2005 STS V8, uneven power under load, last 30K miles...
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Cadillac STS Forum - 2005 through 2012 Discussion, 2005 STS V8, uneven power under load, last 30K miles... in Past Cadillac Vehicle Discussion; I need some help, and I am just about to give up. For the last 30K miles (currently 108K), the ...
  1. #1
    Benjamin Simon's Avatar
    Benjamin Simon is offline Cadillac Owners Enthusiast
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    2005 STS V8, uneven power under load, last 30K miles...

    I need some help, and I am just about to give up. For the last 30K miles (currently 108K), the engine sometimes skips under load. Over time, it is doing to more frequently, but not all of the time. I notice it happens more often when the engine is still reaching operating temperature, but can still skip during a long drive on the interstate. It very often skips up hills that require a load on the engine. Sometime even during driving on a level road it skips very slightly. I can see the RPMs go up 50-75, then back down, 2 seconds later back up 50-75. If i rev the engine, it tends to go away. There is never a check engine light.

    I have tried the following:
    replaced spare plugs with OEM
    ran 100% gasoline for a few weeks
    fuel treatments
    new cats (under warranty)
    replaced throttle body gasket
    injector cleaner at a shop and seafoam at home
    transmission flush
    new air filters (every year)
    replaced fuel filter
    yelling at the car

    When it was under warranty, the dealership (who is horrible) said they way could never find anything wrong. They said it was normal for these engines to do this when they get high up in the miles, e.g., 75K+. It is getting much worse, but I am still not getting a code. I hate to just replace things like sensors, but it is driving me crazy. I might look to see if I can find an independent shop that knows how to work on car, but since it still runs, I have avoided doing that.

    Anyone else experience this? If so, any suggestions?
    Thanks!!!

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  3. #2
    EChas3's Avatar
    EChas3 is offline Cadillac Owners Master
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    It sure sounds like a high speed miss. No correctly running Northstar does this.

    When were your intake manifold bolts last checked for proper torque? They may have been over-tightened by the bogus dealer; in which case, you'll need to replace the gasket as well.

    Other things usually leave a diagnostic code. You are wasting money on air filters; I hope you're using OEM there as well. IMHO, Seafoam is worse than a waste of money. The only fuel treatment of value is Techron and if you use Top Tier fuel, you shouldn't need it. Ever had the throttle body cleaned?

    Let's say it's not engine at all... Have all motor mounts & trans mounts been checked; rear diff bushing, too? You're feeling it, right?

  4. #3
    bsr22 is offline Cadillac Owners Member
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    Quote Originally Posted by Benjamin Simon
    ……..engine sometimes skips under load. Over time, it is doing to more frequently, but not all of the time. I notice it happens more often when the engine is still reaching operating temperature, but can still skip during a long drive on the interstate. It very often skips up hills that require a load on the engine. Sometime even during driving on a level road it skips very slightly.
    Benjamin,
    I have an '08 that has had the same problem twice over the last few years. The first time the symptoms were nearly identical to what you described above. The second time was this week and the symptoms started under a light load and the quickly progressed over two days to a major miss at idle. In both cases the problem was a coil pack.

    Please note that your Check Engine light may or may not be on. The first time it happened to me the light was not on steady, but a code was set. This week with the light miss it also was not on steady. However, as it progressively got worse the Check Engine light began to flash under engine load and then later stayed on steady. You may have a code stored that will indicate which cylinder has the problem. You really need the code to isolate the problem. P0302 in my case.

    Most of the major parts stores will read your codes for free; however, not one had a coil pack in stock in my location (Advanced Auto, Autozone and Orielly's). I ordered from one from Amazon because I could get next day delivery at a very low cost with Amazon Prime ($63 total). The replacement job is very straight forward and can be accomplished in 10 mins or less.

    Good luck!
    Scott

  5. #4
    amgqmp1 is offline Cadillac Owners Fanatic
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    ^^^ A coil pack makes perfect sense.

    All of the symptoms + your extensive troubleshooting have left ignition or other electrical as possible culprits.

    - Check coil packs
    - Replace plug wires
    - Check alternator

    Those would be next three targets.

    Good luck! You've already done a lot of great things and I'm sure you'll get the root cause soon.

  6. #5
    Benjamin Simon's Avatar
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    Thanks everyone on the replies.
    Is there a way to tell if I have a bad coil packs? Or is it something I just replace?
    Thanks again! I really want to keep my STS for another year or two, but this is drinking me crazy.

    ----------

    Hi Scott,
    When you say coil pack, is that the same as the Ignition Coil, ACDELCO Part # D593A? As it is a v8, I would need 2.
    What part did you find for $63? These things are $150 each, so I think I am looking at something else.

    As I understand these ignition systems, they do not have spark plug wires per se. The Ignition coils replace the plugs and the coil with one component.

    I have a Bluetooth OBD2 dongle that can run the codes on my phone, and nothing is saved. It so werid... Sometimes this thing runs like a champ, other times like an old truck.

    ----------

    EChas3,
    I have not checked those items. I assumed the dealership did last spring. However, the problem has gotten worse, so I will see if I can have these checked by a professional. I am certainly feeling the skipping, but, sometimes it runs perfectly. This leads me to think it is electrical, opposed to mechanical; however, I have no freaking idea what I am doing.

  7. #6
    chazglenn3's Avatar
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    Out of curiousity EChas3, why do you feel that way about Seafoam? I use it regularly in my motorcycle with excellent results. However, that being said the bike is carburated.

  8. #7
    Benjamin Simon's Avatar
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    Is there any type of test to determine whether if one or both of my Ignition Coils are bad?

  9. #8
    bsr22 is offline Cadillac Owners Member
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    Quote Originally Posted by Benjamin Simon View Post
    Thanks everyone on the replies.
    Hi Scott,
    When you say coil pack, is that the same as the Ignition Coil, ACDELCO Part # D593A? As it is a v8, I would need 2.

    What part did you find for $63? These things are $150 each, so I think I am looking at something else.

    As I understand these ignition systems, they do not have spark plug wires per se. The Ignition coils replace the plugs and the coil with one component.
    I have a Bluetooth OBD2 dongle that can run the codes on my phone, and nothing is saved. It so werid... Sometimes this thing runs like a champ, other times like an old truck.
    Benjamin,
    The coil designs are different between the 2005 and 2008 in that the '05 apparently has one coil per engine bank, whereas the '08 has one coil per cylinder. However, the resulting symptoms could be very similar for a misfire. The AC Delco part number is correct for your car and mine was lower cost due to the design difference.

    My son-in-law is a GM tech and says often a carbon track will develop between the spark plug boot and the spark plug. The coil discharge will follow this carbon track path to ground (cylinder head) rather than through the spark plug, resulting in a misfire. Unfortunately the coil discharge doesn't always follow the carbon track path, so at times the car will run fine.

    Alternatively, one of your coils could also be breaking down, but I would think the symptoms would have become more persistent over time. That said; ignition issues can be frustrating to pin down when they are intermittent.

    Because of the cost of the ’05 ignition coil and the fact that you do not have a fault code, I would try a couple of things before spending the $ on the coil.
    - Check the intake bolts as EChas3 suggested. I have had to re-tighten mine several times.

    - As a troubleshooting technique for the carbon tracking my son-in-law suggested that I put a heavy dab (think of the size of toothpaste on your toothbrush) of dielectric grease in the spark plug boot which will aid in bypassing the carbon track. Because I knew my misfire was on cylinder # 2 I knew where to try first. I did, it worked and validated the coil or plug was bad (coil in my case). You could try the same thing. Because your coils take care of four cylinders you may want to try the grease on one complete bank of spark plug boots to save time when trouble shooting. Then drive it a few days to see if the problem goes away. If it does not go away, then try the other engine bank by moving the new coil. If it does go away this is not a long ter m fix, but it does tell you the side that needs the new coil.

    - Lastly, if you don’t mind buying a coil without verifying it is definitely the problem (tough without a code), purchase one coil and install it on one side of the engine and then test drive for a while drive to see if it fixes the problem. If it doesn't fix it, put the new coil on the other side of the engine, re-install the first coil you removed and then test drive again. Hopefully one side or the other will fix your problem.


    I hope this helps.

    Good luck!
    Scott

  10. #9
    Benjamin Simon's Avatar
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    How Scott, thanks for the advice and information regarding the coil packs.

    Since the car has 110k miles on it now, I went ahead and replaced both coil packs. That helped 98% for any driving under 45 mph. Power is even, now skipping. However, high speed driving over 45 mph is still causing an issue. It is most noticeable around 70 mph going up a incline. Speed and throttle stay constant, but the rpms fluctuate 150 - 175 every 3 seconds.

    When I give it the gas, power is still there. The Northstar has so much power, I have trouble telling if it is under performing at full throttle.

    In the last few weeks, I have also noticed my torque converter engaging hard. For instance, when coasting the TC disengages. Once I press the throttle and the TC engages, I feel and hear a loud thud. The harder I press the throttle, the harder the thud is when it engages.

    I had the transmission fluid changed 8k for standard 100k service. No issues immediately after the change.

    I'm still shooting in the dark, but perhaps these issues are related. I'm taking it to a good independent stop tommorrow, but fell free for anyone to offer their thoughts.

  11. #10
    1BadCadSTS's Avatar
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    Pull your cam sesnors and clean them. Sounds like what happened with mine.

  12. #11
    Benjamin Simon's Avatar
    Benjamin Simon is offline Cadillac Owners Enthusiast
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    It is sometimes difficult to search forum post for particular topics. Can anyone tell me how to clean my cam sensors? Thanks!

  13. #12
    1BadCadSTS's Avatar
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    Unbolt wipe down reinstall

  14. #13
    Benjamin Simon's Avatar
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    I finally got a check engine light! P0174, Lean Bank 2. It is unclear how others have addressed this code. Replace MAF? Vacuum leak?

  15. #14
    Ludacrisvp's Avatar
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    loose intake manifold bolts. they are to be tightened to 87 inch pounds IIRC. (about 7 ft. lbs.) they are commonly loosened due to engine hot / cold cycles.
    At least this is a potential cause, be careful not to over tighten or you will trash the intake gasket (like my dealer did).

  16. #15
    EChas3's Avatar
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    Re: 2005 STS V8, uneven power under load, last 30K miles...

    8 minutes. Asked & answered.

    Nice job, Luda!

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