P0171 Toyota -

Question

1999 Toyota 4Runner
SR5
3.4L V6
250,000 miles

Issue: When engine is cold it will backfire with the throttle is opened up past 50% or over 2500 rpm. Once engine is at operating temp, the backfiring will stop. Upon acceleration there is hesitation from the engine, like it’s going to stall out. When I opened the throttle manually I can hear a “suck” of air at the throttle body. When the engine is at operating temp, the hesitation is not as severe, but still noticeable. It has stalled out in the past on my wife at a stop light, but that has not happened in the past few months.

The only code that is present is 0171, system too lean.

Here is what I have done myself:
1. Replaced upstream o2 sensor
2. Replaced TPS
3. Cleaned MAS
4. Replaced plenum gasket
5. Replaced IAC gasket
6. Replaced throttle body gasket
7. Cleaned throttle body
8. Replaced spark plugs with OEM plugs
9. Replaced plug wires
10. Replaced fuel filter
11. Replaced PCV
12. Checked all vacuum hoses
13. Replaced valve cover gaskets (separate issue)

I feel like I have addressed all the air issues and the remaining fuel issues are fuel pressure (which I don’t know how to do) fuel pump, fuel regulator. I can do the fuel pump but I would hate to do that if it’s not the culprit.

I am at a complete loss now and extremely frustrated. Any help you can offer would be greatly appreciated.