Idle in neutral and drive - why are they different? Auto trans

Discussion in 'SN95 4.6L Mustang Tech' started by Red03Mustang, Dec 31, 2005.

  1. Red03Mustang

    Red03Mustang New Member

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    Does anyone know why the idle is higher in neutral than when the car is in drive? My car has an auto trans.

    I noticed that in my Predator, the neutral idle is around 760rpm while the drive is set around 560rpm (roughly).

    I'm just wondering, why the difference?

    Also... I'd love for my car to stick at 800rpm, idle or drive. If I set it to this with the Predator, when I start my car for the first time in a day, when everything is completely cold, the car idles somewhere around 400rpm more than whatever I set the neutral idle to, before it finally quiets down after warming up for about 2 minutes or so.

    Why would the engine run FASTER when it first starts? I thought it was worse on an engine to run harder when it is cold instead of better. So the logical sense doesn't seem to be there to me. The only thing I can think of that this would happen is because the engine is trying to warm up quicker, at its own expense?

    Also, with the idle set higher (800rpm or more) the car surges a lot when shifting into reverse. Is this something that just comes with the higher RPM setting, and is unavoidable?

    Thanks in advance for any help.
     
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  2. 01SN8K

    01SN8K New Member

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    the higher idle when a car is cold is from the cars computer. it waits for the o2 sensors to heat up before switching itself from closed to open loop. the difference between the two idles is the load. neutral imposes no load while in drive there is a load dropping the engines rpm's down. if there is no idle problem why mess with it?
     
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  3. triggz

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    its actually better to run a higher rpm at cold-start to warm it up faster. think about it, if you went out and started your car and went back inside.. if it idled at 500rpm for 15minutes, its not gonna heat up much at all..translating to a lot of internal wear. the old RX7s (maybe the rx8 too) is actually designed to turn 2000rpm at cold start because they create a lot more wear at low temps.

    the best way to warm up the engine is to drive it slowly and shift under 2grand until it reaches at least 150*. the temp gauge begins moving at 150*, once it leaves the deadzone, feel free to open it up some more.

    as for the surging, its going to feel a little more aggressive about gettin movin simply because it is revving higher, so youre putting out a little more hp/tq than youre used to feeling at initial gear changes.
     
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  4. mrvax

    mrvax Stay thirsty my friends
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    An automatic transmission has no load on the engine in neutral so the idle speed will be highest. When you put the car in Drive, there is a small load on the engine so the idle will come down some. It is normal.

    Old school way to see if the idle is correct: The car should move at about a slow walking pace when the car is in drive on a flat surface with no gas pedal being applied.
     
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  5. CanadaStang

    CanadaStang New Member

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    What he said ^^
     
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  6. ToplessStang

    ToplessStang Member

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    Correct about all except when an engine is started cold or at WOT it is in open loop and when it is at normal operating temp it goes into closed loop :nice: .
     
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  7. CanadaStang

    CanadaStang New Member

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    Good catch Topless, I missed that when I first read it.
     
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  8. 03ghoststang

    03ghoststang Well-Known Member

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    that is how they are setup from factory but if you were to get cams then the idle would have to be bumped up a little
     
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  9. jstreet0204

    jstreet0204 Active Member

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    Correct except the engine does not have to be at normal operating temp to go into closed loop. Closed loop change-over at start up is time base and dependant on ECT. "Cold Start" in the EEC is considered to be below 10 degrees, "Hot Start" is above 170, "Warm Start" is in between. Warm start delay is 16 seconds, Cold start delay is 160. So it doesn't wait for the engine to completely warm up. :nice:
     
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  10. ToplessStang

    ToplessStang Member

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    I've taken every Ford Injection course they have given and I am not here to confuse people. I was just clearing up a simple mistake he had written in a clear and simple fashion. I could continue writing how the PCM goes into open loop to cool off the cats in certain situations and all the other strategies the PCM is programmed to adapt to but I don't think it has anything to do with this thread. Bottom line cold start meaning COLD ENGINE = OPEN LOOP.

    CanadaStang thanks :nice: .
     
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  11. jstreet0204

    jstreet0204 Active Member

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    For about 2 minutes... My point is it is the delay is longer to get to closed loop, not that it waits until it ECT gets to 180 degrees which is a common misconception. So if you don't want to confuse people you should have probably cleared that up.
     
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  12. Red03Mustang

    Red03Mustang New Member

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    I understand what you're saying, but what I mean is, the car is actually programmed to run fewer RPMs in "D" than in "N". That's the part I don't get.

    I can program both "D" and "N" to 800 RPM, and it'll stay there, no matter which one I'm in.

    I'm just wondering if there is a reason the car is programmed to hold a lower RPM in "D" than in "P" or "N". It's not the engine load if the car is specifically programmed at a certain number RPM, and I'm just wondering WHY it is programmed lower in "D" than "N" from the factory, instead of just a steady RPM accross the board.

    I hope that makes sense.
     
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  13. 03ghoststang

    03ghoststang Well-Known Member

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    even though it is programmed to be like that but it's mainly due to the load put on the engine by the tranny.
     
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  14. Red03Mustang

    Red03Mustang New Member

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    That's what I mean. Why is it programmed like that?

    If I program it to 800-800, then it will stay at 800.
     
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  15. Tommy 00GT

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    I don't know how some of you are getting the riddculous idea that the car is programmed to run at a certain rpm in D and N. It's just that the engine is idling lower in D due to the load while being in a drive gear.

    non computer controlled cabureted cars also idle lower in D so that tells you something.
     
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  16. jstreet0204

    jstreet0204 Active Member

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    Because "it is" programmed that way. This link will show the parameters used. http://www.sctuner.net/techinfo/calibration/#nidle



    To clear it up a little, the programmed settings are there to know what rpm to maintain the idle at. At idle, the eec is constantly making adjustments to the IAC valve in order to keep the idle smooth and not fluctuating. If the idle creeps below the programmed setting, the iac valve will open some, to let more air in, and vise versa. The reason they are not set the same, is because you can maintain a lower idle in drive because of the load. The same idle rpm in N would cause it to stall.
     
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  17. 03ghoststang

    03ghoststang Well-Known Member

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    yes it would but i dont see a reason to change it unless you have cams installed
     
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  18. DMAN302

    DMAN302 My mom says thanks for the pearl necklace.

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    Anyone notice the idle is quite a bit smoother when in drive, than at idle when warm. Warm idle neutral seems to have a little vibration too it. Both 04 auto's I've been in..wifes and another both had vibration felt in neutral that were not there in drive.
     
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  19. PlatinumDevil

    PlatinumDevil Member

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    mine purrs in neutral and feels like an SUV, and then in drive it has a nice rumble, feels like the 68 cougar... I dont know why you'd want your idle in drive to be higher??? to waste gas i guess??? :shrug:
     
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  20. Red03Mustang

    Red03Mustang New Member

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    Yeah, to waste gas. :rolleyes:

    Because the car sounds better, and idles smoother. That's why.
     
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