Background: 2023 Z51 w/ 20K on it. Never had any issues beyond the weird DCT behavior occasionally when coming to a stop or low speed stop-and-go. HSF procedure done and filter changed on time. Car is a semi-daily driver as I have 3 vehicles I rotate around. Car is not modified.

Issue: for the last 2 weeks, the car has acted very strange at low speeds when shifting from R to D out of my driveways particularly. The car will hesitate to move a then slam into D as though I got rear ended at 5mph. This is not just my imagination, passengers have said "WTF was that?". Worst manifestation was the car stalled multiple times trying to leave my driveway. No check engine light, no indicators of any sort. No leaks visible.

My steps: booked the car in for a diagnosis and hoped I could get it there without issue. I warmed the car to a 180 degree oil temp in an attempt to avoid the problem and got it to the dealer. Luckily, both the service writer and tech experienced the rough shift but no stalling. Writer pulls codes, nothing stored which I found odd as the car has stalled 4 or 5 times now. Tech attempts to access the TCM and can't. Says the system keeps "kicking him out". Tech phones GM for guidance and I waited 5 hours drinking bad coffee hoping they would get back to him. Finally left and the car drove ok and they advised they would call me with next steps. I opened a case with GM on my own to work it from both ends.

Question: What is wrong with my car?

 

It sounds like the hill assist or parking brake is being set and you need to apply enough power to overcome it. Are you doing this on an incline?

 

So why when I apply a bit of power does it slip, slip, slip, then SLAM into gear and stall? I don't think stalling is a part of this 'feature'.

The stalling has occurred both times on a small incline but the rough shifting and odd behavior occurs as well on level ground (such as the dealership service lane). Worse when cold and on an incline but still exhibits odd behavior at other times.

Might be worth noting I have owned the car from new so I'm not some noob that is just experiencing the Tremec DCT for the first time.

 

I didn't see that the transmission was slipping or the engine is stalling, that would be a different problem.

The occurrences I've experienced is that when the parking brake is engaged it takes some throttle to disengage it. The engine RPMs will increase without the car moving and then the brake is suddenly released causing a sudden acceleration. It is sometimes similar when the car exits hill assist mode.

You could try setting the EPB and then try driving off with it set to see if it is a similar experience but it sounds like you are having other problems.

 

Have the dealership perform the transmission fast learn procedure, it will relearn clutch engagement and have the transmission feel silky smooth again. I personally do it every time i change the filter/fluid.

 

Good to know. Perhaps he couldn't do this procedure since it kept locking him out of the TCM? They have not called me back all day so I'll give it half the day tomorrow before I start calling. I'll definitely mention this.

 

Update: Service writer called back and said GM finally emailed the tech back. They just blew right past the fact that the diagnostic tools can't access the TCM and advised that I pay to have the fluid and filter changed in the hopes that it cures the issue. I told them to pound sand for 3 reasons:

1) The TCM being inaccessible is a problem in its own right and we are just ignoring that. Further, if it can't be accessed, can the tech even perform the HSF as directed?
2) Just guessing and having the customer pay $1000+ only to have the problem persist and need replacement or extensive warranty work in which the fluid and filter would be replaced anyway is complete BS.
3) The car is not yet due for either service. I in no way want to deviate from the maintenance schedule only to have the trans pack it up 6 months from now and GM tells me to pound sand on the warranty.

I called GM customer service back and told them to get with the service manager for an acceptable solution because this isn't it.

 

Its unfortunate that they think you should pay for the filter and fluid replacement - instead of covering that under warranty. I think you said you did not have a DTC. But, in many many cases when there is a DTC set, the diagnostic procedure (which I would expect to be covered under the B2B and the Powertrain warranty) calls for progressive attempts to remedy the situation that include changing the external canister filter, changing the fluid, even removing the pan and replacing the internal filter, and also the HSF ( not necessarily in the order I listed them here). When that fails then it may be followed by a valve body replacement or a transmission replacement. But after each step there is a check to see if the DTC went away. Not sure what procedure they follow when there it no DTC - but I would have expected it to be covered. I believe that the TCM must be accessed to perform the HSF, so if that is not accessible, that is a problem of its own.

 

The dealer should try pushing a software update to the tcm and ecu, though they don't seem very helpful. If that doesn't work then probably a tcm replacement under warranty. The whole thing seems weird to begin with, I feel the car is pretty sensitive and would store a bunch of codes in regards to tcm communication.

 

I'm going to give it another day or two and see what they come up with. I'm active duty military and don't really have the time to **** about with this endlessly. My brother is a well-known Youtuber with a Corvette heavy channel so I'll just blow this up in GM's face if they continue to jerk me around.

 

Not sure what military has to do with it other than you’re feeling entitled over non mil? Let the system work. It’s a warranty item. That said a lot of people have gotten poor service from dealers and poor warranty support from GM.

Good luck and thanks for your service.

 

I'm the least entitled service member you'd ever meet. Nobody even knows I'm in the military. I just brought it up because being in a Middle East focused special ops unit, you can well imagine that I don't have a lot of time to mess around with my car at a dealer an hour away from my job. As far as the system 'working', it isn't doing anything. No parts have been ordered, there is no plan to fix anything. They seem to think it's fine that the car stalls and is clearly not working. Nor do they seem concerned that the diagnostic tools cannot access the TCM. Not sure what exactly you think is going to happen here if I do nothing.

 

Take the car to the dealer and tell them to fix it. It’s under warranty. Tell them you want a free rental.

Complaining (saying this nicely) here or a persons YouTube won’t help.

Good luck.

 

Tried that. They contacted the GM Field Service rep who told them to have the customer pay for a fluid an filter change (you literally can't do the hydraulic system flush procedure if the tech can't access the TCM). The dealership has no ability to go outside what they've been told to do. This dealer has about two loaner cars and they are out. They are not going to pay for a rental. That is not something you are guaranteed to get. A lot of folks in big metro areas think every car dealer has 50 loaners sitting around. GM has zero obligation to give you a car if yours is broken.

 

Is there another dealer?

 

Yeah. Also an hour away. The service manager from dealer #1 is supposed to call me in the morning. If he doesn't, I'm moving on.

 

Update: Dealer #2 seemed much more keen to diagnose and work with the quirks of whatever was going on. They kept it in the climate controlled shop overnight to replicate cold start stalling. Still not storing codes which is baffling but GM has advised replacing the valve body. So looks like external canister filter and fluid will be on GM.

Of course the valve body is on nationwide backorder.

 

Amazes me how many poor dealer service centers there are. Hope this turns out awesome.

 

I'm optimistic but if the car acts up again, I'm not going through another multi-week debacle with it. I'll trade it on something else. Shame as the car is so good when it works but these transmissions are just too problematic and then the issue is exacerbated by terrible customer service on GM's part and outright incompetence on too many dealers' parts. At least ZR1 customers are getting handled with kid gloves and every issue they are having are getting pushed to the front of the line. I still can't imagine dropping $200K to put up with GM.

Since I don't have a ZR1, my service writer told me to be prepared to wait multiple weeks for the part to be sourced.

 

Hope you get results. My friend, who was the shop supervisor, had a very real vision of his job. It was the salesman's job to sell you the first car, it was his job to sell you the next one by great service.