Ok so now after top end has been rebuilt all of a sudden today the SES light came on telling me it was running lean. Outside air Temp was 25 F. Intake temp was 58.6 F. Leak down test shows no problems. What else could it be? Please see attached screenshots for more information. I am wondering if there may be a problem with the snorkel between the MAF/IAT and the engine. Any Ideas?
99 Blazer ZR2 4.3 Vortec

 

LTFT at 25 is the maximum, it doesn't go any higher Definitely lean on both banks. If there was a leak in the inlet, it would also cause both banks to run lean BUT, it would also set a DTC for intrussive air. You mentioned leakdown test shows no problems, but nothing about fuel pressure. Low fuel pressure will cause a lean condition on both banks.

Install the fuel pressure tester. Install a jumper wire from battery positive to the fuel pump prime terminal in the underhood fuse panel:



When the pressure stabilizes, write it down. Disconnect the jumper wire. Wait 10 minutes and read the pressure, write it down. Post the two readings.