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2013 Cadillac ATS 3.6L
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Discussion Starter #1
2013 ATS 3.6
While accelerating normally there's a "shudder or stutter" between 30-40 mph consistently. If I'm accelerating hard it doesn't happen and if I release the gas and reapply it goes away. It only happens while driving "normally". Doesn't matter if the engine is cold or not. It only happens in that range. I really don't know how else to describe it. Best comparison I have is if you ever tried drive a manual transmission car in high gear at low speeds. Any ideas of what I could start with investigating?
 

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2016 ATS Premium 6-spd MT
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14,168 Posts
Sounds like one or more cylinders are misfiring. Then again, it could be your transmission.

I sure didn't narrow it down much for you.
 

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2017 ATS Coupe 2.0T Luxury, formerly 2011 CTS Coupe Performance
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197 Posts
. Best comparison I have is if you ever tried drive a manual transmission car in high gear at low speeds
That kind of thing happens a lot with the 2.0T and 8spd because it's just geared/programmed to upshift a little too fast.

But none of that applies to you.

Still could be a torque converter clutch problem. You'd kind of think the engine would exhibit some rough running in some other power band or gear at least.
 

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2013 Cadillac ATS 3.6L
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Discussion Starter #6
That kind of thing happens a lot with the 2.0T and 8spd because it's just geared/programmed to upshift a little too fast.

But none of that applies to you.

Still could be a torque converter clutch problem. You'd kind of think the engine would exhibit some rough running in some other power band or gear at least.
The going rate seems to be transmission flush, fluid and filter change. I'm going to try that next week.
 

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2013 ATS 3.6L Luxury
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366 Posts
There have only been a couple of reports of torque converter shudder with the A6 on here, so you get to be a pathfinder to see if flush, fluid filter can fix it on the A6.

How many miles on your car? Are you going to use the latest fluid called for in the TSB for the A8? IIRC, one guy that was going to do it with this fluid never reported back how it worked out.
 

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2013 Cadillac ATS 3.6L
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Discussion Starter #8
There have only been a couple of reports of torque converter shudder with the A6 on here, so you get to be a pathfinder to see if flush, fluid filter can fix it on the A6.

How many miles on your car? Are you going to use the latest fluid called for in the TSB for the A8? IIRC, one guy that was going to do it with this fluid never reported back how it worked out.
58000 miles. I'm going to use the fluid in the TSB for the A8. If it's good enough for the A8 then it should be good enough for me. I won't get to flush it until this weekend but I'll definitely report back. Thanks
 

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2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
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3,040 Posts
Dex VI is still the recommended fluid for the 6Lxx series transmissions, I wouldn't use a different spec fluid formulated for the specific issues of the 8Lxx series in your 6L45 because it just adds another unknown to the shudder issue and it isn't needed or recommended for your transmission. Also avoid the temptation to use any "magic mouse milk" additives from the auto parts aisle which aren't good for your transmission and create a new and more expensive set of issues.

Try a full tank of 91 or 93 octane and see if that changes the behavior towards the end of that tank run. The 3.6 is a rather high compression engine and direct fuel injection allows it to perform well with 87 octane HOWEVER the transmission programming is optimized for fuel economy under light/moderate throttle and it will be in a higher than optimal gear range under circumstances like where you are experiencing shudder symptoms. This high load/low RPM engine operation leads to detonation of the fuel air charge and the chugging you are experiencing can be symptoms of both the detonation and the engine adjusting timing to reduce detonation. Winter fuel blends are often questionable so trying a different octane preferably from a different station is a simple diagnostic.

A failing coil pack often first shows up under the conditions you note. A flashing CEL along with trouble codes won't be set until there is a sustained heavy misfire, this is to avoid nuisance warnings to the driver but it means that you will NOT get a warning for every misfire episode if it isn't sustained (both in time and consistency of misfiring). A failing plug can also cause this issue, heavy engine load at low RPM creates a compressed mixture that is more difficult to cleanly ignite so problems in the ignition system show up under these conditions. If you have to replace a coil pack, also replace the associated plug because a failing plug can damage a coil pack either by allowing excessive voltage to build up (gap too wide) or excessive peak current (plug leakage).

A code scanner with live view capabilities will show real time counts of misfire by cylinder even when you don't have a CEL. With a failing pack or plug, instead of a full misfire you will get a "poor fire" but the scan will show the counts for this by the time it gets serious enough to create significant shudder. Very precise crankshaft speed sensors measure the periodic brief increase in crankshaft speed for every cylinder firing event and if the expected increases don't occur or are weaker for a couple of cylinders that data will be captured. To best diagnose which cylinder is causing the problem, choose conditions that hold the powertrain in the shudder inducing condition for a period of time such as a slight throttle increase on a grade to hold speed but not sufficient to unlock the converter or force a downshift. Putting your transmission in manual mode will assist in doing this since it won't automatically downshift from the selected gear unless the road speed drops too far regardless of engine load.

Torque converter lockup clutch induced shudder is not common with the 6LXX family unlike the 8LXX family which has a very different "squashed" converter so that it can fit in the same space as the older 6LXX family.

GM currently has multiple auto transmission fluid specs for their various transmissions with the older 6LXX series still using Dex VI which is also the spec for the transverse mounted 9 speed FWD shared with Ford , the 8LXX requiring Dexron HP, and the new 10 speed longitudinal series (shared with and design led by Ford) which require Dexron ULV. When troubleshooting an issue, don't add additional variables like the wrong spec fluid which will have a different modifier pack and operating viscosity compared to the correct spec. For the same reason, avoid the multiple spec labeled "universal" ATF.

Rodger
 

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2013 Cadillac ATS 3.6L
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Discussion Starter #10
Dex VI is still the recommended fluid for the 6Lxx series transmissions, I wouldn't use a different spec fluid formulated for the specific issues of the 8Lxx series in your 6L45 because it just adds another unknown to the shudder issue and it isn't needed or recommended for your transmission. Also avoid the temptation to use any "magic mouse milk" additives from the auto parts aisle which aren't good for your transmission and create a new and more expensive set of issues.

Try a full tank of 91 or 93 octane and see if that changes the behavior towards the end of that tank run. The 3.6 is a rather high compression engine and direct fuel injection allows it to perform well with 87 octane HOWEVER the transmission programming is optimized for fuel economy under light/moderate throttle and it will be in a higher than optimal gear range under circumstances like where you are experiencing shudder symptoms. This high load/low RPM engine operation leads to detonation of the fuel air charge and the chugging you are experiencing can be symptoms of both the detonation and the engine adjusting timing to reduce detonation. Winter fuel blends are often questionable so trying a different octane preferably from a different station is a simple diagnostic.

A failing coil pack often first shows up under the conditions you note. A flashing CEL along with trouble codes won't be set until there is a sustained heavy misfire, this is to avoid nuisance warnings to the driver but it means that you will NOT get a warning for every misfire episode if it isn't sustained (both in time and consistency of misfiring). A failing plug can also cause this issue, heavy engine load at low RPM creates a compressed mixture that is more difficult to cleanly ignite so problems in the ignition system show up under these conditions. If you have to replace a coil pack, also replace the associated plug because a failing plug can damage a coil pack either by allowing excessive voltage to build up (gap too wide) or excessive peak current (plug leakage).

A code scanner with live view capabilities will show real time counts of misfire by cylinder even when you don't have a CEL. With a failing pack or plug, instead of a full misfire you will get a "poor fire" but the scan will show the counts for this by the time it gets serious enough to create significant shudder. Very precise crankshaft speed sensors measure the periodic brief increase in crankshaft speed for every cylinder firing event and if the expected increases don't occur or are weaker for a couple of cylinders that data will be captured. To best diagnose which cylinder is causing the problem, choose conditions that hold the powertrain in the shudder inducing condition for a period of time such as a slight throttle increase on a grade to hold speed but not sufficient to unlock the converter or force a downshift. Putting your transmission in manual mode will assist in doing this since it won't automatically downshift from the selected gear unless the road speed drops too far regardless of engine load.

Torque converter lockup clutch induced shudder is not common with the 6LXX family unlike the 8LXX family which has a very different "squashed" converter so that it can fit in the same space as the older 6LXX family.

GM currently has multiple auto transmission fluid specs for their various transmissions with the older 6LXX series still using Dex VI which is also the spec for the transverse mounted 9 speed FWD shared with Ford , the 8LXX requiring Dexron HP, and the new 10 speed longitudinal series (shared with and design led by Ford) which require Dexron ULV. When troubleshooting an issue, don't add additional variables like the wrong spec fluid which will have a different modifier pack and operating viscosity compared to the correct spec. For the same reason, avoid the multiple spec labeled "universal" ATF.

Rodger
A lot to take in there. Thank you for the detailed write up.

I'm flushing the transmission today and I'm going with the recommended fluid for my car without the 'witch juices' added. You're absolutely right about introducing new variables. Thanks for that

I'm also purchasing a new code reader that can do live readings because my current one can't do that. Hopefully these two things will get me a definite answer. Thanks again for your time detailing all this.
 

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2013 Cadillac ATS 3.6L
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Discussion Starter #11
Update for anyone interested: I flushed the tranny and replaced the filter. I drove around for about 90 minutes trying to induce it and it happened once the first 15 minutes and hasn't happened since. Not ready to say it's fixed yet but it's definitely WAY BETTER. Thanks everyone for your input and help.

P.S. the magnet in the tranny oil pan had a lot of transmission on it if you know what I'm saying. I don't know how much is too much but it was alarming for me (haven't seen too many of them so I can't really judge).
 

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2014 ATS 3.6 Premium RWD, 2016 Corvette Z06, 2018 GMC Sierra Denali 2500HD Diesel
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This was probably the first change for you vehicle and there is a fair amount of initial wear material and of course some additional is created over time so it is probably normal.

I have owned several GMC heavy duty series diesel powered pickups and those come with an Allison automatic which has an external spin on filter with a ring shaped magnet. I always change the spin on transmission filter at the initial 1,000 mile point and there is quite a bit of stuff to wipe off the magnet at that change. I do subsequent transmission filter changes every other oil change and there isn't much to wipe off the filter magnet at those changes. The internal filter for that transmission is a change only at overhaul item and I have yet to keep one long enough to get to the roughly 400,000 mile point where the transmission is due for overhaul so I have no idea what it has picked up :)

I hope that this cured your shudder issue!

Rodger
 
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