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Thread: MAP Sensor

  1. #1
    Join Date
    Aug 2009

    MAP Sensor

    Hi guys,

    I have read a lot about car's MAP sensor and it's function but there are some areas that are not completely understood yet!

    Let me put it this way:

    My car's MAP sensor reads 337 mbar (millibar) @ idle (800 RPM).
    When i open the throttle, the reading DROPS (for example at 2500 RPM it shows around 260 mbar)

    With the engine OFF and ignition ON it reads exactly the barometric (atmospheric) pressure of our city which is about 870 mbar.

    My car is a Peugeot.

    The problem is that i saw in a video that MAP sensor reading of a Ford was the exact opposite of my car! When he opened the throttle the reading went HIGHER not LOWER!

    (both cars have naturally aspirated engines)

    I think that my car's MAP shows manifold VACUUM and that Ford's MAP shows manifold PRESSURE not vacuum.

    I know that highest vacuum is created in high rpm and closed throttle (like going downhill)

    In this situation my car shows highest numbers and that Ford shows lowest numbers.

    But someone told me that my car isn't showing vacuum, it shows Absolute Pressure. Someone else said something about Relative Pressure.

    My interpretation of my own car is this:

    @ idle the MAP reads 337 mbar. It means that @ idle my manifold pressure is 337 mbar lower than atmospheric pressure so my REAL manifold pressure is 533 mbar (870 - 337 = 533).

    So my MAP does NOT directly show my manifold pressure but it shows how much my manifold pressure is lower than ambient pressure.

    Am i correct?

    Please shed some more light into these concepts.

  2. #2
    Join Date
    Apr 2003
    "A string is approximately nine long."
    Egg Nogg 02-04-2005, 05:07 AM

  3. #3
    Join Date
    Oct 2012
    You have a MAP sensor. 870 with engine off is correct.
    337 mbar at idle sounds about right.
    260 mbar at 2,500 sounds about right.
    To see an increase, you need to snap the throttle from idle. You will only see the increase while the engine is accelerating. You are using the inertia of the engine to act as a "load". Accelerating on road or driving up a hill will allow you to hold large throttle openings for longer periods to see an increased MAP.

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