Excessive downshifting in 2011 Buick Lacrosse

[GerGa]

My 2011 Buick Lacrosse 3.6 v6 does this thing(rarely) where I am already driving on the highway, go to accelerate, and it barely accelerates, and just downshifts excessively, and downshifts dependent how much throttle I give it.
When it happened the first time, a shop rebuilt the trans and the car did it again the week I got it back. I took it back and he scanned for trans codes, agreed to come back when it happens again. After that, it seemed to do it once every 6 months.
The car did also have trouble shifting into drive when hot pretty much every time, but obviously I should have got a second opinion on the problem from another shop, and kind of got screwed.
At least it will shift into gear when hot now.
It has not done this in a year+, but I haven't driven the car much in the past few months.

I'm wondering if anyone has run into this problem or has any advice on how to fix it.
When it happens, I can put it in manual mode and drive it just fine. It would go back to normal on my way home from work when finally off the highway. Or possibly with car restarted, its been a while, I can't remember...
I'm hesitant to have a professional look at it because I don't know how easy a once every 6 month problem will be to fix when it doesn't throw any engine or transmission codes.
So I'm here hoping for other's experiences. I feel like it is a sensor or something electrical, as the transmission performs great now otherwise.

Thanks

 

[supton]

Is this doing it intermittently, or always? Intermittent can be a pain, but usually is electrical in nature.

Can you start driving it more and see if it's still doing every 6 months, or if it's degraded and can be made to do on command?

 

[GerGa]

It's like every 6 months, I drove every day probably a couple years after the rebuilt/time it first happened. Scared the heck out of me, but figured out quickly to use manual mode when it does happen. So i mean it will happen one trip and not happen again for many months. One time it happened twice in 2 weeks but that's it.

 

[GSCJR]

I feel like it is a sensor or something electrical

I agree, this sounds more like a fuel delivery problem.

 

[supton]

Sounds very intermittent then... hrm.

When you say "manual mode", do you take out of drive and put into (3rd?)? Like, you just drop down a gear with the shifter, and it then will be in the proper gear (I think it's 3rd on this)? Does it drive properly then, but if you shift back into drive, is it fixed or does it again act up?

I could see fuel delivery but don't see how moving the gear shifter does something to change that.

Edit: Ok I see this is a six speed, not 4. 6T70 apparently.

 

[supton]

Dumb thought: Neutral is next to Drive on the gearshift, no? Is it possible that the gear shift mechanism is out of adjustment or something faulty in this area, and it's actually shifting into neutral? Pulling on it to go into any other gear would jog the mechanism and/or sensors, and might restore any fault. If not in the shifter area, then perhaps the transmission internally goes into neutral (all solenoids open up?) and then commanding a gear down "wakes up" the ECU and it retriggers solenoids.

Just a pure off the wall guess.

 

[GerGa]

Sounds very intermittent then... hrm.

When you say "manual mode", do you take out of drive and put into (3rd?)? Like, you just drop down a gear with the shifter, and it then will be in the proper gear (I think it's 3rd on this)? Does it drive properly then, but if you shift back into drive, is it fixed or does it again act up?

I could see fuel delivery but don't see how moving the gear shifter does something to change that.

Edit: Ok I see this is a six speed, not 4. 6T70 apparently.

To both of you, why would fuel delivery be an issue when it wants to shift instead of hold a fear during acceleration, essentially?
Manual mode on this car is just sliding the shifter left while in D. Has a + and -. Pull back to downshift, push forward to upshift.

 

[GerGa]

Dumb thought: Neutral is next to Drive on the gearshift, no? Is it possible that the gear shift mechanism is out of adjustment or something faulty in this area, and it's actually shifting into neutral? Pulling on it to go into any other gear would jog the mechanism and/or sensors, and might restore any fault. If not in the shifter area, then perhaps the transmission internally goes into neutral (all solenoids open up?) and then commanding a gear down "wakes up" the ECU and it retriggers solenoids.

Just a pure off the wall guess.

Possibly, but I don't think so. I've thought about it and it seems like it's grabbing gears but just lower and lower gears as I push the gas more.

 

[supton]

It's drive by wire, right? If it's running out of gas and not delivering the requested power, it is apt to open throttle more and command downshifts to get power by looking for more revs. Should trip a CEL though, unless if it's too short lived.

Not familiar with this car at all. I know on Toyota's going into manual mode, it's still vetted by the transmission controller. You cannot force the transmission to be in a higher gear than it would normally chose to be in--you can force it to be a few gears lower, unless if it might result in engine over speed. But you can't force it into lugging. In a sense, it's more of a "max gear allowed" and the transmission shifts with that limit in mind. Here, maybe you can tell it to upshift, no idea.

Odd problem for sure.

 

[Jetronic]

To both of you, why would fuel delivery be an issue when it wants to shift instead of hold a fear during acceleration, essentially?
Manual mode on this car is just sliding the shifter left while in D. Has a + and -. Pull back to downshift, push forward to upshift.

Downshifts are likely triggered by more throttle opening (like you would get going up a steep enough hill) and gas pedal position (driver wish). Seems like you push the gas depper and deeper and the car translates that as you being in a hurry to get up to speed. I also think it's an engine power issue.

 

[GerGa]

It's drive by wire, right? If it's running out of gas and not delivering the requested power, it is apt to open throttle more and command downshifts to get power by looking for more revs. Should trip a CEL though, unless if it's too short lived.

Not familiar with this car at all. I know on Toyota's going into manual mode, it's still vetted by the transmission controller. You cannot force the transmission to be in a higher gear than it would normally chose to be in--you can force it to be a few gears lower, unless if it might result in engine over speed. But you can't force it into lugging. In a sense, it's more of a "max gear allowed" and the transmission shifts with that limit in mind. Here, maybe you can tell it to upshift, no idea.

Odd problem for sure.

Yes drive by wire. Another thought I have is traction control or stability control, but I don't think I got any lights showing up on the dash. Not 100% sure they trigger lights on this car, I would have to test that driving in the snow.

 

[THORROMIG]

Whens the last time it has had a tune up?

 

[GerGa]

Whens the last time it has had a tune up?

Never, cars been in the family since 20k miles, now 112k. Recently sat for 2+ weeks and had misfire codes but drove fine. Mechanic told me he can't take it for a week, and that the plugs could be the cause but the iridium plugs they use last a long time. Said clear it and see what happens, so I did, been driving it and it didn't come back.

 

[THORROMIG]

Never, cars been in the family since 20k miles, now 112k. Recently sat for 2+ weeks and had misfire codes but drove fine. Mechanic told me he can't take it for a week, and that the plugs could be the cause but the iridium plugs they use last a long time. Said clear it and see what happens, so I did, been driving it and it didn't come back.

Honestly, I would begin with plugs,air and cabin filters, differential fluids, trans fluid -- at least spill N fill. Start making your list and check out Rock Auto for pricing. Hook it up with the 5% off code on the rebate/sale forum and hopefully you can save some money. Good luck and keep us posted please.

 

[GerGa]

Honestly, I would begin with plugs,air and cabin filters, differential fluids, trans fluid -- at least spill N fill. Start making your list and check out Rock Auto for pricing. Hook it up with the 5% off code on the rebate/sale forum and hopefully you can save some money. Good luck and keep us posted please.

Could check air filter. Trans good, was rebuilt. We wanted to do plugs on our own but the back 3 look inaccessible. I thought maybe have to take the intake manifold off but my mechanic mentioned there is a tool to pull the engine forward on its engine mounts. I didn't look too hard into how to actually do the plugs though.

 

[THORROMIG]

Could check air filter. Trans good, was rebuilt. We wanted to do plugs on our own but the back 3 look inaccessible. I thought maybe have to take the intake manifold off but my mechanic mentioned there is a tool to pull the engine forward on its engine mounts. I didn't look too hard into how to actually do the plugs though.

Sorry, you did mention getting the tranny rebuilt. Hopefully with the right tools and some patience -- all goes well for you.

 

[SatinSilver]

@clinebarger may have some good ideas here.

 

[GerGa]

This video shows the plugs being done. Requires taking the intake off, doesn't look that bad.

 

[Jetronic]

Yes drive by wire. Another thought I have is traction control or stability control, but I don't think I got any lights showing up on the dash. Not 100% sure they trigger lights on this car, I would have to test that driving in the snow.

wet grass can do it too.

 

[meep]

hard to say if this is even a trans issue. It could be a trans issue, or the engine could just be down on power and the programming making it seem like to you, the driver, that it’s a transmission issue. How many miles on this transmission fluid?

Without doing maintenance to get the whole thing up to date, you may be chasing ghosts. Air filter, plugs, oil, transmission fluid…possibly plug wires. Note - bad plugs or wires are most prone to cause problems when loaded up at lower rpms. Revving higher is easier for the spark to push through (less dense air).