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Andrew Martin
Andrew Martin,
Category: Chevy
Satisfied Customers: 203
Experience:  Technician at Wile Hyundai
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Vehicle arrived with codes p0300, p0305, p0307, p0308, p171,

Customer Question

Vehicle arrived with codes p0300, p0305, p0307, p0308, p171, and p174. Engine runs rough at idle and lack of power. runs smooth when held at about 1500rpms. Have constant misfires showing on scanner on cylinders 7 and 8, and a couple misfires on cylinders 3 and 5. We have checked spark, replaced spark plugs and wires, checked fuel pressure (about 55psi koer), and checked to make sure injectors are all firing. Long term fuel trim on both banks was in the 20%+ range
I thought I may have heard a vacuum leak type of noise, however, we have smoked the intake manifold and found no leaks. Additionally, watching data on scanner, we verified egr valve was closed. Also, engine stalled as EGR valve was opened (as it should). Verified there was no vacuum leak at purge valve.
Submitted: 3 months ago.
Category: Chevy
Expert:  Andrew Martin replied 3 months ago.

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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Customer: replied 2 months ago.

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

Expert:  Andrew Martin replied 2 months ago.

Hmm. Have you thought about doing a fuel injection cleaning?