Code 0171 - sets only when cold.

1999 3.4L 4Runner

Code 0171 System too lean- sets only when vehicle is cold.

When cold, application of accelerating throttle results in too much air at the throttle body, and the vehicle lugs down. Warm with ECT normal; application of accelerating throttle results in too much air and the vehicle lugs down. Warm with ECT on power; it drives normal. Idle is normal warm or cold.

I have changed fuel filter, both O2 sensors brand new with factory units, checked all hoses for vacuum leaks. OBD checker gets check engine light and code after warm up, but if OBD checker erases code after engine is warm, the check engine light stays off, and vehicle will only run 'normal' in ECT power mode.

Since the thing runs fine warm in ECT power mode, I have ruled out clogged injectors. ECT system seems suspect, but have no background with it, does it modify fuel pressure?. A real puzzler to me..any ideas? Thanks in advance.