I have this 97' ford ranger 4.0 and it keeps throwing code p0171(bank 1 lean)

I've insured no vacuum leaks with smoke and pressure test. (Could still be some but unlikely) the only thing that leaks is the IAC vent as it’s supposed to from what I’ve gathered.

Replaced fuel filter and tested fuel pressure(35psi)
Cleaned maf sensor
Replaced upstream 02 on that side
Checked for exhaust leaks when exhaust was cold between 02 and head, there are none
Replaced pcv valve

Any ideas?

also I’m about to load the back with a 1/2 ton camper + my clothes and gear.
any suggestions for rear suspension help on a budget would be appreciated as well.

 

Welcome to the forum

Lean, or Rich, codes don't mean engine is actually running lean or rich
These codes reference the Open Time for the fuel injectors

Computer calculates injectors open time base on engine size, injector size, fuel pressure, RPM and air weight(MAF)
Say that open time is 100ms(milliseconds)
It then opens injectors for 100ms and watches O2 sensor feed back
(O2 sensors generate their own voltage, 0.1v is high oxygen or lean, 0.9v is low oxygen or rich, 0.4v is what computer wants to see)

If computer sees 0.2v then it changes open time to 104ms(adds more fuel), then checks O2 again, if still under 0.4v it adds more fuel
In this example if the computer reached 116ms(16% more fuel added) it would set Lean Code for that bank of the engine, over 15% calculation error = codes set
This all takes under 2 seconds
At no time was engine ever running lean, any gasoline engine will ping/knock when its running lean, and will smoke if running rich

In this case your computer is programed for a 4 liter engine, so it already knows EXACTLY how much air is coming in at any RPM and throttle position, its just Math
And it has 14 pounds per hour injectors with 35psi fuel pressure

Gasoline's air:fuel mix ratio is 14.7:1.......this is a WEIGHT RATIO
14.7 pounds of air to 1 pound of gasoline
14.7 grams of air to 1 gram of gasoline
This is why fuel injectors are rated in pounds per hour

The reason you need to know all this is because AIR doesn't weight the same at any given time
If you watch the weather reports you will see low pressure and high pressure "areas", lighter air and heavier air
Warmer air is lighter than colder air, i.e. "Hot air Rises", thats how hot air balloons can fly, they float above the cooler air around them
If you drive above 2,000ft elevation the air gets "thinner"/lighter the higher up you go

And this is what the MAF("mass" air flow) sensor is there for, to WEIGH THE AIR, determine its MASS/WEIGHT
If there is a vacuum/air leak then not all the air is being weighed by the MAF sensor, so computer has faulty data

On a V6 engine this would effect both banks of the engine

For just one bank to be effected it could be the O2 sensor, so changing that was a good idea, but changing all O2s that are over 12 years old is best, these are the ONLY sensors that wear out

A clogged up injector on one bank will flow less fuel causing miscalculation in computer, and lean code, add injector cleaner to gas tank, once a year

Misfire, a misfire or partial fire of a cylinder means not all the oxygen was burned up, so that extra oxygen is dumped into exhaust and O2s see it and lower the voltage, so lean on the bank with misfiring cylinder
Make sure spark plugs are clean and gapped correctly, change spark plug wires every 10 to 12 years

False lean
If there is a small exhaust manifold leak it will SUCK IN air along with leaking exhaust, O2 sees that extra air/oxygen and its a false lean

Partially blocked exhaust on that one bank, if each bank has its own Cat Converter then this can happen when the Cat converter starts to break up inside, usually have a noticeable power loss at higher demand/speed

EGR valve leaking, usually effects BOTH banks of a V6 engine but can just show up on one bank to begin with

LTFT, long term fuel trims
The computer saves short term fuel trims as an average and its called LTFT
As any engine ages the compression is less, the fuel pressure maybe a few pounds lower, there would be smaller air leaks, ect............
LTFT is used so the computer doesn't have to re-learn adjusted fuel trims at each startup

If LTFT on one bank has been creeping up from partial misfires, dirty injectors, small air leaks, or failing O2 then that can set a lean code as well
If it was an older O2 sensor issue then lean code would go away with new O2 install
There would be a few days of driving needed for the AVERAGE to come down below the 15% error
Or if you fixed the problem it can take a day or to

You can get a Bluetooth OBD2 reader for under $20, and you can then watch Live Data to see fuel trims and why you are getting lean code
These work on ANY vehicle 1996 and newer, so worthwhile to have one

 

Thanks for the extremely informative response this should lead me to get rid of this light!!

 

For mine I just use Black Tape, lol

Nice looking setup

 

Ok so it’s now throwing a lean code for both banks.

When the truck is hot the idle is surging from 800-1000 kind of a consistent tone of running poorly.

after tapping gas it stalled but usually just drops really low.

I’ve replaced the IAC but everyone I’ve put on leaks from the vent when I put vape smoke into the intake to look for leaks. I’ve read that this is supposed to vent? But regardless it’s unlettered air getting in

 

No, the IAC Vent shouldn't leak when engine is running, its just there to allow air behind the valve to escape when valve moves
Spray a little silicone or other lube on the front of the valve, that should help seal it better

But that wouldn't be enough air leaking in to cause Lean codes

Both banks with lean codes is better, that narrows it down to common shared areas
Fuel pressure is common, needs to be above 25psi, 35psi is expected up thru 1997 Rangers, 1998 and up needed 60psi

Exhaust restriction is common, use a vacuum gauge, hold RPMs at 2,500(approx.) and watch the gauge, if its slowly dropping exhaust is restricted
PCV Valve, they get dirty and stick open

Air leaks between MAF sensor and intake, the Breather hose to oil filler and oil cap
The engine crank case and valve cover areas are sealed, or should be, the PCV Valve applies negative pressure using engine vacuum, that will pull in fresh air through the Breather hose on the other valve cover
If its not sealed or not working then air would be sucked in from the Breather hose

Vacuum leak in upper intake, does RPM change when you press on brake pedal, i.e. booster leaking

With engine warmed up and idling
Unplug the 2 wire connector on IAC Valve
RPMs should drop to under 600 or engine may even stall, either is GOOD, it means no Vacuum leaks
If idle stays the same then there is a vacuum leak

Dirty MAF sensor, should be cleaned once a year

 

Went over everything with two cans of brake cleaner and finally found the leak. It was the upper intake manifold gasket will be replacing and seeing if this resolves the issue thanks for all your help