I have a a 1997 ford ranger 4.0 ohv auto trans 2wd
It has be woking fine until a couple of days ago it got the codes p0171 and p0174 and it wont start when cold and when it does start it idles rough but it seems to clear up on higher rpms also when i break hard it wants to die at stops, ive checked fuel pressure and when the engine is running it gets 28-30 psi and when its off its at 35 psi, i tried unplugging the iac to see if the rpms lowered and they do and sometimes the engine would die so i dont think its a vacuum leak, i also checked live data for the maf sensor it shows 0.62-0.69 lbs/min at idle and it does go up when i rev up i also checked the o2 sensors and they both show 0v for some reason and the STFT shows -100.0%
i dont know what else to try and i would really appreciate any advices

 

Welcome to the forum

Reads like computer issue if you see -100% that can be a disconnected O2 or wrong O2 selected
1997 4.0l will have 3 O2 sensors
O2 B1S1
O2 B2S1
These are the O2s closest to engine, upstream

O2 B1S2 this is the O2 after Cat converters

B = bank, 1 is passenger side, 2 is drivers side
S = sensor, 1 is upstream, 2 is downstream(after Cat)

The O2 sensor part, after warm up, should be 0.1v to 0.9v and change rapidly, 0.2v to 0.6v
O2s generate their own voltage, but need to be heated above 600degF to start working
P0171 System too Lean (Bank 1)
P0174 System too Lean (Bank 2)
That would be correct if O2 voltage was 0v
0,1v = lean
0.9v = rich
Computer tries to keep voltage at 0.45v average

O2s also have Heaters built in, but of a heater fails or its fuse blows there would be different codes
i.e. P0135 02 Sensor Heater Circuit Malfunction (Bank 1 Sensor 1)

MAF looks OK, grams per second is better to use, at warm idle, 700-800rpm you should see 4.4-4.8gm/sc
0.62lb/min = 4.6gm/sc so OK

If this happened suddenly then a computer issue or its connector makes sense
Losing BOTH upstream O2s at the same time is a long shot

You can check O2 sensor wires and unplug both upstream O2s then start engine, should get the "Heater circuit" codes
See how the engine runs after warm up
Should see 99% from both upstream O2s as well

 

The issues with running rough at idle and wanting to die in a hard stop were there since i bought the truck and it had codes for coil primary circuit in all a b c coils so i replaced the coil pack and that got rid of the codes and it was idling better but then sometimes it wouldn't start when cold but it will only do it when it was a little under the quarter tank mark and the same for wanting to to die in a hard stop it will only do it when its a little under quarter tank mark so i gues that would be normal since its the original fuel pump and its old but still didn't got any lean codes until one day when i was driving it suddenly started running rough/missing and wanting to die on me at stops so i got home turn it off and back on and thats when the lean codes popped up