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Hmmm...P0171 and P0174

Discussion in 'E39 (1997-2003)' started by ViolinARC, Jun 2, 2011.

    ViolinARC guest

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    OK...it's been happening for a while now but is always oddly intermittent. I know the codes mean I'm lean on Banks 1 & 2 but after clearing the codes, I can sometimes drive for 60-100 miles without CEL and then, out of nowhere, CEL is illuminated again. At first, it was just P0171 (lean Bank 1) but now it is always both codes. From my research, this leads me to believe that I have a small tear, which is getting bigger and causing a vacuum leak. Of course, that is what I am hoping for since it is the least expensive fix...LOL! BTW, the ONLY symptoms are the codes and CEL.

    It seems that most experience a failure at the air intake boot so that is where I am going to look first. I have recently replaced the CCV as a DIY so I am almost positive that the leak isn't coming from there. However, it is possible that the "Brake Sucking Pump Booster" hose is leaking because it was old, hard and kinda brittle but I wanted my baby back on the road so I opted to reinstall the old one. I guess I should look there too...:rolleyes:

    If I can't find any obvious vacuum leak, another successful MAF (HFM) cleaning is in order and then I am gonna go directly to the cats because they are original and have 150k so it's possible that they are getting clogged. I have received no CEL or strange odor to indicate failure but they are old and probably need to be replaced soon (like 50k miles ago)! I'm always into saving a dollar or two and as long as she passes emissions with flying colors, I'll curtail the expense!

    My question is: Is there a surefire DIY to test for vacuum leaks or should I go to an Indy so they can hook up an HVAC? If the latter, how much should I expect to spend? Thanks...

    Any comments or suggestions are always appreciated as I share these experiences to hopefully save others time, money and aggravation. Peace my e39 Brothers...:cool:
    • Member


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    After confirming you do not have a vacuum leak, check your fuel pressure.
    Might be time for a fuel filter (includes fuel pressure regulator) or fuel pump.
    The lean condition can be either too much air OR not enough fuel.

    ViolinARC guest

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    Thanks but I forgot to mention...

    Thanks...fuel filter has been replaced and fuel system has already been throughly checked so fortunately that will be one of the very final things to re-investigate if all else fails. ;)
    • Member
    • Technical Service Advisor


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    I vote vacuum leak. It is very common on those cars. Have you done all the intake gaskets? Throttle body gasket? Valve cover gaskets? If not, 150k means they need it.
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    My 540 (2000) just passed 100k yesterday. Nice to know what I have to look forward to in the maintenance dept...

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