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Discussion Starter · #1 · (Edited)
I have a 2014 XTS with 120k miles, 3.6 paired with a 6T70 trans.
I did spark plugs and a transmission fluid flush.
I have what feels like a shudder, in 5th and 6th gear under light acceleration.
I do not have cel or tcm code set.
I’ve narrowed it down to what I think is a TC lockup problem.
The shudder will happen at any speed but only in 5th & 6th gear, it will happen at lower rpm 1000-1500rpm, and under light throttle the car will begin to shudder as if the TC can’t make up its mind on staying locked up or not.
I believe it’s a lockup issue because it’s only in 5th & 6th and the cars resolution, is to raise the rpms to about 2k or higher, so I believe it’s disengaging TC lock up.
I’m looking for any input as to whether it could be the TC or the control body, but once again I don’t have and codes to go off of.
 

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2012 SRX Luxury
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597 Posts
I had that exact same thing on a 07' CTS with the 2.8 engine. I would have sworn on a stack of bibles that my trans was the problem. For sure it's a shudder from the torque converter. blah blah blah. NOPE was a misfiring cylinder. Finally set a fault code and I bought the coil pack it needed and new plug for that cylinder and wha la' FIXED IT ! I had that going on for years before it set the code.
 

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Discussion Starter · #3 ·
I had that exact same thing on a 07' CTS with the 2.8 engine. I would have sworn on a stack of bibles that my trans was the problem. For sure it's a shudder from the torque converter. blah blah blah. NOPE was a misfiring cylinder. Finally set a fault code and I bought the coil pack it needed and new plug for that cylinder and wha la' FIXED IT ! I had that going on for years before it set the code.
That gives me some hope, I would have expected a cel light by now if it was the transmission, I just don’t have the software to monitor a small misfire.
I believe I read, that the car will set a fault code only if it detects severe misfiring.
Now prior to this whole problem, I had a flashing cel which indicates a severe misfire, maybe two weeks later I started to get a slight shudder but no more cel, so I did the spark plugs, and the shudder remains.
Then I did a flush and I still have a shudder.
A bad coil pack was my original concern, the shudder feels like it could be a transmission problem but it’s also feels very similar to a misfire under load, but with no cel and no way to measure live data I’m kind of stuck on wether or not to replace the coil packs.
 

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2013 XTS Luxury
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96 Posts
My brand new 2014 Chevy Impala did the exact same thing. Never did figure out why. It had less then 30,000 when turned it back in. It was a lease. Good luck on figuring it out.
 

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Discussion Starter · #7 ·
So all 6 spark plugs have carbon tracking, no carbon tracking on the boots, and all 6 plugs were replaced about 100 miles ago… what did I do wrong…
 

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Discussion Starter · #9 · (Edited)
The problem is fixed!
I went back to OE iridium acdelco spark plugs
Part acdelco #41-109
The bad acdelco spark plugs I bought were almost a 1/8 shorter than the OE ones.
I used some acetone to clean the boots because there was ever so slight carbon tracking on them.
I also used permatex dielectric grease.
And upon pushing the coil pack in all the way I felt ever so slight contact between the spring and spark plug terminal, where as I never felt contact with the previous new (bad) acdelco spark plugs.
I went for a test drive manually held the car in 5th & 6th up inclines and no shudder what so ever!

Im assuming the none OE spark plugs where faulty or not a true replacement part like the vendor states they are. I assume this because I had OE spark plugs with no carbon tracking and within 100 miles of the new spark plugs (non OE) I had problems and discovered carbon tracking. I’m not sure if the problem was a shorter plug or dielectric grease. But on my original set of OE plugs I never had dielectric grease.

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Gesture Finger Font Glove Tints and shades
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2005 Cadillac Deville
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456 Posts
Hey! I got that t-shirt, too!
Light Auto part Electric blue Metal Fashion accessory

New plugs and coils cured my intermittent miss at idle and during heavy load.
Glad that helped.
 

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Discussion Starter · #11 ·
Hey! I got that t-shirt, too! View attachment 606764
New plugs and coils cured my intermittent miss at idle and during heavy load.
Glad that helped.
Interesting thing was I replaced my spark plugs do to a flashing cel, I never had a shudder, I replaced them with non OE acdelco spark plugs, got a shudder, found carbon tracking, went back to iridium OE plugs and some dielectric grease, no shudder.
I can’t imagine all my my coil packs went bad after a spark plug change, and the vendor excepted a return after explaining my problem.
 

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2002 F55 STS, 2014 Explorer XLT, F-150s
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Like it or not, once a boot is tracked it's tracked forever. That little lightning strike track on both the plug and boot is as conductive as a piece of wire. The dielectric grease and cleaning may buy you some time but the miss will return fairly shortly.
 

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2005 Cadillac Deville
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Glad the vendor was cooperative. I don't think my coils were bad, I just didn't know the boots were replaceable until the coils got here. They were just too shiny to resist. New boots may be in your future.
 
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