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Yeah. Honestly try unplugging the MAF and seeing what happens. It will default to just using the o2 sensors and should level out in ~5 minutes after.

But again, you need a live data scan tool to know wether this fixes it.

Cal

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Discussion Starter #22
I'll reset the PCM, unplug the MAF and check the results with my live data scanner. I'm not sure exactly what I'll be looking for so I'll post the results.
 

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You'll be looking for the fuel trim numbers to go to or near 0% long term fuel trim (ltft) or short (stft) 0% fuel trims along with the first 02 sensor fluctuating from .1-.9v means it's running level.

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Discussion Starter #25
I guess I don’t know how to reset the PCM. I thought that just meant to clear the codes. Anyway, first thing I did was unhook the MAF sensor. Started the car and instantly the CEL was off. I erased the codes. Ran the car for 5 minutes and got the attached readings. O2 sensors are reading same as before. #1 is .1-.8v. The other 2 are steady.
 

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I guess I don’t know how to reset the PCM. I thought that just meant to clear the codes. Anyway, first thing I did was unhook the MAF sensor. Started the car and instantly the CEL was off. I erased the codes. Ran the car for 5 minutes and got the attached readings. O2 sensors are reading same as before. #1 is .1-.8v. The other 2 are steady.
PCM reset:

RFP is right at 41psi (282 kPa) which is good. Is this data with the MAF plugged in or not?

Paul
 

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Yeah I've cleaned the MAF sensor. I'll try again. I've used propane all over the engine trying to find a vacuum leak. I can't find one. I don't hear any hissing or sucking either. I put a vacuum gauge on a line and it shows ok.

I wonder if the O2 sensor can sometimes work and not work?

What would be the next step? Test the fuel pressure with a manual gauge? Why can't you trust the fuel pressure readings from the scanner? (or can you?)
The reason we don't trust the FRP sensor reading, is that we don't know if it's accurate until the reading from it is verified with a manual pressure gauge.
 

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Discussion Starter #29
Does it seem normal for the CEL to turn off immediately when I unplugged the MAF?

I erased the CEL code and drove 30 miles with the live data scanner on and the MAF plugged back in. Light did not come on yet.

The O2 sensor 1 cycles between .1-.8V like it should. So that seems fine. O2 #3 and #4 are mostly at .8V sometimes a little lower like .5 maybe I saw. So they seem good.

The LTFT bank 1 is always around 17%
The STFT bank 1 is mostly around 5%

The STFT sensor 1 is about +-8%
The STFT sensor 2 is 99.2% and it NEVER moves.
The STFT sensor 3 is 99.2% and it NEVER moves.

Should I be focusing on STFT bank 1 or sensor 1, sensor 2, or sensor 3?

Thanks for the help by the way! I’ll reset the PCM as well.
 

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Discussion Starter #30
One more thing. From what I read if my MAF sensor is working properly when I unplug it and drive the car is should drive poorly. If no change then the MAF might be bad.

Does this sound right?
 

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Discussion Starter #31
Another thing I thought of. My exhaust resonator is leaking. I think it's the resonator. It's the one just before the rear axle. Long, round cylindrical. I know an exhaust leak can cause an issue like this but this is a leak near the rear of the vehicle. I wouldn't think that far back would cause the O2 sensor to misread.
 

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Discussion Starter #32
I drove another 30 miles home and the light is still off. STFT is +-0% and LTFT is 10%

O2 sensors are reading good.

So who know's. All I did was unplug the MAF sensor, run it for 10 mintues and plug it back in. We'll see what the future brings.
 

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I drove another 30 miles home and the light is still off. STFT is +-0% and LTFT is 10%

O2 sensors are reading good.

So who know's. All I did was unplug the MAF sensor, run it for 10 mintues and plug it back in. We'll see what the future brings.
[thumb]

The only thing I would check now is that all the diagnostic monitors are complete. Your scan tool will tell if they are/arent.

Paul
 

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Hi,

Came across this while hunting my own p0171.

Tried the MAF unplug and it cleared nearly everything up for me, but when unplugged my car now shifts super hard. Did this happen with yours?

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Discussion Starter #36
Hi,

Came across this while hunting my own p0171.

Tried the MAF unplug and it cleared nearly everything up for me, but when unplugged my car now shifts super hard. Did this happen with yours?

Sent from my Moto Z2 using FF Mobile

That did not happen to me. I have a 5 speed. I only had it unplugged for 20 minutes of driving. then plugged it back in and cleared the CEL. it's stayed off for now.
 

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That did not happen to me. I have a 5 speed. I only had it unplugged for 20 minutes of driving. then plugged it back in and cleared the CEL. it's stayed off for now.
Good to know. Turns out Ford uses the other sensors in the MAF to determine shift points in the autos.

Found an eBay seller (looks like a dealership) selling genuine unused Ford MAF sensors for $30 instead of $200, so I bought one. I appreciate all the details you provided in your posts because it helped me figure out I probably have a similar issue (and I discovered a block heater while I was unplugging the MAF)

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Discussion Starter #38
Good to know. Turns out Ford uses the other sensors in the MAF to determine shift points in the autos.

Found an eBay seller (looks like a dealership) selling genuine unused Ford MAF sensors for $30 instead of $200, so I bought one. I appreciate all the details you provided in your posts because it helped me figure out I probably have a similar issue (and I discovered a block heater while I was unplugging the MAF)

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Good luck. You don't have a leaking PCV hose? That's what everyone told me was my issue. I changed it and it was perfectly fine. I also changed it without removing the intake manifold when everyone said it has to be removed.
 

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Good luck. You don't have a leaking PCV hose? That's what everyone told me was my issue. I changed it and it was perfectly fine. I also changed it without removing the intake manifold when everyone said it has to be removed.
I haven't replaced mine yet because I don't have the time to, but all signs point to it not being the pcv hose (steady vac pressure, no hissing, etc) plus even when I'm driving if I give it a shot of starting fluid right to where that hose is, it doesn't react.

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