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P0171 p0174 lean code

  • greaserlarry
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11 Feb 2020 22:15 #37394 by greaserlarry
P0171 p0174 lean code was created by greaserlarry
Hello I'm a new member I mainly work on heavy diesel trucks and equipment but also some light trucks I have a dumb question I'm working on a 2002 chevy 1500 5.3 it has a check engine light the only codes it has is a p0171 and a p0174 lean codes watching the freeze frame it show mainly set at a idle short term fuel on bank 1 and 2 are 0-2 long term fuel are at 25 watching live data short term are both at 0 -3 long term are both at 25 as I increase rpm bank 1 long term gets better bank 1 stays the same at 25 I did a water test for leaky intake manifold the idle gets a lil rough and steam comes out of tail pipe so I believe it is a leaky intake gasket causing my codes my question is why would only one bank long term get better with rpms and why when I hook up a regular vacuum gauge does it not show the leak thank you
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12 Feb 2020 17:50 #37437 by scannerjohn
Replied by scannerjohn on topic P0171 p0174 lean code
my first thought is the leak on one bank is not a severe as the other bank so when you increase RPM, you overcome the smaller leak first. continue the RPM increase and it may overcome the larger leak as well. But, if the leak is too severe it may not.

you may ask, why are the LTFT's the same? because they are both max'd at 25.


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  • TheTechWhisperer
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14 Feb 2020 12:29 #37483 by TheTechWhisperer
Replied by TheTechWhisperer on topic P0171 p0174 lean code
If an RPM increase makes your trims better, you have an unmetered air leak and you are going to need to fix that regardless.

Let me ask you this first - is it misfiring at all, or are there any exhaust leaks?

As long as the answer to both of these is "no", then the next thing I would do is check the bank 1 pre-cat sensor's voltage. If you work the throttle to load up the engine with fuel, I expect to see the pre-cat sensors react to that via a higher voltage (going above .5v). A sure fire way would be to manually richen the mixture via adding propane to the intake. You want to make sure that the pre-cat o2's are capable of getting above .8v, otherwise there is a concern of a lean-biased sensor. Another clue is to compare the pre and post cat o2 sensor voltage for bank 1. If you are richening the mixture, you expect to see both the pre and post cat voltages creep up initially. If only the post-cat o2 goes to the higher voltage range (.5- 1.0v) but the pre-cat sensor on that same bank stays pegged lean (0v-.5v), that would point to a lean-biased pre-cat sensor. You cannot be extreme lean and extreme rich on the same bank simultaneously. Often the post-cat sensor can be used to tattle on a junk pre-cat sensor in that regard.

But like I said, make sure it's not misfiring and there are no exhaust leaks. Because those will both report false mixture readings.

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14 Feb 2020 19:41 #37496 by greaserlarry
Replied by greaserlarry on topic P0171 p0174 lean code
The engine is not miss firing and there are no exhaust leaks the pre cat o2 sensors are working correctly
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14 Feb 2020 19:43 #37497 by greaserlarry
Replied by greaserlarry on topic P0171 p0174 lean code
I also tried checking off the mass air flow sensor the STFT increase but the LTFT stay the same at 25
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