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Have a 2011 E with 117k miles on it. Started "shuddering" on either light acceleration or maintaining speed. Also has the fluctuating rpms as I've seen mentioned here. Local service rep thinks it's likely the torque converter, which does check with what I've been able to research (here & other sources). He says it's about $2500 to replace the converter, which also seems to check out. I asked about possibly replacing the timing chain while the engine is out if I'm going to keep the vehicle (which is likely with what new ones cost and/or the unavailability of them) [No codes or problems at this point - strictly preventative]. Said that would up the cost to about $4000. That seemed a bit much since the engine was already going to be out, but a mechanic I am not.
What I'm considering trying before making the service appointment is to replace the transmission fluid (doing the "double drain & fill" technique) and having the coils checked to see if any might be misfiring without the CEL coming on. (Plugs were replaced at the beginning of the year as part of a 100k mile service I had done, also the water pump & balancer pulley before that). Is this a waste of time & money, or something that makes sense?
 

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we had similar shudder shifting gear around 35-45rpm. began like 135k ish on the 2009. drove it til 167k til multiple other things began failing. (there’s a detailled post on the rebuild) no tranny codes for us either.

i did remove my tcm and had some membranes fail to see if that was it.... rebuild w sonnex kit. still not different.

added shudder fix, no luck.

next common step is to replace engine mounts. at 167k, my second set of mounts were failing. easy to check. drive car up on curb so u can get ur head under engine. have someone pushing car back and forth in park with w engine off. check the 2 primary fluid filled mounts on bottom, and the small arm next to the fuse box . if they flop around. replace them first

i took the car out of service at 167k and dumped $7k in parts replacing all the reasonable parts while i had the engine out in my garage

we never got codes related to the chains, but the car leaked oil like the exxon valdez. once i got down to the chains, i noticed the tensioner had room to expand and the guides had plenty of teflon left on then. at 167k, our car didn’t NEED chains done, it needed all the rubber seal replaced. we changed oil 4-6k w half of them synthetic and other half dino. yes, synth is better, but imo after having engine apart, it was very clean. , as long as u change oil often and have no codes, you can get more life out of them, imo.

i assumed my shudder issue was TC as well. i ended up buying a low mileage tranny/TC. problem went away. paid $550 for a 2011 tranny w 67k, reused my techm.

i think it was the TC in my case, but can’t prove it

if you can live w the shudder, consider getting another 10-30k out of it like i did. then when u pull the engine, you’ll have a lot more to replace..... camshaft actuators, ac compressor, leaking steering rack, leaking oil seals.... i strongly recommend to replace the $5 oil pick up tube in the oil pan. mine was gum rubber and lost its seal. most likely starved my actuators of oil = “sewing machine “ rattle at start up

i have the 2009, which appears to be the worst reliability year model to own?? sounds like 2011 is better

i had felt the issue could have been engine misfire related as well, but i ultimately believe it was the TC after rebuilding the car.

oh, i added an external tranny cooler and filter. i flush my tranny by removing the tranny outlet tube, run the engine 20-50 seconds til line gurgles, add more fluid, repeat til about 13-14 quarts until bright red dex6 comes out. imo, this is the best way... engine pumps fluid through on it’s own, no excessive pressure, chemicals, and you do this once vs multiple 4quart “dilution” changes. i did this thinking (and the $7k) i might?.. might get another 60-90k out of this car???

good luck
 

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The transmission in my '11 failed after the radiator developed an internal leak that allowed coolant to contaminate the transmission fluid. This was around ~155k miles or so. Water and coolant are enemies of the friction material inside the transmission and torque converter. GM has a test kit for the dealer called Gly-Tek meant to test for coolant contamination. AcuStrip makes a test strip that you can dip in the trans fluid to test it as well. If the transmission has coolant in it, you will need a new radiator and transmission with torque converter if you want a long lasting repair.
 
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The fuel mapping could be commanding very lean under cruise situations causing the shudder due to weak coils or spark plugs which misfire under these conditions and usually goes away when you accelerate. Connect a scantool and check the misfire counter for each cylinder. If you see a cylinder or two with misfires, a bad coil or plug could be your problem. I've seen this more than once and without a Check Engine Light on. The Enclave 3.6 V6 engine requires the intake manifold to be removed to replace spark plugs and coils. With 117k on your E, I would replace all the coils if one is found to be the problem. Stay away from cheap coils, I think there are some threads on this forum on which aftermarket coils are the good ones.
 
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I own a 2011 Enclave AWD and I certainly can't say whether my shuddering issue is yours or not. Mine only shuddered between the 2 - 3 shift and on light acceleration. I was in talking with the service manager at the Buick dealer and the trans mechanic came around the corner just as I explained the problem. He immediately said that it was due to a failed retaining ring in the transmission and that it was a faulty design. I was getting 50 rpm fluctuations whether I held the gas steady or it was in cruise I could stop the shuddering by accelerating harder. The only time that did not happen was going up or down hills.

That's the long of it. The short of it is that they got a GM remanufactured transmission (that took several months to find) and for the sum of $5500 it took care of all the problems. The remanufactured transmissions from GM are guaranteed for 3 years or 100,000 miles.

I use to own a garage and still do all my own work on my cars and it's always hard to let someone else do it but changing out a trans is just too difficult the way they are made now.
 
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