89 K2500 Lean (Code 44)

[Mixelplix]

I have an 89 K2500, auto, which was swapped to a 305 (from 350) from a previous owner.

It throws a 'service engine soon' light after ~15 mins of driving, accompanied by a loss of power.

After checking the code, it's throwing code 44 - engine running lean.

The fuel pump, tank, and fuel filter is new. The injector spray pattern seems normal, and there's no residual drips after shutting the motor off.

The timing of the service engine soon light, and the error code lead me to believe the O2 sensor was feeding bad data to the ECU when it went into closed loop.

After a moderately difficult time, I replaced the O2 sensor.

Unfortunately, after taking the truck for a test drive the service engine soon light (code 44) persisted. (after about 20 mins of drive time)

I've looked over the exhaust both to the front and the rear of the o2 sensor, and there does not appear to be any leaks.

What's my next move here? I'm kinda stumped.

-Des