95 STS hi-rpm and auto-acceleration problem

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Northstar Engines and System Technical Discussion Discussion, 95 STS hi-rpm and auto-acceleration problem in Cadillac Engine Discussion; Hello! I'll preface this by saying I do not know a ton about car repair--I understand some basics but havent ...
  1. #1
    PDXCruiser is offline Cadillac Owners Member
    Join Date
    Feb 2006

    95 STS hi-rpm and auto-acceleration problem


    I'll preface this by saying I do not know a ton about car repair--I understand some basics but havent done much tinkering. Just going through these forums the last few days has given me a better understanding of a few processes--I am impressed by the automotive passion and knowledge on these boards. With that said, on to my problem! Thank you in advance for any and all help! =D

    I drive a 1995 STS with 115,000 miles on it. Its been driven mostly for city/highway commuting, currently averaging a 30/70 mix I'd say. Recently it has been having a funky problem where it just seems to want to take off. It will start up fine in the morning and usually idle around 1k. In a drive lasting longer than 10-20 min the RPMs go up as well as the idle. The weird thing is that I can take my foot off the gas and the car will actually accelerate (speed of acceleration dependant on current speed). This isn't too big of a deal on the highway, but at low speeds and stop-and-go traffic it is really a hastle. It is like I am fighting the car to stop, I need to essentially fully depress the brake pedal to ensure the car doesnt move.

    When decelerating, extra brake power than usual is needed to slow down the car, and around the 5-15 MPH (depending on speed of deceleration) the car shudders and gets a quick boost of power, lurching forward. I would assume this may be the car shifting down roughly to a lower gear as I slow down. RPMS when stopped at this point will average 1.5k. If I put shift to Park, Reverse, or Neutral the car jerks, and the RPMs shift. Park will average 1.7-2.2k, and go as high as 3k briefly. Maybe the high RPMs are making it hard on the transmission, and something is catching making the car jerk?

    Today after a stop light I let my foot off the brake, and the car accelerated to 50 MPH without touching the gas before I reapplied the brake. I would guess it was accelerating at about 5mph/1-3sec rate. This problem makes smooth braking more difficult, and there is occasionally a throbbing feeling from the brakes (a rhythmic low thud sound), which also gets catchy at low speeds. It's difficulty to control makes me worry about accidently rear-ending someone. The problem is not constant but seems to be happening more and more (maybe I'm just noticing it more now...).

    I checked and it gives me the following codes:
    P056 Present
    I027 History
    S044 History
    S061 History

    I had just taken the car into the shop a month ago for the Service Engine Soon light, and the panel will flash Service Ride Control intermittently throughout most trips.

    From what I've read so far in these forums, it seems a common problem. I don't have too much experience with working on engines so I would probably need to either get detailed, written-for-dummies directions, or something to tell my mechanic. Would anyone suggest a progression of procedures for addressing this problem? From what I've been able to find, it could be a vacuum leak in the intake, problems with the throttle body, bad O2 sensor, or a faulty Input Speed Circuit (unsure on this, all I really know is it is a "$20 part located $2000 deep in the transmission...). I also read something about a free replacement of a vinyl fuel rail with a stainless steel fuel rail? Where would be the best place to start? Anything thats fairly easy for someone with little experience to check on their own?

    Sorry for the long-winded post, if I left anything out or have any questions please let me know... Thanks for the help!

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  3. #2
    eldorado1 is offline Cadillac Owners Connoisseur
    Join Date
    Feb 2004

    Re: 95 STS hi-rpm and auto-acceleration problem

    The most probable cause of your acceleration problem is a sticky throttle caused by deposit buildup. You can start by cleaning the throttle body, it's a simple task that can be done by yourself. You just have to take off the rubber intake boot, and scrub the crap off the throttle body bore and blade with a toothbrush and some throttle body cleaner. Open the throttle and get it all.

  4. #3
    Ranger's Avatar
    Ranger is offline Cadillac Owners Member
    Automobile(s): White Diamond '03 DHS (with DTS floor shift)
    Join Date
    Jun 2004
    Woodstock Ill.

    Re: 95 STS hi-rpm and auto-acceleration problem

    Yeah, I'd start with a good TB cleaning, then check for vacuum leaks and finally get it in to a dealer for the fuel rail recall. If that doesn't help, Start looking at the ISC (Idle Speed Control) motor.

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  6. #4
    wph101larrya is offline Cadillac Owners Member
    Join Date
    Sep 2005

    QuestionRe: 95 STS hi-rpm and auto-acceleration problem

    I have a 95 eldorado with the same problem. Mine gives me a code P039 and has for over a year, long before this problem started.

    I have taken the following steps to isolate the problem. First I have replace the PCV and hoses, checked for vacume leaks, and cleaned the throttle body. I have also checked the throttle plate shaft to see it it is sticking by watching it while the engine speed was changing. I have appliced lub to the cables and other mechanical parts.

    I observed the Idle Control motor and it was not moving while the speed changed. Then I removed the connection to the Idle Control motor and started the car and imediately the engine speed went to 2000 rpms. Next I started the engine with the Idle control motor connected and touched the accelerator and let off so the engine would return to normal idle speed (650-700 rpm). Then I removed the connector with the engine running. It essientially had no effect. So I think I have ruled out the Idle Control motor.

    Any ideas?


  7. #5
    95stsrider is offline Cadillac Owners Fanatic
    Join Date
    Jun 2005

    Re: 95 STS hi-rpm and auto-acceleration problem

    TTT....my 95 sts is doing the same damn thing...any more ideas?

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