06 STS N* stumbles below 2000 rpm
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Cadillac STS Forum - 2005 through 2012 Discussion, 06 STS N* stumbles below 2000 rpm in Past Cadillac Vehicle Discussion; Sympton is there is a notable lack of power, essentially hesitation below 2000rpm. Problem is more prominent with the AC ...
  1. #1
    GREGB1954 is offline Cadillac Owners Member
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    06 STS N* stumbles below 2000 rpm

    Sympton is there is a notable lack of power, essentially hesitation below 2000rpm. Problem is more prominent with the AC on. There are no trouble codes set. Sometimes when it is hesitating it will actually have surges in power. I am very surprised that a code is not thrown when that happens.

    If you give it more gas causing it to downshift and get the rpm above 2000 it is fine. If you drive in manual mode forcing it to run below 2000 rpm it barely will accelerate. Then it runs with expected power once it gets past 2000rpm.

    I have tightened to torque specs the upper intake manifold with some improvement. This makes me wonder if it has developed a vacuum leak that is enough to cause the problem, but not enough to throw a trouble code.

    I have had 2 Caddy dealers look at it and they say it runs fine. One suggested a fuel system cleaning for $150 which I did. No improvement at all. So I have paid the 2 dealers $350 with no improvement.

    I have had the car for 3 years and it started 1 year ago. My son noticed the problem as well. It has 72k miles on it.

    My conclusion is that the dealers can only address these type of problems if a code is thrown. I do not know what to do at this point.

    Any ideas?

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  3. #2
    EChas3's Avatar
    EChas3 is offline Cadillac Owners Master
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    Re: 06 STS N* stumbles below 2000 rpm

    Is the throttle body clean? I wonder if the intake bolts might have been overtightened and the gasket deformed. As you suggest, a small intake leak may not cause a code but still cause symptoms at idle & just above.

    There's also an issue that while can't be called common, isn't rare, either. The wiring harness of early STS may lack sufficient slack/clearance near the connection to the throttle. Issues with this almost always throw a code so it doesn't really fit, but it did happen to mine just after I bought it in 2008.

  4. #3
    GREGB1954 is offline Cadillac Owners Member
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    Re: 06 STS N* stumbles below 2000 rpm

    Well I was told that the fuel system flush included cleaning the throttle body but I did not check. I am tempted to spray carb cleaner around the manifold while it is running to see if it changes the rpm indicating a leak.

  5. #4
    1BadCadSTS's Avatar
    1BadCadSTS is online now Cadillac Owners Connoisseur
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    Check the oil level

    If low it will mess up the VVT which will effect power.

  6. #5
    EChas3's Avatar
    EChas3 is offline Cadillac Owners Master
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    Re: 06 STS N* stumbles below 2000 rpm

    ^^^ Always, especially after spirited driving or mountain roads. (See WOT in Sticky!)

  7. #6
    mckellyb is offline Cadillac Owners Enthusiast
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    Re: 06 STS N* stumbles below 2000 rpm

    I vote intake manifold bolts being a tad loose.

    Does it 'shake' at idle? If so, is it worse when cold (engine temp, not ambient)?

  8. #7
    GREGB1954 is offline Cadillac Owners Member
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    Re: 06 STS N* stumbles below 2000 rpm

    It does "shake" at idle when the AC is on. It did not use to do this.

  9. #8
    1BadCadSTS's Avatar
    1BadCadSTS is online now Cadillac Owners Connoisseur
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    Did you check anything you were advised to?!

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    GREGB1954 is offline Cadillac Owners Member
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    Re: 06 STS N* stumbles below 2000 rpm

    Oil level is fine, intake bolts torque are to spec, and it has a fresh air filter. I have had this problem going on 2 years with 2 trips to the dealer including the fuel system flush.

  11. #10
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    Re: 06 STS N* stumbles below 2000 rpm

    Quote Originally Posted by EChas3 View Post
    ^^^ Always, especially after spirited driving or mountain roads. (See WOT in Sticky!)
    I just looked for the wot in the sticky with no luck. Link?

  12. #11
    curtc's Avatar
    curtc is offline OCD - Obsessive Cadillac Disorder
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    Re: 06 STS N* stumbles below 2000 rpm

    It's in here; http://www.cadillacforums.com/cadillac-tech.html

    I spared you the time though and copy/pasted it below;

    Occasional
    Full-Throttle Acceleration Is Good For Your Engine

    There are many advantages to occasional full throttle accelerations with a Northstar and any engine.

    It keeps the carbon cleaned out of the combustion chamber. This is maybe a little more important with the Northstar than some other engines due to the tight squish volumes between the piston and the cylinder head. It's designed this way to promote good in-cylinder mixture motion (good combustion) but it has the down side of providing a ready place for carbon build-up to touch the piston - causing noise. Ever heard of the Northstar "cold carbon rap" problem?? Simply put you'll hear a rythmic, piston slap-like noise when the engine is cold. Very prominent and very annoying. Cause: excessive carbon build up causing the the piston to contact the carbon on the
    head - causing it to rock in the bore and "slap" Much more evident when the engine is cold and the pistons haven't expanded to full diameter yet. Simplest and easiest "fix" for this: A few good WOT (wide open throttle) accelerations to clear the carbon out. That is all it takes to eliminate the problem and prevent it from re-occurring.

    Occasional WOT accelerations also help seat the rings to the ring lands and exercise the rings and keep them mobile and from becoming stuck in carbon in the ring lands. At high RPM and WOT the rings move around on the piston - they actually rotate on the piston and will polish away any carbon and seat themselves to the sides of the ring grooves. This isespecially important on the 2000 and later Northstars which had hard anodized top ring lands on the pistons. Very hard and wear resistant - also harder to break-in and seat the rings to the sides of the ring-lands to promote the best possible seal. Many oil consumption complaints on the 2000 and later engines are related, to some extent, with the rings never seating to the sides of the ring-grooves due to lack of load as the engine was babied around forever. Even engines with rings stuck in the ring-grooves due to carbon build up can eventually be freed up with enough high RPM operation.

    WOTs warm up the engine thoroughly and clean out the exhaust due to temperature in the exhaust and high flow rates blasting particles, rust and such out of the system.

    Frequent WOT operation will not hurt the engine or the transmission. They're designed for that. The healthiest engines that I have seen at high miles are always the ones that are run the hardest. Rings are free on the pistons and sealing; no carbon buildup.

    The exercise that I think works best for many things is to select manual 2nd gear on an isolated stretch of expressway. This takes the transmission shifting out of the question if you are worried about hurting it. Start at 55 MPH or so and go to WOT in 2nd gear and hold it until the RPM reaches near the normal shift point - i.e. 6500 for an L37 and 6000 for an LD8. Hold the throttle wide open until the engine reaches, say, 6200 for an STS and then just let completely off the throttle. Leave the transmission in 2nd so that the engine brakes the car and creates some pretty heavy over-run conditions at high vacuum levels. Let it slow until it is about 55 or so and then go to WOT again and repeat. This exercise really loads the
    rings, allows variable RPM operation at WOT for several seconds continuously, creates heavy over-run which tends to unload the rings and make them move and thus exercise them in the ring grooves and it will blow-out carbon and the exhaust - all without creating a spectical of yourself and attracting the attention of cops. You can do it on most any freeway and stay within the 70-75 MPH range allowable. Once a week like this will keep the engine cleaned out and healthy and is DEFINITELY recommended for the Northstar in particular.


    The Northstar engine was designed/developed/validated to be run hard. It was expected that people would use the performance of the engine - which few people seem to do. The biggest single problem that many issues stem from is lack of use at full throttle by the owners. It just doesn't like to be babied around. The rings are low-tension by design for good high RPM operating characteristics and low friction/good power. They work best if "used" and kept free.In every conversation with owners I have had, once the owner started doing the WOTs and using the power of the engine they report no more carbon rap, better oil economy, no "smoke" when they do light it up (keep the exhaust cleaned out. If you notice a "cloud" at WOT then you are not doing enough WOTs...) etc... A bit of judicious use of the other end of the throttle travel is a GOOD thing...
    Past Cadillac's

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  13. #12
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    Re: 06 STS N* stumbles below 2000 rpm

    ^^curtc you're awesome. Thanks

  14. #13
    Guy.Seminerio is offline Cadillac Owners Connoisseur
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    Re: 06 STS N* stumbles below 2000 rpm

    When my grandfather died we got his 99 Deville. Real beauty. White diamond, vogue tires, chrome rocker panels, gold vertical grille, and more. When I started with the WOTs, massive amounts of buildup blew out the tailpipes. I eventually cleaned it out. Took a while. I didn't use this method exactly although I'm sure it's better all I did was mash it whenever I felt like going fast, like on ramps.

  15. #14
    GREGB1954 is offline Cadillac Owners Member
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    Re: 06 STS N* stumbles below 2000 rpm

    I admit that I have not tried this preventative maintenance step yet. Not that I don't go WOT occasiaionally Very interesting indeed. I am not experiencing excessive oil consumption nor the slap, but then again my hearing is not that good either.

    I will give it a run and report back. Thanks for the insight.

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