Chevrolet Repair Questions? Ask a Chevy Mechanic for Answers ASAP
P0171 and P0174 are both codes indicating a lean condition on bank 1 and bank 2. If the fuel trims are that high it is dumping fuel in. If you have verified there are no vacuum leaks, check to see if there are any exhaust leaks before the secondary oxygen sensors. If there are none, I would venture to guess you have an issue with either the mass air flow sensor not reading properly, or the oxygen sensors not reading properly. Since it is both banks I'd lean toward the mass air flow sensor. Inspect the mass air flow sensor for debris, and if possible watch a graph of the grams/second of air on a scan tool. It should be a steady angle up on acceleration, if it is erratic replace the sensor.
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hey andrew thanks for responding. Checked the MAF while accelerating full throttle (mostly) up through 3rd gear. MAF gm/sec increases steadily with no dropouts as rpms come up. I really feel silly not being able to figure this one out. Usually it would be so obvious. After the truck runs 20-30 minutes it smooths out a lot by feel, but the misfire counters on 7 and 8 just climb constantly
Hmm. Have you thought about doing a fuel injection cleaning?