Sometime I get a lean code on bank 1 only and other time it only bank 2. Once in a while it bank 1 and 2. Sometimes it runs great other times not so great. It a 4.0 in a 2002 ranger 5 speed. have replace iac and mass air sensor. Change plug wire and plugs. Some time rpm will not fall till you come to a complete stop. stays around 1200 to 1800. , other times it normal. Change pcv hose no vac leaks that I can find. Happen more in hot weather then cold . Any ideas? been problem for 2 years ford cound not tell me what problem is.

 

Welcome to the forum

2002 Ranger 4.0l SOHC engine with Manual trans

First, RPMs should stay high with clutch pedal in, its an emissions thing, then once speedometer is under 5MPH RPMs will drop to normal level, about 700 if engine is warmed up.

Lean code does not mean engine is actually running Lean
The computer calculates the amount of fuel to add to the Air that is coming in to the engine, based on RPM and MAF sensor data.
The weight ratio of air to fuel is 14.7:1, so if there is 14.7 grams of air then computer will add 1 gram of gasoline.

When that calculated ratio is burned in the engine, the O2 sensor "reads" the Oxygen level in the exhaust, if too much oxygen then burn was Lean, if not enough oxygen then burn was Rich.
The computer then adjusts for what O2 sensor "sees", if Lean then computer adds a bit more fuel, until oxygen level is Just Right
All this happens in milliseconds, super fast
So at no time is engine actually running Lean

But.........if computer is having to add too much gasoline over the calculated amount it will warn the driver that there is a problem with the calculation by setting Lean Code.
Computer doesn't know what the problem might be, it just knows it is Opening Fuel Injectors longer to add more fuel than it calculated.

Yes, that could be an air leak, if all the air being sucked into the engine isn't flowing thru the MAF sensor then computers calculation will be off
The 14.7 grams of air flowing thru MAF sensor might actually be 15.7 grams air IN the engine if there is a vacuum leak

2002 Ranger will use 60psi fuel pressure at the injectors.
Thats the pressure the computer "knows" and uses in its calculation, if fuel pressure is actually 40psi, then when fuel injector opens 1 gram of fuel may not flow out, may only be .9 grams, so Lean, and computer sets Lean code.
Computer doesn't have fuel pressure sensor

None of this should make engine run rough, as said Lean code doesn't mean engine is running Lean.
But if you are having Mis-Fires, that can cause Lean codes, O2 sensors read Oxygen levels, not fuel, if a cylinder Mis-fires then NO oxygen is burned and it is dumped into exhaust manifold and O2 sees higher oxygen levels.
Computer adds more fuel and sets Lean code because of Mis-fires

What did the tips of the old spark plugs tell you about each cylinder?
Were any blackish color
What about very white?
Very Light brown means cylinder is running well

Fuel filter in the frame rail under drivers seat area should be changed every 5 years or so
I recommend adding a can of fuel injector cleaner to the gas tank once a year, I use Seafoam, but other brands will work as well

Simple test for vacuum leak
Warm up the engine fully
Let it idle
Open the hood and unplug the 2 wire connector on IAC Valve
IAC valve will close all the way and engine RPMs should drop to about 500, engine may even stall, either is GOOD, it means no vacuum leaks.
If idle doesn't drop or stays at 700 or higher then there is a vacuum leak

Underside of PCV hose elbow is a common leak point on the 4.0l SOHC

Also check the large air tube that runs from MAF sensor to the intake, if it has cracks or leaks then that will also cause lean codes

 

plugs lite brown, rail fuel filter changed thinking it might be fuel pump.
some times it has no power and is hard to start