Piston Slap: A New (Wave Plate) Sensation?

Keith writes: Mr. Mehta, My apologies if this has been covered, but I'm looking for advice on my soon-to-be out of warranty 2008 GMC Acadia. I'm at 64K and 4.8 years, so bumper to bumper is gone but power train is still good for a few months. Two fellow Acadia owners I know have reported tranny problems at roughly 60k and the forums seem to indicate numerous others with similar issues. Most often its an issue with the wave plate, particularly with the 07-09 models. Within the last few weeks I'm also starting to get an intermittent stabilitrak warning light promoting me to get the brakes serviced. My question for you is should I 1) hope the tranny drops in the next two months 2) shell out $3k for a 4/48 extended service contract or 3) trade it in on something similar. I love the car(truck) and was hoping to get 8-10 years out of it, so maybe #2. Or I could take the $3k and couple that with what I think is still pretty good resale value and get a new ride. I'm generally pretty cynical about extended service contracts/warranties, but I have no experience on those for automotive. Any thoughts, advice or general musings would be greatly appreciated. TTAC is wonderful resource. Keep up the good work! Sajeev answers: Now's a good time to remind my dear readers that I am not a mechanic by trade, I've just been in "your shoes" in the past. Perhaps an oversimplification, but let's do this thang and dig into your tranny. This is the first I've heard of this problem, ditto the "wave plate."  I suspect most of you are in my shoes, so a little research: this thread points to the wave plate vs. conventional clutch plate of the 3rd gear drum (i.e. direct drive, 1:1 ratio) of an older GM 4-speed.  Which I then recalled while hunting for a good rebuilt-upgrade for my Ford AOD. I learned about an upgrade to 3rd gear, choosing an aftermarket Blue Plate Special (yes, really) clutch pack for mine. With that in mind, reading one of the comments in the LS1 Tech link said it all: "The waved steel keeps the splines from taking a hard hit by pre-loading it. In other words, the waved steel takes up the slack before the clutches are completely applied." So if the waved steel clutches aren't the right "wave dimensions", there could be a problem shifting into that gear. The problem might look like this. Note how the speedometer never slows down as the rpms fluctuate: indicating that the transmission is slipping that frickin' hard on the upshift: Click here to view the embedded video. FINALLY: a transmission almost as horrible as the one in the Smart Car, without the need for Smart Car ownership! (childish giggling) Unfortunately we don't know if this video is indicative of your problem. Or if this thread on the Saturn forums also applies. Or if you have a problem yet…is nothing ever easy in this world??? My advice?  If/when the wave plates start ruining your ride, see if your homework (including the stuff I posted) can get you a little credit with GM service: pleasant, level-headed customers can easily get their out-of-warranty work covered under the blanket term of "goodwill." Because nobody wants to lose a good customer, if possible.  If not, get a reman transmission that specifically addresses this problem. I suspect both GM and big name rebuilders (like Jasper) will have you covered. Even if it's gonna happen after the warranty expires. Perhaps you should just give up and get a Crown Vic Best of luck, as always.   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. from The Truth About Cars http://www.thetruthaboutcars.com

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