Definition of Closed Loop vs Open Loop

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  1. #1
    Senior Member Array Boosted98GT's Avatar
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    Default Definition of Closed Loop vs Open Loop

    Eric,

    Could you take a moment to describe the difference between Closed Loop and Open Loop. Also, please mention how the ECU knows when to switch between the two.

    -Joshua

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    Premium Member Array Eric Brooks's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    Quote Originally Posted by Boosted98GT
    Eric,

    Could you take a moment to describe the difference between Closed Loop and Open Loop. Also, please mention how the ECU knows when to switch between the two.

    -Joshua
    Open Loop and Closed Loop are two terms used to describe the fueling mode the computer is in.

    In closed loop fueling, the computer takes readings from the oxygen sensors which read +/- of 14.64 AFR (Stoich) and makes corrections for it. If your car is idling @ 13.5 AFR, the 02s can see this, and will know to pull out enough fuel to get back to stoich.

    In open loop, the fueling is purely based off the MAF transfer function and the open loop fuel table. This table has to be calibrated any time you make MAF changes, and changes that affect the airflow of the MAF.

    When a car is started, the computer is in open loop until certain criteria is met. The o2 sensors need to be heated to work properly, so this is the main reason there is a delay.

    After the sensors are heated up, the car goes into closed loop and the O2 sensors make their corrections.

    The computer will go back into open loop under a variety of conditions. Throttle Position (TP) is one way.

    There are parameters in the computer that determine what the TP is for Open Loop, and other things.

    When that TP is reached, and the car goes open loop, and richens up.

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    Senior Member Array Boosted98GT's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    What are the criteria the ECU looks for in order to go into closed loop?

    Coolant temp is one of them correct?

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    Premium Member Array Eric Brooks's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    No, many people used to think that though.

    I remember years ago, the magazines said that too cold of a thermostat would cause the computer to not enter closed loop.

    Its basically on a timer as to how long the car stays in open loop before going into closed loop at startup.

  7. #5
    Senior Member Array Boosted98GT's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    Thanks for clarifying that. This forum is helping me already!

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    Premium Member Array Eric Brooks's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    Good! Thats what I hoped for.

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    Senior Member Array Maniax Racing's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    so whats the down fall for tuning a car totally in open loop and bypassing the 02's.........

  10. #8
    Premium Member Array Eric Brooks's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    Emissions and Fuel economy.

    When I tune a car that has bad 02's I just tune the car in open loop, shutting the 02s off, and tell them to get new 02s and I give them a file with the 02s turned back on.

    On EEC-IV cars, like 5.0s, I generally shut the 02s off anyway, since there is no real way to datalog what the PCM is doing anyway. I tune them totally in open loop.
    Last edited by Eric Brooks; 07-31-2006 at 04:49 PM.

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    Senior Member Array Maniax Racing's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    Quote Originally Posted by Eric Brooks
    Emissions and Fuel economy.

    When I tune a car that has bad 02's I just tune the car in open loop, shutting the 02s off, and tell them to get new 02s and I give them a file with the 02s turned back off.

    On EEC-IV cars, like 5.0s, I generally shut the 02s off anyway, since there is no real way to datalog what the PCM is doing anyway. I tune them totally in open loop.
    How bad will it affect your gas mileage and will it need to be re-tuned for different climates since it is running totally off of mass air transfers

  12. #10
    Premium Member Array Eric Brooks's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    Yes, I corrected it.

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    Premium Member Array Eric Brooks's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    Quote Originally Posted by Maniax Racing
    How bad will it affect your gas mileage and will it need to be re-tuned for different climates since it is running totally off of mass air transfers
    I am not sure how bad the mileage would be, I don't think it would be horrible, if the MAF is dialed in.

    No, a MAF system does not need to be retuned. For different climates or altitude. Once the MAF transfer function is dialed in, all is good.

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    Senior Member Array Maniax Racing's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    i might need to pm you or call you on a problem i am dealing with, maybe you can help

  15. #13
    Premium Member Array Eric Brooks's Avatar
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    Default Re: Definition of Closed Loop vs Open Loop

    Sure, shoot a PM.

  16. #14

    Default Re: Definition of Closed Loop vs Open Loop

    Since someone mentioned something about the coolant temp I havea question. When tuning a car, how much timing do you set it up to pull when it gets warmer. EG- Every 10 degrees coolant, pull X-amount of timing?

  17. #15

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    That would explain why my truck seems to be rich on startup. From what you have said, the pcm is in closed loop during idle and part throttle. Then back to open loop during WOT or heavy acceleration right?

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