hypntyz

Update, been a few months and no shuddering has returned after the fluid flush I did.

If I ever have to pull the converter for any reason, it's very likely that same one won't be going back in anyway. But, that's the first I have ever heard of that "trick". I'll try to keep it in mind for the future.

I have an 18 sierra denali with the 6.2 and 8sp trans, 18k miles, one of the very late build 18's (I had hoped that for this reason mine wouldn't be afflicted with any trans issues). After a recent long trip to go pick up and tow my c8 back from out of state, I noticed some vibration in the truck. It was light but apparent, it seemed like I was going across rough patches or rumble strips in the road every 2-3 seconds rhythmically, and I felt it in the steering wheel more than anything. Research told me that this was likely to be the torque converter shudder that is somewhat common on the 8sps. The "fix" for this is detailed in a GM document that I found elsewhere on this forum, which states that 20qt of blue label Mobil 1 LV synthetic HP ATF should be flushed through to remove all the factory fluid. The GM procedure uses a flush machine that connects directly to the transmission after disconnecting the stock trans fluid cooler piping. Others here discuss dropping the transmission pan which will get about half the fluid out, but you still have half the fluid held in the torque converter, valve body, and trans cooler/lines. Plus you have to reseal the trans pan and hope it doesnt leak. I didnt like that solution so I wanted to do it close to the GM method of flushing, but without an actual flush machine. I wanted to do this procedure myself at home and I was able to get the fluid from rock auto for about $185 shipped. I learned a couple things during the process that I thought I'd post here for others to consider. I disconnected the transmission fluid pipe up front beside the radiator, I used the one on the drivers side, behind the headlight area, under the PCM/computer. There's a plastic retainer that you slide out of the way, exposing the quick disconnect C clip, once that's removed, you can separate the piping there. Use a couple towels to catch run-out. The pipe closer to the front of the truck will have fluid being pumped out of the trans and fluid cooler, and the pipe you pull back toward the engine would be the fluid return to the trans. My intention was to pump fluid out of the trans by idling the engine, catching and measuring that fluid in an old 5qt oil jug, then use the other pipe to pump an equal amount of fluid back into the trans. I scoured the shop and found an old small pipe fitting that I modified and it slid right into the open fitting beside the radiator...a piece of 1/4 or 5/16 tubing would probably have fit also. I then clamped on an extension hose to my little adapter pipe...just a random piece of rubber hose that ran down into the oil collection jug. For the fresh fluid return pipe, another hose was going to my fresh fluid pump. The GM flush procedure states to flush out 3-4 quarts of old fluid then shut the engine off and pump in an equal amount of fresh fluid, then repeat basically until you've done all 20qt. I was able to pump the first 3qt of old fluid out as planned by starting the engine and watching the amount measured in my collection jug, then shut it off. But then I found I was unable to get any fluid to go back into the return pipe. I tried a small electric fluid transfer pump, and a homemade pneumatic pressure vessel to push fluid, but the return pipe wouldn't accept any. Further research revealed that there may be a thermostat in the trans that prevents fluid from flowing (apparently via the return pipe side, which seems an odd place to put it instead of just stopping the trans from pumping out fluid to begin with) and thats why I couldnt get any to go back in. The GM procedure has you remove a bolt from the front of the oil pan. This bolt leads to a pipe that is the fluid level check location...you're supposed to use an adapter fitting to fill fluid into this location,. The bolt/hole is tiny so a regular plumbing fitting won't fit there. The adapter that threads into that hole and has a barb for a fluid hose on the other end, even on the aftermarket, is $30, and you have to wait for it to be shipped in. I searched my garage and found a short length of copper tubing, about 5mm OD with a 2mm ID hole, and it fit pretty well in the hole in the trans pan so I put it up in there a couple inches. I then used some silicone vacuum hose and other tubing to connect that pipe to my electric fluid pump. I was able to, slowly, pump fresh fluid back into the trans that way. The pump pushed about 1 qt every 5 minutes but it did work. so I just put the pickup tube for my pump into one quart and then used other quarts to refill that one as needed. Every 3-4qt I pumped in, I would start the engine and run it a couple minutes to pump out 3-4qt of old fluid into my oil jug, using the marks and clear strip on the side to measure what I drew out. The first 8-12qt was really brown and dirty looking, even though the truck is rarely driven hard and has only 18k miles. The next few qt was cleaner, probably a partial mix of old and new, and the last 4qt I drew out was clean and bright, so that was probably mostly new. The idea of the process is that you have to waste some fresh fluid to ensure as much of the old stuff is out as possible. I had 20qt of fresh fluid but I figure I lost 1/2qt in the whole process of trying to pump fluid in, failing, changing containers and tubing, etc. so I drew out 19.5qt total and put all the fresh fluid (about 19.5) I had left in. Then I reconnected the fluid pipes up front and reinstalled the c-clip and retainer, removed my pipe from the trans pan and replaced the bolt, sprayed the areas down with brake cleaner to remove spillage, and went for a drive. I shifted through the gears manually several times and the low gears were kind of jerky the first couple of times but after that everything seemed fine. So far, I have only driven a bit but the shudder seems gone. Of course, I will need to drive it more and get the trans heated up a lot longer to be sure it doesn't come back.

Its the way the newer trucks are. I went from an 02 yukon denali with the 6.0 and 4sp, to an 18 sierra denali with the 6.2 and 8sp. The new truck makes 100 more hp at peak, is much quieter, and gets about 6mpg better on the highway, but it's city driving response is far worse. The trans seems to shift "nervously" for lack of a better word, sometimes at low speeds (say in a 25mph neighborhood when making turns onto short streets) is becomes jerky as it shifts back and forth and hunts for the right gear. The goal of these is to shift as soon as possible and keep engine rpm down, sometimes as low as 1000rpm while driving under load. That's good for economy and noise but awful for throttle response.

I have an 18 sierra denali 6.2 bought new in march with 6800 miles on it now, and it seems like I'm one of the lucky ones without the shudder based on the descriptions I am reading here. I do distinctly feel the grab and release of AFM/V4 mode but otherwise it seems relatively smooth. There are very specific circumstances at low speed when the trans seems jerky. Such as if I'm rolling to a stop sign from say 20-30mph and I slow down to maybe 10-5mph but not completely stopped, then I apply throttle to pull out and accelerate again. The trans jerks and its as though it isn't sure what gear to be in for a couple of seconds. Also I park at the bottom of my driveway so every time I get in to leave I have to climb a bit of a hill, a couple of times it is like the trans is in neutral until I give it more throttle then suddenly it grabs and jerks and takes off abruptly until I let off the throttle. One time I got in and pulled it into D and gave it gas and nothing happened at all, it was completely in neutral with the engine free revving under throttle even though the dash indicator showed D. I put it back in P then back to D and it went normally. Other than those two oddball issues, my trans seems okay. I would still like to change the fluid just as a preventative measure, even if that means I have to pay for it and do it myself. I did inquire at the dealer about the possibility of evaluating it for a fluid flush for the known issues, and the service writer told me something like "we cant do anything, gm isnt sending us any fluid until spring" which sounded like BS.