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A few questions regarding P0171 -- System Lean Bank 1:

  1. If there is a problem with the fuel-air ratio, which this is basically about, why is it limited to only one of the two engine banks of a V6?

  2. Are there guidelines/heuristics to determine whether the cause of this is air intake (such as dirt around the MAF sensor or vacuum leak caused by a hose puncture) or the fuel pump is weak inadequate? Since I am also getting P0101 - MAF, I am inclined to think the former. I replaced the MAF sensor a couple of months ago so it shouldn't be that.

  3. I noticed the fuel efficiency is slightly off. Could this be the reason why?

  4. How safe is it to drive the vehicle?

Other than the CEL and 3. above, the truck (2007 Tacoma V6) is running fine.

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  • Have you checked for an exhaust leak before the O2 sensor? That would affect only one side and would give the computer a lean condition (too much oxygen in the exhaust for that bank) and in doing so will cause the ECU actually to go uber rich trying to fix the lean.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Apr 7, 2023 at 23:57
  • @Pᴀᴜʟsᴛᴇʀ2 dunno how to check other than the cat coming out of the engine does not seem to leak any exhaust or smoke
    – amphibient
    Commented Apr 8, 2023 at 2:43
  • @Pᴀᴜʟsᴛᴇʀ2 -- also, could it be that one or more of my cylinders may be leaking, which then in turn makes ECU send more fuel there?
    – amphibient
    Commented Apr 8, 2023 at 2:44
  • Is your long term fuel trim on bank 2 running rich?
    – Bill Sullivan
    Commented Feb 19 at 14:15

1 Answer 1

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  1. Its limited to bank 1 because unmetered air is only going into one side (3 cylinders). So basically air that is not being registered by the MAF is getting in after the intake manifold divides. Other possibility would be that bank 1 fuel rail/injection pressure is low, but i would think it would affect performance if that were the case. You could check it with spraying propane or starter fluid around that side and listening for a surge, although due to fire risk i prefer using water and listening for a drop in power (check out a scanner danner video).

2.you could try some MAF cleaner as well as cleaning the connector. Idk of specific hueristics that would pinpoint the exact cause. Also check your air filter.

  1. Yes, this is exactly why. System is running lean with too much air, so it tries to compensate by dumping more fuel in combustion.

  2. Safe? Yes, sort of. your fuel economy will suffer and with all that extra fuel your catalytic converter could eventually melt.

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  • do you think it could be that one or more of my cylinders may be leaking, which then in turn makes ECU send more fuel there?
    – amphibient
    Commented Apr 9, 2023 at 17:55
  • I’m not sure what you mean by leaking. But whatever the case, yes the ECU is seeing that there’s too much air getting into one half of your engine (bank 1) and sending more fuel there. Aqain, pour some water around the bank 1 side and listen for a subtle drop in power. Start with the engine cold and start trying to pinpoint where air is getting in. May be something as simple as an old leaky intake gasket.
    – Dont Know
    Commented Apr 9, 2023 at 23:50

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