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Discussion Starter #1
2003 F150 V6 4.2 L

Had a check engine light for a MAF problem P0101. Ended up replacing it with another unit I ordered online. After swapping the units out I now get PO171/PO174 - lean mixture on banks 1 and 2.

The old MAF had #F6DF-12B579-EA
The new MAF has #F6ZF-12B579-AA (Specifically stated to fit my truck)

Not sure if that number difference is relevant or should I assume I got a defective MAF ?
 

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Discussion Starter #2
Well, for what it's worth, I ran a voltage check on the new MAF and it came back normal and the trouble code never returned. So I conclude the new MAF is good.

As for the P0171/P0174 - lean mixture on banks 1 and 2, I replaced the PCV since it was about due because many folks reported it to be a common cause of the codes, but no help there.

Others were advising me to look for an air leak. I didn't find any bad connections or hoses. I ran a home smoke test but couldn't find any leaks either.

The freeze frame data showed the codes only re-appear under load each time after I cleared them to see if my work had any effect. So, I decided to look at the fuel side of the equation.

Running a fuel pressure test revealed low pressure and pressure that would not hold. Snapping the throttle showed a 5 psi increase and removing the vacuum line on the regulator showed a 10 psi increase. Conclusion is that the fuel pressure regulator is good and the fuel filter or pump is failing.

I replaced the fuel filter with no change, so my problem appears to be I need a new fuel pump.

Lessons learned: check the freeze frame data, follow up on air or fuel diagnostics based on whether the P0171/P0174 codes appear under load or near idle.
 

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Another cause of lean codes has been the Baro sensor..... Usually incorporated inside the MAF sensor. I got burned years ago on this. I always verify a Baro reading in Live Data Before chasing Lean codes. Although Fords are the only vehicle that seem affected.


GLuck. Jay
 

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You might also check for an exhaust leak. Depending on where it is, it's possible to draw clean air into the exhaust, making the O2 sensors think the engine is running lean.
 

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Another cause of lean codes has been the Baro sensor..... Usually incorporated inside the MAF sensor. I got burned years ago on this. I always verify a Baro reading in Live Data Before chasing Lean codes. Although Fords are the only vehicle that seem affected.
You might also check for an exhaust leak. Depending on where it is, it's possible to draw clean air into the exhaust, making the O2 sensors think the engine is running lean.
Good to know. I definitely have a low fuel pressure problem. If I can get that resolved and the check-engine-light comes back, I will pursue other possibilities.
 

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On my Jeep there are a few things that when they get replaced you need to do a ECM reset. Not sure about Fords but on Chrysler's you disconnect the Positive cable and hold it on the Ground so the Capacitors in the ECM drain then the ECM will do a relearn.
 

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On my Jeep there are a few things that when they get replaced you need to do a ECM reset. Not sure about Fords but on Chrysler's you disconnect the Positive cable and hold it on the Ground so the Capacitors in the ECM drain then the ECM will do a relearn.
Not aware of anything in the Ford service manual about anything like that.
 

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A failed DPFE can fool the computer by under-reporting the amount of EGR gas flowed into the intake. That can lead to a leaner than normal mixture, or misfire, either one leaving the O2 levels in the exhaust a bit higher than desired. If the O2's are higher, the computer may decide the mix is too lean... and goose the fuel flow, reaching a max in the adjustments, and then report a p0171/p0174. Having both can be more of an indicator than having one side report a problem.
tom
 

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Discussion Starter #9
A failed DPFE can fool the computer by under-reporting the amount of EGR gas flowed into the intake. That can lead to a leaner than normal mixture, or misfire, either one leaving the O2 levels in the exhaust a bit higher than desired. If the O2's are higher, the computer may decide the mix is too lean... and goose the fuel flow, reaching a max in the adjustments, and then report a p0171/p0174. Having both can be more of an indicator than having one side report a problem.
tom
I've heard about failed DPFE's. I may end up there too. I can only hope the resolution of low fuel pressure is the answer.

The p0171/p0174 codes are among the worst to get.
 

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Discussion Starter #10
I got the fuel pump replaced and the system is now holding pressure.

Unfortunately, the trouble codes 171/174 came back. I followed up on
JayzAuto's suggestion and he was right !

The "baro" readings were way off and resulted in under-reporting the air flow. So my problem was a bad sensor. Replacing the MAF appears to have solved the problem.

Thanks to all for your contributions.
 
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