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Post code definitions with the codes, since you already looked them up
P0171 - System too Lean (Bank 1)
P0174 - System too Lean (Bank 2)
P0401 - Exhaust Gas Recirculation Flow Insufficient Detected

And never "google" a code number, look it up on a full list, like this one: https://www.therangerstation.com/how...trouble_codes/
Computer can pick from many codes regarding the same system, so the codes it does NOT use can tell you as much as the code it did use


Now to the problems
Simple test for a vacuum leak
Warm up the engine and let it idle
Unplug the 2 wires on the IAC(idle air control) Valve, it will close and RPMs should drop to 500 or engine may even stall, either is GOOD, it means no vacuum leaks
If RPMs do not drop then there is a vacuum leak, find it
If both banks have Lean codes then it will be an upper intake leak

Air tube from MAF sensor to intake can crack, that is not technically a vacuum leak but it IS an air leak so would cause Lean codes on both banks

What was the MAF sensor code?
P0100 Mass or Volume Air Flow Circuit Malfunction
P0101 Mass or Volume Air Flow Circuit Range/Performance Problem
P0102 Mass or Volume Air Flow Circuit Low Input
P0103 Mass or Volume Air Flow Circuit High Input
P0104 Mass or Volume Air Flow Circuit Intermittent

A couple of these codes point to possible WIRING issues, guess which ones, and thats why you should look up codes on a Full List, just in case, and it can save you mioney

Generally speaking P0401 means EGR tube or EGR valve may be clogged up, so even at wide open not enough exhaust gas is flowing to the intake
Do you get Pinging/knocking when accelerating?

If not then it may be the sensor, the "flow" is determined by the DPFE sensor, it has TWO hoses that run to the EGR Tube, check the hoses, the DPFE sensor detects the Pressure Difference between the two hoses, as EGR valve opens the hose closer to the valve loses pressure faster than hose farther away, this difference is how computer determines how much exhaust is "flowing" into the intake
P0401 - Exhaust Gas Recirculation Flow Insufficient Detected

 

 

 

 

Warm engine idle on 1997 4.0l OHV should be 650 manual trans or 750 automatic
Engine should almost die with IAC Valve unplugged, shouldn't be hard at all to hear a difference
Previous owner may have adjusted the Anti-Diesel Screw, thinking it was an "idle screw", fuel injection can't use an idle screw, goofs things up, which is why an IAC Valve is used
Look on the throttle linkage for what "looks like" an idle screw
With engine warm and IAC Valve unplugged turn that screw counter clockwise to see if RPMs drop, if not return it to where it was, and check for vacuum leak
If RPMs do start to drop, someone messed with it, turn it until engine is barely running, 500RPMs, no vacuum leaks, plug IAC Valve back in and RPMs should come back up


LTFT-Long term fuel trims are high, that is what is setting the Lean codes, 17%
LTFT is cumulative over a long period, and can't be Cleared using OBD Readers, these can take a few weeks to drop down after a repair

STFT-short term fuel trims are also a bit high 10%, -5% to 5% are normal, these are instant feedback readings so they can go as high as 20% when accelerating

The computer calculates Air/Fuel mix on the fly, every second engine is running computer is doing the math very very fast
Fuel Trim is the Open time for the fuel injectors
Computer "knows" engine displacement, so it knows a 4 LITER engine will suck in 4 liters of air every 2 RPM at Wide Open Throttle(WOT)
So it "knows" how much air is coming in at any RPM and at any throttle position, its just Math

Problem is, the air/fuel ratio is a WEIGHT RATIO
For gasoline it is 14.7:1 ratio
14.7 POUNDS of air to 1 POUND of gasoline
14.7 grams of air to 1 gram of gasoline

And computer can only calculate volume of air not its WEIGHT
Gasoline, for the most part, weighs the same at any temperature
AIR on the other hand has widely different weights depending on temperature, and Altitude above sea level
"Hot air rises", why?
Because it is LIGHTER than cooler air

This is why there is a MAF sensor and also an air temp sensor(IAT)
MAF uses a "heated wire" that is cooled by air flowing past it, the heavier the air the more it cools, and that also needs to be offset by RPMs, then offset by air temp as well
If you set MAF data to gm/sc, grams per second, on the OBD reader, it should show engine displacement at warm idle, 700rpms, so 4.0g/s, +/- .5

So computer has the AIR WEIGHT for its calculation, and that is STFT 0
It then opens the fuel injectors for that calculated time to get 14.7:1 ratio
Computer gets feed back from O2 sensor after this calculated mix is burned, O2s "sees" oxygen levels not fuel levels
If there is too much oxygen it was a Lean burn, so computer adjusts Fuel Trim to +2%, opens injectors 2% longer that calculated, then sees if that works, if not then +4% is used, ect.........
If there was too little oxygen it would be a Rich burn so computer would adjust Fuel Trim to -2%, reducing the open time by -2%

With feedback from the O2 sensor the computer adjusts the injectors open time until O2 shows correct oxygen levels in exhaust, it does this for each Bank of a V6 engine separately, so the chance of BOTH O2 sensors failing at the same time are not likely, possible but not likely.
This adjustment takes seconds, so engine is never actually running Lean or Rich
O2 sensors last about 100-150k miles, and they will reduce MPG as they get older, so after 100k I would replace them just on spec for best MPG for the next 100k miles

When the computer "sees" a consistently high(or low) fuel trim it will set codes notifying the owner that its calculations are not correct, 0 STFT is not working.
Lean codes are usually caused by "unreported air", air that is coming in without going thru MAF sensor so computer has the wrong data, so its calculations are wrong.

LTFT is the average of STFT over long periods, this is needed because engines change over their lives, MAF gets dirty, seals wear out and rings, fuel pressure may drop a bit, all normal stuff.
So computer saves LTFT to use on cold starts, O2 sensors only work above 650degF, and to offset STFT after warm up
Computer works best with fuel trims between -15% and +15%, so LTFT offsets the 0 STFT in the calculations
+15% LTFT and +10% STFT means injectors are actually open 25% longer which is why computer set the Lean codes
25% error means something is wrong in the system

1997 fuel pressure should be 30psi, but should be fine down to 20psi, below that you would get Lean codes, injectors have to be open longer to get correct amount of gasoline added, computer doesn't have a Fuel Pressure Sensor, it "assumes" 30psi

 

 

Good finds, thanks for the update