Piston Slap: 6L80E…eeeeeek?

Leather is better. (photo courtesy: image.automotive.com)
Nathaniel writes:
Long time listener, first-time caller. I’m responding to your plea for new Piston Slap questions. I purchased a gently-used 2008 GMC Yukon Denali AWD a couple of months ago. Other than its appetite for fuel, the only negative is that it has 141,000 miles. I believe the previous owner changed the transmission fluid at 100,000 miles (Carfax shows that the transfer case fluid was changed at this point, and I can’t imagine doing that and not doing the transmission). The fluid was relatively clean but I changed out several quarts via the dipstick tube using a fluid extractor after I purchased the vehicle, replacing them with the specified Dexron-VI. I believe the fluid level is correct but it’s difficult to read.
On a recent road trip, the 6-speed automatic (6L80E) transmission stumbled during the 2-3 shift while driving through the mountains and went into a failsafe mode. The check engine light came on. I pulled over, turned the ignition off and on again, and the truck operated normally. The CEL remained on for the next several ignition cycles. When I called OnStar to obtain the fault code, they could not retrieve it because the CEL was no longer on.

I was able to reproduce the problem by giving the truck wide open throttle. The 1-2 shift occurred normally. The 2-3 shift was delayed (the engine revved higher than the normal shift point of about 5,500 rpm), it eventually shifted into 3rd, and stayed there. The CEL lit up and the truck slowed down to about 40 mph, but it eventually allowed me to speed up, staying in 3rd the entire time. I pulled over, turned the ignition off and on again, and again it was back to normal. I called OnStar again, and they pulled codes P0700 (a generic code indicating a transmission problem) and P2714 (clutch pressure control solenoid stuck off). The CEL disappeared after a few ignition cycles.
A few TSBs, #PIP4304A and #PIP4184B, and 09-07-30-004A, appear to be on point. I’m not that familiar with how automatic transmissions work, but the TSBs appear to involve disassembly of the transmission and the possible replacement of valves, control modules, etc., which sounds fairly involved. I have not noticed a delay in the engagement of forward gears after putting it in drive, which I think is mentioned in one or more of the TSBs.
There appear to be no other mechanical problems with the truck. I see the options as follows:

Do nothing for now, try to avoid full throttle upshifts, see whether the problem gets worse (how much danger is there in doing this?);
Bring it to the selling (GM) dealer, beg for mercy on the cost of any repairs since they only sold me the car a couple of months ago (on the theory that any assistance they might provide is less and less likely as time goes by); or
Take to an independent transmission shop. (Does anyone actually do repairs anymore or do they try to swap in a remanufactured transmission?)

Any advice?
Sajeev answers:
Fantastic assessment, especially the two hyperlinks. Kinda sad, however: the 6L80E is quite an impressive gearbox most of the time, like the robust GM 4-speeds before it. However, if your vehicle is (one of?) the first GM trucks using this gearbox, perhaps teething problems are par for the course.
Since you did the work for me, I’ll be brief:

Not a bad idea, because this problem will likely take a long time to get bad enough to warrant removal/rebuild.
A better idea: talk to your salesperson and ask for a referral to the service manager. Granted its a 6-7 year old vehicle with well over 100,000 miles, but there could be some goodwill in your purchase. Maybe a steep discount, hopefully a free diagnosis of PIP4184B (as per your Corvetteforum.com thread) with their fancy pants Tech 2 diagnostic gizmo.
Consider this after 1 and 2 run their course. Also consider something like a Jasper reman gearbox shipped to the mechanic of your choice; whatever works best in your part of the country.

I’d start with #2. If the dealership deems the fix within their tolerance for goodwill repairs, that’ll be great! If not, stick with #1 until the gearbox’s bad behavior is unbearable: save your repair/diagnosis for a bigger problem. Adding a new valve body to a gearbox that might have 140,000 hard, clutch-wearing miles of towing by the previous owner isn’t my idea of money well spent. You never know!
Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.
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