Too bad you replaced all that stuff
First test
After engine is warmed up to normal temp and idling, unplug the IAC Valve, it is the tube mounted on the upper intake at the front.
This is what the computer uses to set stable idle, it is basically a "controlled vacuum leak"
When IAC Valve is unplugged it will close, engine RPMs should drop to 500 or engine may even stall, either is good, it means no vacuum leaks.
If RPMs stay above 700 then you do have a vacuum leak.
Leave IAC Valve unplugged
Now disconnect one vacuum hose from intake at a time and plug intake hole with finger.
If idle drops to 500rpm or engine stalls then that hose has the leak.
If all hoses check out OK then it is a gasket leak
Since you are only getting p0171 and not p0174 as well it means it probably isn't a general hose or upper intake leak(or MAF sensor) since these would cause BOTH banks to show Lean so you would get both p0171 and p0174 code.
Lower intake leak on passenger side(p0171) would cause it, O2 sensor on that side as well.
Exhaust manifold leak on passenger side sucks air in as well as letting exhaust out.
O2 sensor can only "see" oxygen(air) so an exhaust leak on that side would make O2 sensor show Lean(too much air).
Lean code doesn't mean engine is actually running Lean.
Computer controls fuel injectors, it opens a fuel injector for a calculated amount of time to let in the calculated amount of fuel needed to get 14:1 air:fuel mix.
Computers calculation is based on MAF(mass air flow) sensor data, if MAF says 14 pounds of air came in then computer will add 1 pound of fuel spread out along the 6 fuel injectors to get......14:1 ratio
The 14:1 ratio calculation = 0 Fuel Trim
After fuel is burned the O2 sensor shows if there is too little oxygen(rich) or too much oxygen(lean) in the exhaust on that bank(side) of the engine.
If there is too much oxygen(lean) computer opens fuel injectors on that bank longer, adding more fuel, Fuel Trim changes to +1, then +2, then +3, ect.......until O2 sensor is showing correct oxygen level in the exhaust on that side.
(if O2 sensor showed too little oxygen then Fuel Trim goes to -1, -2, -3, ect.......at -15 you would get Rich code on that bank)
Computer and O2 sensor communicate several times a second, Fuel Trim is in constant adjustment.
If Fuel Trim should get above +15 for any length time, computer will set the Lean Code, if it is just the passenger side of engine that is above +15 then it will only set P0171, if both then it will also set P0174(drivers side).
Computer can't monitor Fuel Pressure, it expect 65psi of pressure at the injectors on 2000 Ranger Returnless Fuel system.
If Fuel Pressure was actually 30psi computer's calculations would be off and it would have to open injectors longer to get fuel needed, so higher fuel trim numbers.
BUT.....you would get Lean codes for both banks, because all fuel injectors would have lower pressure.
Computer is always adding extra fuel so engine isn't running Lean, the code is to tell owner that something is wrong with the computers calculations.
Since only one bank of engine is showing Lean it limits possible causes.
I would pull out the spark plugs on the passenger side.
If they look dark then that side is actually running Rich, which would mean faulty O2 sensor or exhaust leak, these report Lean to computer so computer adds more fuel to that bank and it is really running Rich.
If one spark plug on that bank looks different than the rest then that cylinder or injector could have a problem.
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and just changed the Plenum/Upper intake gasket. The vehicle runs great but can't get the problem fixed withe the PO 171 code. It's some thing I want to solve, any suggestions?
