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Oscillating idle on my 308 QV

Discussion in '308/328' started by Tifosi2011, Jun 6, 2011.

  1. Tifosi2011

    Tifosi2011 Formula Junior

    Apr 3, 2011
    307
    Burnaby, BC Canada
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    John
    I have a 1985 308 QV. I've noticed a slight oscillating while idling when the engine is fully warmed up. It oscillates 1000-1050 +\-. should I be concerned? Or is this something that is common? Thanks for your input everyone. Go Canucks!
     
  2. PhilB

    PhilB Formula 3
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    Feb 17, 2004
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    If you have the cat(s) on it still and the oxygen sensor in place, it should be the "too lean, too rich" cycle......normal operation.
     
  3. Badman

    Badman Formula 3

    Mar 4, 2007
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    Bruce Wayne
    US Spec car? It's the lambda circuit oscillating. I believe this is normal behavior.
     
  4. miketuason

    miketuason F1 World Champ
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    Check your O2 sensor , unplug it (green wire) and see if the idle change.
     
  5. teatime

    teatime Rookie

    Mar 8, 2009
    33
    NW Burbs, Chicago
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    Todd A.
    I just replaced the Cat on my car and now I have the same issue. When the O2 sensor is unplugged the idle doesn't move, as soon as it is plugged in, the idle begins to hunt.

    Does everyone agree this is this normal?
     
  6. stevel48

    stevel48 Formula 3
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    Jan 4, 2005
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    Steve (85 308 Owner)
    my car does it and I have no idea. I think some swing is normal but too much isn't. "Too much" needs to be defined though. A guy like Fastradio has the smarts do define it.
     
  7. DWPC

    DWPC Formula Junior

    Mar 10, 2011
    733
    Sedona AZ
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    Dennis
    Mine does the same. I believe its normal and caused by the time lag between the response of the O2 sensor in the cat and the frequency (metering) valve's reacting with mixture change at the fuel distributor. The K-Jet Lambda fuel metering accuracy seems to be +/- 50 RPM.
     
  8. dakharris

    dakharris Two Time F1 World Champ
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    Jun 7, 2001
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    Normal. Wide fluctuations are caused by other issues, such as a vacuum leak.
     
  9. PV Dirk

    PV Dirk F1 Veteran

    Jul 26, 2009
    5,401
    Ahwatukee, AZ
    Same here.
     
  10. spirot

    spirot F1 World Champ
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    Dec 12, 2005
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    Normal. as the cat wears out, you may see large variance... or at least that is what mine did. If you dont have emisions - unplug your cat and it will idle fine.
     
  11. Tifosi2011

    Tifosi2011 Formula Junior

    Apr 3, 2011
    307
    Burnaby, BC Canada
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    John
    What exactly does one mean to 'unplug' the cat? Is it simply removing the O2 sensor? Since I've originally posted this thread, my idle has smoothed itself out. All I did was add Lucas upper cylinder lubricant and fuel treatment. Plus the obvious other details like oil change and such. But since the oil change, my fast idle at cold start has disappeared. Still trying to figure that one out. But overall, the car runs great.
     
  12. tuttebenne

    tuttebenne F1 Rookie

    Mar 26, 2003
    2,991
    Bay Shore, NY
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    Andy
    Fluctuating idle on an injected car isn't normal. It indicates the system is hunting for the correct mixture. The system is plenty fast enough to adjust itself without it being noticeable. An exhaust leak can cause this. I think a bad O2 sensor can as well.
     
  13. spirot

    spirot F1 World Champ
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    The O2 sensor is connected to the "computer" via a cable that you will find under the intake on the right side of the car - kind of next to the fuel dist & slightly under the right rear fender - corner by the trunk. there is a rubber connector that you can "unplug" and you will see an immediate change in the idle. Essentially you are setting the CO ratio to 14:1 at all time. ( slightly rich mixture ). You may notice that you go thru a tank of gas faster and I guess technically you get a tiny - tiny bit more power... but it also may be illegal in your state to "tamper" with emission control.

    if your idle has "smoothed out" to me it may be some other electrical connection... that has "healed its self" for the moment, and or you have an air leak.

    I know from personal experience about the O2 sensor plug.
     
  14. fastradio

    fastradio F1 Rookie
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    Apr 26, 2006
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    As much as I hate to say this..."they all do that". (If not, the system is running open loop or is a Euro). The degree of hunting is related to just how far away the base mixture is from the reference mixture. When the two are equal, the idle will hunt the least due to the hysteresis curve within the lambda computer. With a very slow responding O2 sensor, coupled with 1970's processing speed, this is the best we can expect. I like to see less than 100 rpm of hunting...
     
  15. stevel48

    stevel48 Formula 3
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    #15 stevel48, Oct 17, 2011
    Last edited: Oct 17, 2011
    I thought that's what you said last year when my idle was hunting more than 100. Last year i could feel my car surge when driving. After the winter work and all new hoses it's more like 50-70 RPM now and the surge is gone.
     
  16. Dr Tommy Cosgrove

    Dr Tommy Cosgrove Two Time F1 World Champ
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    May 4, 2001
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    #16 Dr Tommy Cosgrove, Oct 17, 2011
    Last edited: Oct 17, 2011
    Perfectly normal.

    Some here may say that there is a "problem", albeit a minor one, which should be attended to. Don't waste your money.

    You will spend 3 figures attempting to smooth an idle issue that simply unplugging the O2 sensor will cure 100% of the time. Go ahead and see if you don't believe me. If you think that simply correcting the CO mixture will fix it you are in for a big surprise. Adjust away. Perhaps it did cure things in 1985 on a brand new car but not now. Too many years and too much crud in there now.

    I want to make this point perfectly clear so read carefully: UNPLUGGING THE O2 SENSOR ON THESE PARTICULAR CARS IS 100% HARMLESS.

    Do not allow anyone to make you believe otherwise.

    Mine did the same thing so I unplugged it - in 1997.
     
  17. Tifosi2011

    Tifosi2011 Formula Junior

    Apr 3, 2011
    307
    Burnaby, BC Canada
    Full Name:
    John
    I smoothed out my oscillating idle. How you ask? Simple...buy the highest octane fuel available. Since acquiring my 308 back in April, I only used 91 octane fuel from whatever station that was convenient. Mostly from Esso mind you, but Chevron as well. Anyways, I just purchased the Chevron 94 octane. And holy cow, what a difference. My idle has now smoothed out completely. It does not hunt/scavenge at all. It sits at 1000 rpm with no fluctuations. I guess the adage of 'you are what you eat' applies to cars as well. Especially if your car is a horse like ours.
     

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