New plugs, now won't idle
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Cadillac Seville / Cadillac Eldorado Forum Discussion, New plugs, now won't idle in Past Cadillac Vehicle Discussion; I changed out the plugs with AC 41-987 and it won't idle. I didn't remove any vacuum hoses so I ...
  1. #1
    capthead is offline Cadillac Owners Member
    Automobile(s): 2004 Seville
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    New plugs, now won't idle

    I changed out the plugs with AC 41-987 and it won't idle. I didn't remove any vacuum hoses so I don't have any idea what the problem is. It runs smooth at fast idle, no miss or hesitation, just won't idle and dies when you take your foot off the gas.

    Any ideas?

    2004 Seville

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  3. #2
    EcSTSatic's Avatar
    EcSTSatic is offline Cadillac Owners Connoisseur
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    Re: New plugs, now won't idle

    Maybe not intentionally. Reinspect, something got knocked loose

  4. #3
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: New plugs, now won't idle

    Codes. Still do-able in 2004. MAF or throttlebody sensor(s) ?

  5. #4
    capthead is offline Cadillac Owners Member
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    Re: New plugs, now won't idle

    No codes yet
    I looked the top over and don't see anything loose. I had my wife start it so I could listen and I heard nothing hissing. It must be vacuum can't imagine it's anything else.

  6. #5
    Submariner409's Avatar
    Submariner409 is offline If it won't run, stick on fender vents
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    Re: New plugs, now won't idle

    If you have a vacuum leak THAT large you would hear the whistle........... Hmmmmm.............. PCV dirty air line to the right (rear) cam cover ?

  7. #6
    capthead is offline Cadillac Owners Member
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    Re: New plugs, now won't idle

    This is crazy. I couldn't find anything so I took it for a drive. Drove it a mile or so and brought it back to the house. It's idling great now.

    Is it possible the computer needed to adjust to the new plugs? I removed a different brand and they were iridium made in Japan. Those came with the car new.

  8. #7
    Submariner409's Avatar
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    Re: New plugs, now won't idle

    I find it hard to believe that a PCM would need to do an idle speed relearn with a simple plug change.

    Iridium ?? or DENSO platinum ??? Interesting - pictures of the old plugs ? 2004 - what does the Specifications section of the owner's manual say for plug type/number ?

  9. #8
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
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    Re: New plugs, now won't idle

    Quote Originally Posted by capthead View Post
    Is it possible the computer needed to adjust to the new plugs?
    No. Hard to say what caused it. Chalk it up to "one of those things".

  10. #9
    capthead is offline Cadillac Owners Member
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    Re: New plugs, now won't idle

    It says the AC Delco 41-987 is the right plug. I removed DENSO iridium IT16 which looked like new but the car has 106,485 miles on it.

  11. #10
    Submariner409's Avatar
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    Re: New plugs, now won't idle

    The iridum plugs were not normally used as original equipment ......... did you buy the car new ?

  12. #11
    capthead is offline Cadillac Owners Member
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    Re: New plugs, now won't idle

    Took it on the freeway and the car runs good. Too soon to see if MPG changes but I'll make note of any changes here. IMHO if the Denso plug idled good and the new AC died, maybe the Denso is the way to go for this engine. It's idling smoother now than with the old plugs so a change was needed and I'm happy at this point with the AC's.

    ----------

    My father-in-law bought it as a brass hat car, (A GM employee car), and he sold it to my wife. He never changed plugs and he bought it with 30,000 something miles on it. So as far as I know, it came with those. Usually the driver is a factory rep in the brass hats. I worked at a dealership and we got them from time to time.

  13. #12
    Submariner409's Avatar
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    Re: New plugs, now won't idle

    FWIW, DENSO was the original supplier of the platinum plugs for most of the pre-2004 (???) Northstars - The owner's manuals give a DENSO number - but some time in about 2004 - 2005 there was a change - the plug physically remained the same (platinum), but became "AC Delco" made, labeled and boxed in Japan - your guess as to which Japanese company is under contract to GM. I have three sets of "AC Delco" #41-987 plugs bought over the last 6 years - all are boxed "Made in Japan".

    My original plugs were DENSO with a GM designation of PT16EPR-C13

    Convert your DENSO IT16 to an AC Delco number ......... 41-921 !!!

    Here's the direct cross reference ............. http://www.sparkplug-crossreference.com/convert/AC_DELCO/41-987

  14. #13
    capthead is offline Cadillac Owners Member
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    Re: New plugs, now won't idle

    The car is running like new. It starts, idles and runs great. The idle is a lot smoother with the new AC plugs. FWIW, the electrode is much larger on the AC over the Denso and maybe it's a fatter spark, I don't know but it does run much better.

    I also installed a K&N air filter for the first time on the car. I'm hoping for improvement all around.

  15. #14
    Submariner409's Avatar
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    Re: New plugs, now won't idle

    Hate to tell you this, but you just wasted some money. Air filters have been discussed to death in here and in Engines. If the K&N you installed is the wetted type, be ready for problems - Your (our) engines DO NOT like oil vapor on the MAF heated resistors. Been there, done that.

    A good ol' AC Delco, PartsMaster, Purolator, Fram, WIX pleated paper filter filters down to the micron level and will flow far, far more air - while cleaning it - than the engine can use at WOT redline.......... and for $6.95 you throw it (and the dirt) away every 12,000 miles and start over.

    Want some ideas on how to get better airflow and clean up the intake ducting a bit ??? Look in my albums, search for the butt plug modification, and PM me. It has all been posted before ..............

    Google "cadillac forums butt plug seville".

  16. #15
    Aztec ETC ECS's Avatar
    Aztec ETC ECS is offline Cadillac Owners Connoisseur
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    Re: New plugs, now won't idle

    Quote Originally Posted by capthead View Post
    I changed out the plugs with AC 41-987 and it won't idle. I didn't remove any vacuum hoses so I don't have any idea what the problem is. It runs smooth at fast idle, no miss or hesitation, just won't idle and dies when you take your foot off the gas.
    Any ideas?
    2004 Seville
    I understand it's idling good now, but did you either let the battery die or disconnect it before the plug change?

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