So I have intermittently had a CEL with codes p0171/p0174 (both banks lean) since i got the truck ('97 4.0 4wd) a few mos/11k miles ago. If i reset them, it usually takes at least a few hundred miles for it to return. Last time around (maybe 500 miles), i also got a P0402 (too much EGR flow) code. Replaced some vacuum lines, though didn't see a leak. Truck runs fine. I also replaced the DPFE, which had been reading higher voltage than spec. With my new fancy scan tool, I'm getting some curious readings:
1. There are no longer any codes, pending or otherwise. All of the I/M readiness codes are OK.
2. All of my O2 sensors read 0v. All of my FT readings (both SHRTFT and LONGFT, both banks) are pegged at 100%. It's been like this for a while. Not sure why this isn't triggering a code. I'm getting voltage at the sensors, and I removed one and torched it, and I got a signal from it.
3. My scan tool reads "modules". I don't understand what these are exactly, but here are the readings that fail:
$41 ID 12 Measured value FF38
$52 ID 00 Measured value 2AO
$10 ID 21 Measured value FFFF

So...I figure that
a) I really am running super lean. However, on startup, shouldn't I get different values, esp at the O2 sensors while I'm running in open-loop before the car warms up?
b) There's a problem maybe in the harness somewhere, where the computer isn't reading the O2 data correctly, and perhaps adjusting the FTs based on faulty O2 input of 0v? Although again, I would expect the computer to trip some kind of sensor code if they were misbehaving.

I'm at a loss as to where to go next...again, no driveability issues at all. I took it on a 9200 mi road trip with a slide in camper, several times over the mountains to 10k feet in 90' heat without a problem. I feel like that pretty much eliminates any fuel system problems.

 

Relax, I have a 97 exploder with 360K on her and have had the same anomaly for the past 10yrs of ownership. There seems to be OBDII compliance problems (96-98).

If I use my computer with the Ford Enhanced diagnostics I can read the PIDs for the o2's and record switching graphs, yet all the Generic OBDII scanners and computer software that I have are blacked out for those sensors. Oddly enough some will show my FT and since I am consistently LTFT -2.1 to -2.9%, STFT very active, I know the CPM is reading the O2 and using that info correctly. I can trigger MIL/CEL either by inducing propane or opening Vacuum. I have occasionally seen the 100% FT reading, but if I switch to a different scanner I see the correct reading.

 

Hah! strange that i haven't been able to find this info elsewhere--this just didn't make sense. I actually thought it might be a code reader problem, so hooked it up to my wife's '05 mazda6, which showed all correct/fluctuating values. shouldn't obdII compliance problems trigger a recall? isn't this info a gov't mandate? still don't know why i'm occasionally triggering a CEL though--i had bought the scanner with the live data in order to see whether it was a vac leak or something else triggering the codes...

 

HI Mike,

Most common cause of the p0171/p0174 (both banks lean)codes are

Dirty MAF, and
Intake manifold leaks (vacuum)

 

I had the same problem with my '01 Ranger did everthing but change fuel filter didn't help, installed a new filter and have no problem since.

 

Don't know which scantool you have nor whats going on with it not being able to read your OBD-1 systems PID's that you've outlined.

I have an Actron CP-9150 that reads all the PID's on my 94 Taurus OBD-1 system & 99 Rangers OBD-2 systems that you've mentioned, so they should be accessable & readable.

May be a software problem in the scantool, so call their Tech hotline & see if they have some answers. Most can download software updates for the scantool & you sure could make good use of an operating scantool in your trouble shoot right now.

On the lean both banks code, maybe look to the PCV valve. If it's stuck open, or intermitently sticking open, it'll mess with fuel trim.

Also maybe consider re-torquing the intake manafold fastners to spec, as these year 4.0L's have a history of intake manafold fastners loosening up & sucking the gasket in & causing lean codes.

The O2 sensors & the EGR system aren't on line at a cold start up.

More thoughts for consideration, let us know how it goes.

 

Like PawPaw mentioned take a look at you PCV. Our sable threw the same codes p0171/p0174 (both banks lean). Turned out the PCV hose was both crushed and split. Replaced the hose and PCV and have had no more problems.