Check engine P171

Ranger.. 1997 2.3 4x2 5 speed w/AC

Here's the story.. My truck runs lean. It throws the P171 code about every 500-800 after being cleared. The last two times I've looked at the freeze frame and it happens at low RPMs 1030-1070. When I run the truck plugged into the scan tool, my long term file trim is high (24) and the short term is more normal (4) but short term will increase slowly as well. Both O2 sensors are new and I cleaned the mass air sensor.

I'm thinking maybe an injector. I put some sea foam in and will continue to. Also am going to change gas to see if that makes a difference.

If anyone has any other thoughts that may be helpful it would be much appreciated.. this problems is getting annoying.

Thanks

First the skinny on Lean/Rich Codes, it doesn't mean engine is running Lean....or Rich.
Heavy pinging/knocking when accelerating means engine is running Lean, black smoke coming out tail pipe means engine is running Rich.

So what do the codes mean?
Lean code means computer's 14:1 air:fuel calculation is not correct according to the feedback from O2 sensor.

Computer gets air data from MAF sensor, it then calculates the 14:1 air:fuel ratio based on that.
It opens injectors for a specific amount of time(pulse width) to add the calculated amount of fuel.
Computer gets Oxygen level in the exhaust from O2 sensor, too much oxygen = Lean burn, too little oxygen = Rich burn.
Computer then adjusts the injector pulse width accordingly, Short Term Fuel Trim numbers are + when computer is opening injectors longer(adding more fuel), and - when injectors are opening less, reducing fuel, 0 trim is the 14:1 calculation.
Computer likes to hover around -5 and +5 while driving, at idle engine needs a richer mix so +3 to +7 is normal.

Lean code is set when computer trim gets up around +20 to +25, engine is not running Lean, extra fuel is being added, Computer is just warning the driver that the calculations are not correct so there is a potential problem.

Could be MAF sensor is dirty, so is not reporting the correct amount of air, MAF reports 14 parts air but 16 parts are really flowing past it, so extra air is coming in, so more fuel needs to be added, Lean code set.
Same with an air leak, vacuum leak, extra air is coming in and by-passing MAF sensor, computer has to add more fuel than expected so Lean code set.

If there is a reoccurring misfire, then oxygen is not burned, it is dumped into exhaust manifold and O2 sensor sees high Oxygen, Lean, so computer adds more fuel, Lean code set.

Exhaust manifold leak also sucks in air, O2 sensor sees that as extra oxygen in exhaust, Lean, so computer adds more fuel, Lean code set.

Other side is the fuel:
Fuel pressure is low, computer expects 35-40psi of fuel pressure on a '97, if pressure is lower then less fuel comes in when injector opens, so computer has to open injectors longer, which = higher trim number, Lean code is set.

Injectors are dirty so not passing as much fuel as expected, Lean code is set.

Computer and O2 sensor failure is also a possibility, however there are usually other signs and certainly more than just a P0171 code would show up.

Most common Lean code causes are dirty MAF sensor or an air leak, vacuum leak.

After engine is warmed up, unplug the IAC(idle air control) valve's wires, idle should drop down to 500rpm or engine may even stall, either means no vacuum leak, if idle stays above 700rpm then there is a leak somewhere.

Look for a vacuum leak at the intake manifold above the 2 and 3 injectors. There is a vac line that runs from there to the evap box on the left side under the batt.
I had this and was hard to find.
Spray brake clean, carb cleaner or start fluid under there and see if your idle changes indicating a leak.
My leak was at the nipple where it transitions to the rubber hose. Very small and did not effect idle or drivability, but the computer sensed it.


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