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2004 GMC Sierra 5.3L: P0171&P0174



Hi, I have a 04 gmc sierra with the 5.3l v8 and 12,049 miles. I just got a new diagnostic program and hooked it up and P0171(bank 1 too lean) and P0174(bank 2 too lean). The truck also has a stumble at idle which has gotten more noticable lately and a slow return to idle when switching back from drive to reverse(when parking...). What should I begin to start looking for, contaminated MAF and vacuum leaks? I tried to pull up the BARO PID in Hz but would only read in KPA's. I wanted to compare it to the barometric pressure at basically sea level(155Hz+/-) and see if the MAF was suspect. STFT bank 1&2 at idle were staying right at 0 and LTFT's 1 & 2 were about 9-11%. O2 B1S1&B1S2 were switching as they should be but when I would hold the RPM's @ 1,200 and let off, the truck's rpm would dip to 450 and both O2's would flatline lean for a few seconds. Not 100% sure if this is normal with GM's but thought I would add it in anyways. All the air intake tubes are connected and in the proper places and there are no obvious problems. Have you seen similar things like this at the dealership? I really do appreciate all the help you and everyone else gives!!!




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I would start with making sure fuel pressure was right at spec. Then I'd be looking for unmetered air entering somewhere. Seems unlikely that an '04 with only 12k miles on it would have an intake leak, but anything is possible.

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Start off troubleshooting with the simple checks - they often are the problem...


Running lean indicates that you engine is not getting enough gas. I believe the 2004s still have a replaceable fuel filter mounted on the drivers side frame rail. (The 2005s just have a screen at the fuel pump inside the tank) You could have gotten some bad gas. Have you removed the filter and seen if it is plugged?


This is a good place to start and an easy fix if it is the problem.

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