2000 Ford Ranger P0171 and P0174 Codes

Could be one of the following:
* Dirty/damaged MAF sensor: try cleaning it with MAF sensor cleaner.
* Defective DPFE sensor
* Vacuum leak: check following link for the procedure to determine if you have a vacuum leak.
http://www.thedieselstop.com/faq/9497faq/tsb/tsb/files/041704.pdf
* Low fuel pressure: check fuel pressure to rule it out.

Hope this helps

That has to be something caused by your scan tool. There is no PID for bank 3 and 4. The DPFE really has nothing to do with those codes. You'll see a PO401 when you have a bad DPFE. You can test it by bringing up the DPFE PID on the scanner and power brake it. It should range between 0 and 5v if it's working.

90% of the time when you find these codes on a Ford, it's caused by a bad vacuum leak. They are notorious for sucking holes through the rubber vacuum connectors at the manifold. If you don't have a smoke tester, try some carb cleaner.

Don't worry about LTFT/STFT 3 & 4; it doesn't apply.
Your STFT/LTFT 1 & 2 simply show that you were running lean (no surprise :) ) when the PCM set the codes.
Vacuum leak is definitely a possiblity, but you said you have no problem at idle (which is usually when a vacuum leak is more noticeable) so I wouldn't dismiss a DPFE. Actually, P0171/P0174 can indicate a bad DPFE. These sensors have a high failure rate after a while. Here's a troubleshooting procedure (you'll need a DVOM, probes, and vacuum pump):
http://www.tomco-inc.com/Tech_Tips/ttt36.pdf

Since the DPFE has no vacuum associated with it whatsoever, You may use a vacuum pump to test it but the only thing passing through it on the vehicle is exhaust pressure. I don't see it being capable of causing a vacuum leak. If the EGR valve itself stayed open, it could leak but that would give you a lot worse symptoms that merely a vacuum leak.
The DPFE does have a high failure rate but vacuum leaks are not one of the symptoms of a bad one nor will it set lean codes.

If the DPFE loses its sensitivity, it will not report the correct amount of gases passing thru it, this will trick the PCM into thinking it needs more air, the PCM will command the EGR valve to open/stay open longer which will increase even more the amount of air entering the engine resulting in a lean condition
(this is a vacuum leak caused by a defective sensor)Sometime this will trigger a P0401 code but not always.
The vacuum pump is used along with a DVOM to check how the sensor reacts (voltage increase/decrease) under a certain amount of vacuum.

The PCM will not command the EGR open when the TPS reads closed throttle.
An open EGR at idle would be a completely obvious condition anyway.

I wasn't describing a situation at idle. Of course the PCM won't command the EGR open at idle. I was talking about a car under a certain load with a DPFE under reporting the amount of exhaust gases passing thru it.
Look, I'm not in disagreement with you here; I'm not even sure why we're arguing: Like I said in all my posts Cturnock problem can very well be a " œtypical' vacuum leak as you suggested. My whole point is this: a bad DPFE is another possibility if no vacuum leak is found. Ironically, and like I said yesterday, a bad DPFE would create a sort of " œtemporary' vacuum leak, while the EGR system is functioning, by tricking the PCM into leaving the EGR open longer than necessary. When the car comes to a stop and the EGR is not used then the vacuum leak condition would stop (since the erroneous DPFE inputs would not be taken into consideration by the PCM) and the car would idle smoothly (which is what Cturnock reported).
It's just a scenario; not necessarily the right one. I'm just bringing up all the possibilities I can think of in an attempt to help him. That's it. In the end you and I are just trying to help; there's no competition here.

Yes, your right. I have just seen a zillion bad DPFEs and never saw one set lean codes.

I don't think he mentioned even having any EGR codes.