Intermittent P0171 and P0174

Hello,

Just pulled upper and lower intakes. Did the gaskets. Checked for vac leaks and found none. (1997 4.0)

Typically I'll throw just a P0171 and not P0174 but the two have been coming on together lately. I have been using torque pro to attempt to monitor whats going on. I have cleaned the MAF and it's readings are inline.

Now when I go to look at the fuel trims the all (short and long term banks 1 and 2 come up) -100%. My O2 sensor voltages come up as 0.0V.

Huge help huh. So looking at the mode 6 test results this is what I see:

Mode $06 report generated by Torque for Android
================================================

Vehicle VIN: Not present
Vehicle Manufacturer: Unknown
Vehicle Calibration ID: Not present

Unit and scaling information are not supplied with the data from the ECU for this type of vehicle. Consulting the manufacturers service book for this information is recommended.

Test report:
------------------
TID:$01 CID:$11
- Rich to Lean sensor threshold voltage(constant)
Min: 512
Test result value: 1,024
PASS
----
TID:$01 CID:$21
- Rich to Lean sensor threshold voltage(constant)
Min: 512
Test result value: 1,024
PASS
----
TID:$03 CID:$01
- Low sensor Voltage for switch time calculation
Min: 0
Test result value: 461
PASS
----
TID:$03 CID:$02
- Low sensor Voltage for switch time calculation
Min: 0
Test result value: 512
PASS
----
TID:$10 CID:$11
-
Max: 54
Test result value: 51
PASS
----
TID:$10 CID:$21
-
Max: 51
Test result value: 65,535
FAIL
----
TID:$41 CID:$11
-
Min: 63,744
Test result value: 65,459
PASS
----
TID:$41 CID:$12
-
Max: 512
Test result value: 65,459
FAIL
----
TID:$45 CID:$20
-
Max: 21,556
Test result value: 13,368
PASS
----
TID:$4a CID:$30
-
Test incomplete or dependant test failed
--


End of report.

What does this mean. LOL are the weird readings related to this? Anyone use torque pro on one of these fossils and get the fuel trims and O2 voltages to work?

STFT(short term fuel trim) and O2s don't work until engine is warm up completely, Computer operating in Closed Loop

And STFT will/should be high at idle, O2 sensors are not used at idle, warm or cold, and not at WOT(wide open throttle) either.

LTFT(long term fuel trim) is saved in memory, can't be cleared by most OBD2 readers
LTFT is a longer term average of STFT and downstream O2 sensor, it is used to run the engine when it is cold an O2 sensors can't work, computer has Factory preset air:fuel mixe tables to use at specfic RPMs, the LTFT offset those so older engine can still run it's best

After engine is warmed up and idling, unplug the IAC Valves 2 wire connector
IAC valve will close and idle should drop to about 500rpm, or engine may even stall, either is good, it means no vacuum leak.

If idle stays high then you have a vacuum leak.


Upstream O2 sensors last about 125k miles, unless there is an engine problem then that can shorten their life, they use a chemical reaction to generate voltage and that chemical gets used up, so they fade away, causing high STFT

For BOTH Banks(171 and 174 codes) to show Lean means a common problem for BOTH Banks exists.
So MAF sensor is common to both, air tube from MAF to intake, upper intake is common to both.

Fuel pressure is common to both, 1997 Ranger should run 30-40psi fuel pressure
Computer doesn't have a fuel pressure sensor, it was programmed for 35psi fuel pressure.

So what the computer does in Closed Loop is to get the MAF air flow, calculate how much fuel to add to that air to get 14:1 air:fuel mix.

It "knows" the fuel pressure and it "knows" the size of the injectors, so from that it can calculate how long to open each injector to get the correct amount of fuel added to the available air
That calculation is 0 STFT
After air:fuel mix is burned in the engine, the computer gets the O2 feedback, if O2 shows Lean(too much oxygen) the computer Opens the Injectors longer, thats +1 STFT, if Rich(too little oxygen) then you get -1 STFT
If O2 still shows Lean then +2, and +3, ect....

Normal STFT is -5 to +7, as an engine gets older +3 to +10

So STFT is actually the Open Time, for the fuel injectors

Now if fuel pressure was 20psi instead of 30psi then computer would have to open injectors longer, so you would get higher + STFT
If STFT average gets close to or above +20 then you get Lean code on that bank

Engine is never running Lean, the codes mean computer calculations for 0 STFT are off, so it is letting driver know with CEL and codes

And you can get False Leans if there are exhaust manifold leaks or misfires
Exhaust manifold leak sucks in air, O2 sees that as too much oxygen so Lean
Misfire doesn't burn up, use any air, so it all gets dumped into exhaust and O2 sees it as Lean

Check spark plug tips, if they are getting dark you are really running Rich, because of a False Lean

So I'm going to guess its either a fuel pressure or O2 sensor issue. It's just weird that I'm not seeing an O2 code I think.

I know my fuel filter is good. I did it a few months ago. Also I'm not seeing any misfires which I think would show weak fuel pressure.

Is it possible that all 3 of my O2's are dead and that's why they're reading 0.0V on my scan tool as well as -100% on fuel trims?

Or is my PCM dead. Wish someone could chime in with the meanings of the two failed mode 6 tests.

Engine can run fine down to 10-15psi fuel pressure, just get lean codes
Fuel injectors can get dirty as well, limiting flow, so computer has to open them longer, i.e. so higher STFT

Both upstream O2 sensors going out at once is a long shot, and yes you would usually get "slow switching" O2 bank 1 sensor 1 if that one was starting to fade away

I would try out the OBD2 scanner on another vehicle
OBD2 uses PIDs(packet IDs)
There is a standard for PID numbers, seen here: https://en.wikipedia.org/wiki/OBD-II_PIDs

What an OBD2 reader does is to "request" a PID from the computer, computer then sends back that PID data, reader then displays that data.
Like PID 06 = STFT Bank 1

OBD2 reader simplifies this to english, lol
But reader or computer can get confused, so reboot of both can be what to do if things are not making sense

For Mode 6 you need a Reference chart to look up what the TIDs mean
Have a read here: https://petemeier.wordpress.com/2010/09/23/deciphering-ford-mode-06/

I've done that and all of the numbers made sense on both vehicles. 01 Jetta and an 04 Jeep.

I checked the resistance on the injectors when I did the manifold gaskets and they all came up good. I replaced one brand new since it had a cracked pintle cap and I couldn't find a stupid .05 cent cap anywhere.

Fuel pump could be on its way. I guess I'll try to find a pressure tester and check it out before I replace the O2's (which I believe to be original)

For most of last year I also has intermittent lean codes for my 3.0. When I checked fuel pressure, it was slightly low. Should be 54 - 72 for mine, and it was showing 52. Ran fine so I left it be and just kept clearing the codes when they popped up. Then in December it completely failed on me while out driving around my favorite curvy road. I managed to creep it home. When I checked pressure, it was down to 32 psi and the codes were back for good.

New fuel pump and I've not seen them since late December. Even though I've had intermittent crank, no start conditions, they're related to not being able to keep my damn injectors clean. Only way I avoid the problem is keep the tank loaded up with injector cleaner. I forget one time, BAM! it won't start.

Mine is a 98, and recent diagnostics for the starting problem, showed none of my O2 sensors as being bad, even at 19 years and 148,000 miles. I've replaced all the other sensors at least once, and in some cases twice (IAC too many times to remember), so did not expect any of them to be a problem.

In 2015, I had the Christine mystery happen to me several times. In the garage idling, all of the sudden it would just start revving up, going berserk, vroom, vroom-vroom-vroom, then die. Or driving down the highway at 70 mph and it would just die all of the sudden. Idling at a stop sign and it would just start revving to the moon. In all cases it would die and wouldn't start immediately, but 5 minutes later start like nothing had happened. Replaced the IAC several times thinking it was the culprit, how else can the computer cause the engine to just rev all by itself? And after buying a really expensive Hitachi IAC, the problem more or less went away. Hasn't happened again in a long time. I originally thought it was a bad JET chip, but when I recently put it back on nothing untoward happened (new PCM btw).

So I found something interesting. One of those cased hard plastic lines had snapped. So I wasn't seeing the smoke blowing through because it was being diverted. Fixed that.


Still 0V at O2 sensors and -100% fuel trims. Verified with another scanner.