97 Deville - rough idle no codes

swwjr05swwjr05 Member Posts: 9
My '97 Deville has developed a rough idle recently. RPM varies between 625 - 700. DIC shows no codes. Seems to run fine other than idle. Gas mileage is about the same as always 25 - 27 mpg on highway. It has about 185,000 miles on the clock. Might use about a quart of oil per 5000 miles. Does anyone have ideas what might be causing the idle problem.


  • bolivarbolivar Member Posts: 2,316
    edited August 2013
    I've had to replace both coil packs on my 2005 Deville, with much less mileage than you have. Both times is had a slight miss you could hear at idle and did not set any codes. I'm not sure, your 1997 might have different coils than my 2005. Each wire has it's own coil, maybe?

    At this age and mileage, plug wires could also be failing. I'm assuming the plugs have been changed? The mileage for plugs is 100,000 miles.
  • swwjr05swwjr05 Member Posts: 9
    Yes '97 has 8 coil packs. I had to change one a few months ago. When it went out the Service Engine Soon light came on and there were codes set. Changed plugs and wires when this first started a few weeks ago. Any other idea and suggestion will be appreciated.
  • bolivarbolivar Member Posts: 2,316
    Ok, throwing something else out that I've heard about.

    There is a big rubber boot (cylinder) between the air cleaner and the intake. (Maybe between a MAP sensor and the intake?) When shut down, the unburned gas, etc from the intake condense out in this tube and over time in older Cadillac will rot a hole in the bottom. This screws up the air/fuel mixture. Just reach down there and poke around. If it has a hole, or even especially 'soggy' feeling, it might be pulling in additional air, causing a miss.

    You put the stock AC plugs in, didn't you? If you used some multi-prong non-AC plugs, the Northstar might not like this.
  • swwjr05swwjr05 Member Posts: 9
    Ok, I will check that this afternoon. One other thing, I put a bottle of Techron in the gas last week and the problem went away for a couple of days. But it came back. I'm not much of a believer in fuel and oil additives, but I'm wondering if I should run a couple more tanks with Techron.

    And yes, I did use stock A/C plugs. Thanks for your suggestion!
  • bolivarbolivar Member Posts: 2,316
    Well, that's sounding like a dirty injector. Another round of cleaner couldn't hurt.

    I would look at the wires again. If they were recently changed, there might be a loose one...
  • swwjr05swwjr05 Member Posts: 9
    Believe it or not, I put another dose of Techron in, drove 50 miles now idle is steady as a rock at 650 rpm in gear, 675 in neutral. Hope that fixed it! Still kind or wondering why computer never showed any codes.
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