Lean Code 171 will not go away!

What would cause a P0171 Code (Lean), causing the MIL to come on, when idling but not at running RPM?

If I keep the RPMs up to 2500+ >>> no codes & no MIL
If I idle for a minute or two >>> P0171 code & MIL comes on (steady not blinking).

I am tuning a 2004 2WD Prerunner w/ 2.7L & auto trans. I have checked and/or replaced almost everything on the motor.

It has to be air intake leaking or exhaust leak right?

 

Sounds like you may have a vacuum leak does it idle rough? Exhaust leak should not cause this. Buy a can of carb cleaner and spray it in various spots and see if the idle changes.

 

It does idle a little rough but not as bad as it has...and I only get the MIL when idling.
I tried the spray and got nothing.
The only intake or vacuum hose that has not been replaced is the hard plastic, molded piece, from the air box to the throttle body with the resonator(?) box.
All the small vacuum lines, PCV and other valve cover hose, as well as an entire air box, are all new. The evap and fuel reg. lines are new. New MAF & IAC. New plugs, coils and injectors. New fuel reg, filter, pressure sensor, screen and ran pump test. All of this during a head replacement and valve adjustment. It's not running hot, loosing oil or water. Cylinders all have good, not perfect but all close, pressure.
It sounds like it has a small exhaust leak. The studs on the manifold were tough to get tightened, so that connection to the exhaust pipe is iffy. It has a new gasket.
I am down to only a couple of parts it could be. I'm trying to decide whether to tear the intake apart again and try to test for leaks or pull the exhaust manifold and check for cracks.
Intake you think? It's a coin toss for me...I'm exhausted myself.

 

O2 sensor?

 

This might be a stupid question... but have you disconnected your battery's negative cable long enough to allow the ECU to reset itself?

Here's a complete description of your code.

P0171

Possible causes
- Intake air leaks
- Faulty front heated oxygen sensor
- Ignition misfiring
- Faulty fuel injectors
- Exhaust gas leaks
- Incorrect fuel pressure
- Lack of fuel
- Faulty Mass Air Flow (MAF) sensor
- Incorrect Positive Crankcase Ventilation (PCV) hose connection
What does this mean?

Tech notes
If the P0171 is combined with the P0174 code, it's very likely that the problem is caused by an intake leak. If there are no intake leaks, the next step is to replaced the air filter and clean the air flow meter. If the problem persist the front oxygen (O2) sensor may need to be replaced.


Read more: http://engine-codes.com/p0171_toyota.html#ixzz3AxHyyu2g

 

I haven't read the above posts and don't know if you solved the problem but it is the MAF. I myself had the same problem and know others as well. A replacement MAF from an autoparts store will fix it. The ECU will take some driving to register the new MAF