How to describe this....delayed shift to low at stop maybe?

Got a couple new (and I'd say related) odd symptoms with my '08 V6 automatic....
Started Sunday, truck wasn't fully warm, in D, driving on flat road at about 30-35 mph I felt a fairly strong 'buck', almost like an engine miss, or running over a large stone. It did it a second time after about 10 seconds. I immediately turned back, got my scan tool and checked for OBDII codes - nothing.
So, drove it to work this week, but kept transmission in 4, figured something related to the shudder issue (I did Lubegard a couple of years ago, planning to change some fluid this weekend). I put about 150 miles on the truck since I noticed this 'buck', or whatever it is, and it's not done anything odd, until tonight......
Came to a complete stop, truck was rested back and not moving. Felt what I'd call the transmission dropping into 1st gear. Never had this symptom before.
It did this a second time, from highway speed in 4th to complete stop (after truck rested back and not moving at all), thump. It felt very much like an automatic that had been in neutral and you just put it in drive.
I did later drive it on a slower road, 25 mph and didn't feel that downshift thump when stopping after driving in a lower gear.
I considered driveline (axle wrap, spline sticking, etc), but truck had stopped completely for a few seconds.
I'm kind of nervous about driving the truck now until I change the fluid.
Does this sound like torque converter issues? Guess what I'm asking, does this sound like a good reason to park the truck for a few days?
Thanks. BTW, in the past year it's had injectors cleaned, new plugs, air filter, oil change, and runs smooth.

 

So there's a couple things I think to take away from what I'm interpreting here...

Firstly, if you feel any bucking or something like running over a large stone at 30-35 mph, I can't say that is normal. With you mentioning changing fluid and adding Lubeguard, I'm not going to make any guesses there. But in a nutshell I'd say it's not normal to have bucking/rock hitting feelings when you are moving in gear.

Second, that "bucking" you feel after coming to a complete stop, where it feels like you are shifting into gear or a slight lurch forward but you didn't do anything yourself, that is....dare I say, normal? Lots of people have experienced this. Lots of people make guesses as to what it actually is. Personally I have felt it, and I can't reason exactly what it is myself. Personally I think it does have to do with the axle un-wrapping. Personally I had this happen all the time on the OEM rear suspension, then happening zero percent of the time for the first year after upgrading to full aftermarket suspension, and now it has returned to a small degree along with squeaking in my poly leaf bushings (meaning I need to throw some lube on them bad boys and I bet it goes away again). Some other people say it's the A/C compressor kicking on and that's enough to jolt the axle to un-wrap or whatever. Some people say it has to do with the rear driveshaft slip joint not sliding properly and then un-binding. Or some combination of any of those things - just pick the one you believe in the most and run with it.

To answer your question about "Should I stop driving for a couple days?" well, only you can answer that. But I'd see how it drives and if there are any "bucking" sensations when you are driving normally on flat ground as you described then yeah I'd start looking for more answers. Don't know how many miles you have or where you are at on the maintenance schedule, but the fact you have an '08 makes me wonder if your Taco is getting up there in mileage and maybe it could be the start of something.

Good luck finding your problem. Be sure to share back here what you find, threads like these usually are invaluable to the next guy down the line. Cheers.

 

Thanks for the info. I'm just concerned since neither of these symptoms ever occurred with this truck before. Since it's due for transmission fluid service anyway (in my opinion, I only drained and filled 30k miles ago due to the shudder), I'll do another drain and fill, lube the driveline and see how it acts. I can't recall if I lubed the splines on the slip joint when I had the driveshaft out two years ago, that could be the problem. I'll definitely post what fixes the trouble.
Thanks again!

 

I’m not sure but maybe accumulator sticking caused the hard shift to overdrive. Will see how it goes.

 

My 2012 has done this for years, where it feels like it's down shifting after coming to a complete stop. Ive serviced the driveshaft, changed leaf springs, changed transmission fluid and nothing has really made a difference. It doesn't do it all the time but when it does i can definitely notice it and its different from the feeling when the A/C kicks on. Please report back with any solutions. I just keep driving the thing and it hasn't gotten any worse with roughly 200k miles.

 

Hey Daniel, thanks for your reply, I’ll definitely let you know if I figure anything out with this. It’s reassuring to hear it’s possible this symptom may not be urgent to address. I have to fix my sliding glass door and some other non vehicular repairs this weekend and need the truck.
If I get a chance this weekend I’ll replace the three or so quarts in the trans pan and see if it helps.

 

I'll concur some bucking is normal. If it's getting worse I would look at broken or worn motor/tranny mounts.
Also, crawl underneath and grab your driveshaft by the carrier bearing and shake the shit out of it. There shouldn't be any play. A bad carrier bearing will cause bucking.

 

I think I had something very similar. My truck would "jump" or "buck" after coming to a quick stop and then releasing the brake. For awhile I thought it was axel wrap, but I finally figured out my ATF was low. I had a leaking output shaft seal. Replaced the seal and then changed the transmission fluid. Much less transmission fluid came out than what was supposed to be in there. Problem was fixed after that.

 

Look at this pdf for the article on the A750 trans.

https://gearsmagazine.com/resources/issues/e457d_2010_4_all.pdf

You may have an SLT Solenoid problem. I have the same thing but it rarely occurs so I haven't looked into having it fixed it yet.

 

@tomrad Do you by chance have the tow package? I don't know about the 08 model, but I believe it has something to do with the hydraulically assisted brakes. For the longest time my truck would do the bump after coming to a full stop but not all the time, only noticed after heavy application of the brake pedal or a drastic deceleration. One time I got bumped so hard I thought the vehicle behind me rear ended me. They didn't get out nor gestured like they did anything wrong so I just continued my journey. Long story short, after I did a brake job and replaced the brake fluid, I didn't experience another bump like that for a long time. Maybe it was coincidence, not sure. As far as axle wrap, that usually occurs after you come off the brake and transition to accelerate is when you'll notice the little bump/push as the suspension unwinds but it sounds like you had a similar issue as me where the bump would occur while your foot is on the brake pedal. If you're noticing a bump while actively driving, that could be a different issue all together.

J