P0171 and engine falls flat on it face when pedal is pushed to the floor.
 

I have a 2000 5sp manual Toyota 4Runner and have the CEL for about 4 months. 205K miles, replaced sparks and timing last august. The OBDII scanner noted two errors detected related to code P0171 "System too lean". Noted a couple things while driving.
1. Noticeable loss of power during acceleration up hills and maintaining speed up hills
2. On flat roads, when I push the pedal to the floor fast, my engine will stall, like its overworked. When release it back to 75%, it will pick up the acceleration again. I can evenutually get it to the floor, but I must do it gradually and build up the RPMs.
3. Start my car in morning after sitting overnight or at least 5 hours, it will rough idle for approximately 30 seconds and then the computer will adjust the engine and kick it up to 2000 rpms for another 2 minutes before it comes back down to 800. Additionally, if I try to drive cold right after I start it, it will jerk in 1st and 2nd gear like a person who's trying to drive stick for the first time. You can press down on the gas and there's no immediate response and then you get the lunging effect.

P0171 has long list of possible trouble areas. Here's what I have done so far.
1. Air Induction - Checked, noted no leaks.
2. Injector blockage - Just added more cleaners. Seemed to help.
3. MAF - Cleaned and held off from replacing so far.
4. Engine Coolant sensor - replaced last year when timing was done in Aug.
5. Fuel Pressure. Checked lines, no leaks. Replaced Fuel Filter - seemed to help. Have not touched fuel pump or fuel pressure regulator yet.
6. Gas leak in Exhaust - Nothing done. I think this would be noticeable if there was a leak.
7. A/F sensor - replaced bank 1, a year ago.
8. PCV valve and hose - replaced, check for vacuum and okay.
9. EFI main relay - checked with multimeter. no issues noted.

I'm trying to pinpoint where my errors are. Any suggestions or help would be greatly appreciated!

When was the last time you cleaned the throttle body?

Follow up:
I have cleaned my throttle body and also changed my fuel pump (yes, I had to drop the gas tank to do it). Cleared the CEL via the scanner and started up the engine. Rough idled for 3 minutes. Pressed the gas pedala and couldn't get the engine to respond pass 1500 rpms. Kept revving engine for approximately 5 minutes and built up enough rpms to 3000.

Ran scanner again and now in addition to P0171, now have P0300 (multiple cylinder misfires), which is probably the intial result of the changing of fuel pump and fuel pressure to the engine.

Other things I noticing:
1. I am still losing power steadily when I climb hills even if I downshift and my engine will "buck" as if i wants more gas.
2. I can feel the engine "bucking" as I gradually accelerate on a flat surface like a tapping feeling.
3. The engine is still "stumbling" when I press the pedal to floor quickly, but the engine will pick up eventually as I back off the pedal slowly.

Next steps:
1. Change MAF?
2. Change vaccum hoses?
3. Change O2 sensor again?

Any suggestions would be appreciated.

Follow up:

I brought the truck to a mechanic and he smoked tested the air intake and there was no leaks in my vaccuum hoses. He then tested my Mass Airflow sensor and saw that it was no reading correctly. We replaced the MAF and I'm happy to say that cleared all my issues. All my power is back. No stumbling when pedal is to the floor and best of all, no CEL.

So in summary, I replaced by fuel filter and fuel pump for no reason. The MAF costed $200. Hope this helps out others in the future.