

I wish I were more familiar with datalogging, but I'm not.

Also, it seems like everything I do has an effect, good or bad, but nothing resolves it.

Switching to the old OEM platinum plugs made it significantly worse. Switching to the MSD coilpacks made it slightly worse. Gapping the iridium plugs narrower didn't seem to affect it at all.

It seems like the problem is getting worse. And the fact that there are no codes at all is really getting on my nerves.

So where I'm currently at:

Switch plug wires back from FRPP to OEM.
Test fuel pressure to rule out pump/regulator.
MAF?
Take it to Ford to diagnose OR drive it off a cliff.

Both banks seem to behave very closely with one another so I'm leaning to something that affects the entire system, not just a single cylinder.

Edit: I could send you the log file if you have a means to view it. It's about 8 minutes long or so. Or the file converted to CSV.

Try swapping back to the original wireset first. That involves spending no money. if after doing so you do not experience the issue then you've found the problem.

Update. So I switched out the FRPP wires for the original 1997 wires and put the Autolite iridiums (gapped to .054) back in 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 FRPP wires since I've only had them for a few months.

Sounds good! It just goes to show that even if parts are new, there are no guarantees.

Glad you got your issue resolved.

+1 on the wires. I had the same misfire issue with my 97 and after checking all the normal suspects (including the wires) plus many atypical ones, it turned out to be the wires after all. A new set of OE style Ford wires solved the problem and the engine has been running right for a 1000+ miles now.

One of the original wires had a small burn hole in the boot that is hidden in the valve cover.

So do OEM motorcraft seem to be the consensus for good wires? I'd still really like to replace them, as it doesn't feel quite as snappy as when I first installed the FRPP wires.

What about MSD 8.5mm wires?

Also, should I try the MSD coils now or just sell them?

Generally FRPP wiresets are considered the top quality. But like any part, there is a chance for failure.

Something to consider on new wires - I believe Ford guarantees their OE wires & plugs for 2 years.

FRPP wires must be going down the tubes on quality. LMR has two ratings for the blue and they're both bad.

https://lmr.com/item/M12259C464/1996-98-Mustang-46L-4V-Blue-Plug-Wire-Set

The red ones have two good reviews from 2014, but the one from 2017 shows a misfire starting in a week.