Have am 02 expy 5.4 that's been setting lean codes on both banks and really has me stumped, only will set them once it gets right around the 0 degree mark or colder out, colder it gets further positive the fuel trims go. tonight at -13 the ltft was sitting at 18+ and stft was between 13-15

Have checked and cleaned the Maf( baro pid is between 147and 150hz, which is a little low for our 2000ft altitude here but I don't think it's the Maf itself causing it) have checked for vacuum leaks as good as I can with carb cleaner while watching the o2 sensor and stft data and can't get the typical spike when the system sniffs the carb cleaner (wishing I had access to a smoke machine about now) typical PVC elbow is in good shape, hence no change from spraying cleaner.

Also in addition to the codes it has a high idle, will only come down to 850 when warm when the PCM is requesting a 650 idle, iac is not stuck open either as the PCM is commanding it a very low duty cycle and you can even unplug it and the idle doesn't drop at all (verified it wasn't stuck open either by pulling the supply hose from the intake boot and was not pulling any air at all)

I'm sure I'm overlooking something but this thing has me stumped

 

Have you tested or replaced the coolant temperature sensor? If it is off, the ECM could think the engine is warmer than it is, which in turn leans the fuel mixture.

 

When cold the cts reads same as the iat sender and once it finally warms up it levels off between 185 and 192 (tstat cycling open n shut since it's so cold up here)

 

Water in the fuel filter?
What is the fuel pressure at the rail when it is cold?

When does the freeze frame data indicate that the code set?
Low rpm and light throttle? (Vacuum leak).
Or higher rpm and heavier throttle? (Fuel supply issue).

 

Check the PCV grommet on the valve cover, PCV should be tight.

 

Haven't checked fuel pressure but I'd be surprised if it's low since in the higher load areas ft's are pretty close to zero, code always sets under low load conditions, hence a vacuum leak I'm missing somewhere I think

 

Let me throw out my recent complications under similar condition. Only mine reacted at about 20 degrees or less out.

First, I replaced a small vacuum line next to the battery (I believe this line was completely unrelated...but the codes caused me to look which made me find it) I replaced the PCV grommet, it was very loose and am positive I was leaking off vacuum there. I believe it made things better, but not fixed. The codes came back.

Second, I cleaned the MAF sensor and inspected both PCV elbows one at the intake and one at the PCV. I replaced the one at the PCV because it was pliable enough to pinch shut. Light came back on. Noticed no change.

Lastly, cleaned the IAC and replaced a dry rotten vacuum hose that ran from the top of the intake to the Purge Valve on the drivers side of the fire wall. So far after several days I have not gotten the codes back. It is idling and running better and I have no signs of vacuum leak any longer.

In my case, Im sure that the codes were a combination of a few things and fixing one intensified the other. I also believe that that extreme cold caused the codes due to the contraction of these worn parts. The rotten hose on the top of the intake had cracks deep in the the wall that you could see once it was pulled off and since that is a main vacuum line it made sense that a small leak could cause the problem. I believe the dirty IAC was also a cause of the over all poor idling as the gummy crap in there would easily harden as it gets colder out.

Anyways, hopefully it helps some.

 

Having the same codes on a 2004 5.4 expedition. cleaned the maf, I know what the pcv is but what is the IAC? Will also check the vacuum lines and fittings as soon as it reaches 20 degrees. No sense in doing more damage than I have too. Thanks.

 

Idle Air Control valve. It controls the idle when your foot is off the gas pedal.

 

I don't want to hijack this thread, but I'm having the same code - P0171, left bank lean. We were snow skiing in the NC mountains and the overnight lows were ~ -5 F and the highs were in the low teens when it threw the code. I don't see or hear a vacuum leak, but I did notice an elbow tube that seems to pinch close when the truck is idling (idles around 650 rpm). Below is a general picture of the location of the tube and also an upclose picture of the elbow. I'm pretty certain I've replaced this elbow before. In fact, I still have the hard plastic tubing that leads up to the elbow.

Could this pinched elbow be my problem? What is this part (so I can find it on the fordparts.com).

Thank You

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