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Fuel Trim - Short and Long Term

When my MAF CEL comes on the, the "frozen" data on my analyer shows both Short and Long Term Fuel Trim at -100.6 What does that tell me?
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  • edited August 2009
    It's mainly saying that it's not able to tell you anything. With the light on, I think the frozen data is stuck on a meaningless number. It could also be the default setting when the MAF or the computer isn't working.
  • edited August 2009
    Thanks - From the Sept 2006 issue of Motor Magazine there is a great article on fuel trim. I had my Acutron on continuous read and when my MAF error code came in, it locked in the other readings. With both being VERY negative, it would appear the engine was trying to lean out..a lot. Not sure why....too much fuel pressure?

    From the Motor article:

    Short-term fuel trim
    (STFT) and long-term fuel trim (LTFT)
    are expressed as a percentage, and the
    ideal range should be within 5%.
    Positive fuel trim percentages indicate
    that the powertrain control module
    (PCM) is attempting to richen the
    fuel mixture, to compensate for a perceived
    lean condition. Negative fuel
    trim percentages indicate the PCM is
    attempting to lean out the fuel mixture,
    to compensate for a perceived
    rich condition. STFT and LTFT percentages
    are the adjustments made by
    the PCM to maintain the 14.7:1 ratio.
    No matter what the driveability issue
    happens to be, the fuel trim window
    should be used first to check the
    STFT and LTFT parameters
  • edited August 2009
    With real, not simply indicated, very lean conditions, the engine should be having some severe driveability problems, like, misfire. What symptoms does it have?
    An EGR Valve popping open, suddenly, could cause a sudden leaning. A PCV Valve which flows too much can lean an engine. A brake booster which loses it's vacuum can lean an engine; etc.
  • edited August 2009
    Fuel trim "counts" be it long or short term can range between 0-255. Any value below 128 indicates a rich condition and the fuel delivery is being reduced (reduced injector pulse width). You should test you 02 sensors wave form and voltage swing. So you are seeing a rich condition,I don't know how to classify it as being "slight" or "moderate".

  • edited August 2009
    If the analyzer showed -100%$ on the ST and LT Fuel Trim, didn't the PCM think the engine was way too rich and it was trying to go full lean?

    I've been fighting the MAF CEL for months. Today, for the first time, the engine really bogged down during a moderate acceleration in 2nd gear. The CEL flashed for about 10 seconds, then the engine picked back up. I then had a new MAF CEL and a "pending" Random/Multiple Cylinder Misfire Detected.

    Most of the Saab sites would point the finger at the MAF...8 years old and 52k miles.

    Not sure how how to troubleshoot the MAF. I've cleaned it twice. Pulled it out and got 4kohms and 9kohms across various combinations of the three pins. With the engine idling, the plug for the MAF show 12 volts and 5 volts.
  • edited August 2009
    There are voltage values, from the Mass Air Flow (MAF) Sensor, which correspond to the air flow that the senor is measuring, and sending those values to the engine computer. There is a voltage to air flow table in the tech data for your car. In the "Emissions and Driveability" section for your car, at autozone.com, you might find the table. These values will tell if the air flow rates the MAF sensor is sending to the engine computer are correct.
    Addum:
    The MAF on your car may put out a signal of a variable frequency (Hertz) to represent air flow. Repair manual, please.
    See this article on the MAF: thttp://www.aa1car.com/library/maf_sensors.htm
  • edited August 2009
    I would try another tack. Reading the trim values tells you that the ECM is commanding full lean in closed loop situations. How is driveability in open loop conditions i.e. on warmup, throttle transitions, and WOT? I am wondering if you might have a fuel pressure regulator problem that is leaking extra fuel into the intake. Correct me if this engine does not have a vacuum referenced fuel pressure regulator.

    Always learning.
  • edited August 2009
    Driveabilty is typically good 95% of the time. Starts and idles fine. Accelerates well and the boost will push right up to the yellow-red line. Had one odd event on Sunday. Moderate acceleration in 2nd gear....engined bogged down, check engine flashed for about 5 seconds, then the problem cleared and it ran fine. Wound up with MAF code and a misfire code.
  • edited August 2009
    You need to determine which sensor(s) is mistaken: the one saying there is too little air (the MAF); or, the one saying there is too much fuel (the oxygen sensor). You need to look at the signal values from both, to see which one is incompatible with real conditions.
    A gas analyser, at the tail pipe, will show if the engine is running rich, or not. http://findarticles.com/p/articles/mi_qa3828/is_200503/ai_n13486252/?tag=content;col1
  • edited August 2009
    Just ran some checks with the OBD reader. The initial ST/LT Fuel Trim all fell in the +/-5% range as I ran from idle to 1500 rpm, 2500 rpm, 3500 rpm. The MAF flow increased from 4.35 to 15.92. The O2S1 reading (upstream of CatConv?) varied from .06 (lean) to .80 (rich) as I held the RPMs.

    Of particular note, the ST Fuel Trim dropped to -16% (negative) after I dropped back to idle and the engine stumbled occassionally. Too rich...trying to go lean.

    Not sure what to make of it. I was thinking maybe an idle vacuum leak, but that would be showing too lean?
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