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TECH1994, Technician
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2001 Mercury Sable: lean..MAF sensor..The PCV elbow hose..fuel filter

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My 2001 Mercury Sable 3.0L is giving lean codes on both banks - P0171 and P0174. I have replaced 3 parts. They are the MAF sensor, The PCV elbow hose (which had a huge leak in it), and now the fuel filter. It stumbles on the road as well as the interstate. It is doing something really strange now. In park, The idle cycles in repetition from 1600 RPM down to about 600 and then back to 1600 and then it repeats like a metronome. It just cycles back and forth but the engine dies when I put it in gear. It stalls at red lights unless I keep it highly revved up about 1600 - 2000 RPM.
When the battery cable is first connected, it seems to run perfectly for about 20 minutes or so and then it just goes crazy. What could this be?

Hello, welcome to just answer

Here is what each code means

Code P0174 = Engine started, engine running at cruise speed for 3 to 4 minutes, and the PCM detected the Bank 2 Adaptive Fuel Control System reached its rich correction limit (a lean A/F condition).

Code P0171 = Engine started, engine running at cruise speed for 3 to 4 minutes, and the PCM detected the Bank 1 Adaptive Fuel Control System reached its rich correction limit (a lean A/F condition).

Here is a list of possible causes to set these and cause the symptoms you are having

  • Air leaks after the MAF sensor, or leaks in the PCV system
  • Exhaust leaks before or near where the HO2S is mounted
  • Fuel injector(s) restricted or not supplying enough fuel
  • Fuel pump not supplying enough fuel during high fuel demand conditions
  • Leaking EGR gasket, or leaking EGR valve diaphragm
  • MAF sensor dirty (causes PCM to underestimate airflow)
  • Vehicle running out of fuel or engine oil dip stick not seated
  • Carefully inspect the following areas for potential air leaks:

    • Hoses connecting to MAF

    • Hoses connecting to throttle body

    • Intake manifold gasket leaks

    • PCV disconnected

    • Vacuum lines disconnected

    • Improperly seated dip stick and tube

    • Exhaust leaks at flanges and gaskets

    If any problems are found and you repair them, then clear codes and unhook negative battery cable for approximately 2 minutes. Reconnect negative battery cable and start vehicle to let computer relearn idle.

    The reason it is okay for about 20 minutes after hooking up battery is because vehicle is running in open loop and does not care what the oxygen sensors are saying. It delivers a predetermined amount of fuel to engine until operating temperature comes up. Once up to operating temperature it goes to closed loop and now you have problems because it takes inputs from all sensors to make fuel adjustments and with a bad vacuum leak persay it will cause the problem you are having and cause those codes to set.

    TECH1994 and 6 other Car Specialists are ready to help you
    Customer: replied 9 years ago.
    Reply to TECH1994's Post: One thing I forgot to mention is that fuel consumption has increased but I can't find any fuel leaks externally. What does this mean?
    What is the best way to find vacuum leaks?
    Also, what does it mean when the engine at idle cycles between 1600 RPM and then back down to 600 and then 1600, then 600, and it just repeats up and down, up and down like that, then it dies when i place it in gear? What does that indicate?
    Customer: replied 9 years ago.
    But if it was an exhaust leak before or at the o2 sensor, wouldn't that only give a bank 1 or bank 2 error and not both? I am receiving both 171 and 174.
    Yes fuel consumption will increase with vaccum leaks. Also the engine will idle erratically and rpms go up and down and vehicle can die with put in gear. Everything you have mentioned indicates a problem or vacuum leak in one of the above areas I mentioned. The best way to check for vacuum leaks that I prefer is to spray carb cleaner around intake very carefully and vacuum lines. If idle changes then there is your problem. You could also try water in a spray bottle but I find the carb cleaner more successful or brake cleaner. However, be extremely careful when doing this as if it hits an open spark it could ignite. I would go as far really to recommend you letting a qualified shop perform these tests. The labor to check for these leaks should not be anymore than an hours labor.
    Customer: replied 9 years ago.
    Yes I also think it is a vacuum leak. Your advice has been helpful. Thank you.

    Good luck, if you need anything in the future you can always request me or speak to another just answer expert.

    Thank You