Hi all, I know I have no credibility or post history here. I have been looking into buying a 25' z51 C8 SR in a few months and have been doing serious homework and research. It helps I have been off work the last week due to an ear surgery, with nothing better to do than read everything I can find.

Okay, so a few things struck me while reading thought all of the DCT issues I could find, which also seems to be supported by the highway drive/OBD2 pre-fluid pre-filter change flush routine. It seems those who have had valve body related shifting issues all fall into the occasional use category. I have also found a supporting corollary, those who daily drive longer miles seem to have far less reported issues. So my theory is this, those who are seeing higher probability of valve body issues could be due to particle build up from lack of sufficient fluid flow and filtration. Lots of short drives not using all the gears or not at adequate speed for sufficient duration for proper filtration.

I could be way off, only GM and Tremec engineers may know for sure, but it sure seems like a possibility. Well, thanks for letting me muse my thoughts.

By the way, coming from a 2023 CT4V A10 Blackwing that I daily commute 127mi/day, 60 of those miles is 2 lane canyon road. Love the BW, but I really want another Corvette.

 

There is no pre-fluid change flush routine, The Hydraulic System Flush is done prior to the replacement of the DCT external canister filter.

 

Yes, sorry. That's what I had meant to type. Thank you for the correction.

 

My 2021 HTC had the transmission filter replaced at 4000 miles and again at 7500 miles. The transmission fluid was replaced just before 3 years of ownership. I’ve driven the car from Florida to Ohio twice. Didn’t matter. Transmission still needed a valve body replacement at 13,000 miles when it decided to skip the even gears.

My C8 is my first and likely last Corvette I will own. The DCT has been my biggest disappointment with the car. The maintenance requirements are outrageous, I don’t like the shift points, and the reliability stinks. As long as Chevy continues to put a Tremec DCT in the Corvette, I’m out.

Are you keeping the 4BW or trading it in on the Stingray? I also own a M6 5BW. The 5BW is the better car in my opinion. The only place the Stingray outshines the BW is handling because of the mid-engine design. Ride, power, and build quality are all superior on the Cadillac.

 

The only person I know personally who had a DCT failure had it happen on his way home from museum delivery, with about 400 miles on the car. I doubt that was because the car didn't get driven enough....

 

I think your theory is as sound as any I've read. At this point, who knows for sure, and I also believe the issue is less of one then the internet would have us believe. While there have definitely been C8 Tremec DCT issues with some cars, no doubt, it reminds me of the dreaded valve guide issue for our C6 Z06 7.0 motors......the chatter on the forums and internet in general made people feel like it effected 90% of engines. Love my particular C8, and have never had a stutter........yet

 

So, the "particles" build up while the car is not running? Every drop of fluid that goes into the valve bodies goes through both the coarse internal filter and the external canister (fine) filter first.

We know that in some cases the Hydraulic System Flush will resolve an issue, so that strongly suggests that some issues are a result of debris that was lodged somewhere and free up by the procedure. But where did the debris come from? How did it get past the filter?

Is the debris coming from the clutches (the two main concentric clutches and also the clutches in the differential)? If so, then it only comes from running. More running, more debris. How did it get past the filter? Some say the filter is too small and goes into bypass. Some say the filtration is not adequate. If that's the cause, then the way it gets past the filter is from running. More running causes more debris to get past the filter if that is what's going on (I'm not saying that is what is going on).

Or, was the debris already lodged in the lines and valves from faulty manufacturing? If so, that has nothing to do with short trips, does it? Maybe running cause the debris to shift.

See if you can complete your theory by explaining where the particles come from, how they get past the filter, and why short trips would cause more particle build up then longer drives, and why shorter trips would allow more debris to bypass the filter and get into the valve bodies. Keep in mind that in normal driving, regardless of the length of the trip, the fluid is flowing through the valve bodies at regulated volume rate, because the internal operation of the transmission requires constant, regulated hydraulic pressure regardless of length of the trip, or the speed of the car. I do not know how the plumbing is set up, but I think its possible, even likely, that not all of the fluid that goes through the filter goes through the valve bodies. Its likely that some of the fluid simply goes through the filter and is returned to the sump. If so, then that may be related to the so called "highway" routine that Josh mentioned. When cruising at highway speeds the car could ramp up the volume of fluid through the filter, some of which supplies the valve bodies with the required hydraulic pressure, and the remainder just being returned to the pump. So, that would be consistent with what Josh said - it "forces more fluid" through the filter. And if that is true - the longer trips or more highway driving would indeed result in more filtering. But it still doesn't explain how the debris gets past the filter and into the valve bodies unless there is something wrong with the filter.

 

My uneducated guess? Mostly trash/debris clogging the solenoids in the vb. Clutch fibers and manufacturing contaminants are what I imagine the culprits are.

 

Manufacturing debris is possible. But how does clutch debris get past the filters and into the valve bodies? Is the fine filter inadequate? Some think so.

A theory that would work if the filters are indeed inadequate is this. Start with the assumption that when fluid flows through the fine filter, some of it goes into the lines and valve bodies, with well regulated pressure, and some of the filtered fluid is just returned to the sump. So, during steady cruising at highway speeds, and consistent with what Josh Holder said, more fluid is being forced through the filter - more filtering is taking place. And, since its steady highway cruising, there is very little or no clutch action. So the fluid is getting maximum filtering and little or no clutch particles are being shed. Compare to spirited driving on back roads, or short trips - now there is plenty of clutch action, and now we don't benefit from the higher volume of overall filtering we get during highway cruising. Does this result in build up of debris in the fluid? So much that it causes the filter to go into bypass? Without that last step, it does not explain how the debris gets past the filter.

Another thought (entirely made up and not necessarily what I believe) - the highway cruising routine, if it forces more fluid through the filter (Josh's words), means that there is more hydraulic pressure on the upstream side of the filter (more pressure to push more volume through the filter, right?). So, if that is not designed or implemented perfectly, perhaps the excess pressure during the highway routine causes the bypass to happen. There are forum members that firmly believe the filter goes into bypass mode, and there are some photos showing debris that settled on the canister filter cap, which is after filtering. Is that evidence of bypass, or evidence of some other source of debris? I do not know.

All of this is made up, and just to illustrate that we can create hypotheses - but we really don't know.

 

If the problems were due to an inherent design problem there wouldn't be so many DCTs that don't have problems. And the failures really don't have a single source. While some of the failures are due to valve body problems there are also other problems with the DCTs. And many of the failures are infant mortality.

The DCT is very complex. The valve bodies themselves are complex. The problems are likely due to quality control like residual contamination from manufacturing and just common quality control problems with the multitude of components that go into the DCT. The individual components themselves may not have a high discrepancy rate but all that it takes is one component to fail. It will take a higher level of component reliability to improve the overall DCT reliability which takes time.

 

I think the pressure bypass may be part of the problem. If I were king for a day, I would install a series of filters with delta P indicators and a warning light when the last in the series went into bypass mode. Then oil and filter analysis… treat it like a new acquisition aircraft.

 

Completely agree with this. Just going by forum posts (which is all we really have) it seems the majority of failures are early on and less than 15K miles unless I am missing those or just ignoring them. I also completely agree that this is a complex transmission which is going to be prone to more issues. I am confident that GM is sorting it out with much of it being in the quality control department rather than in the component redesign department.

 

How many of you think the Tremec TR-9080 DCT will be in the C9?

 

Yes, I also agree that debris is not the only issue. Most of my posts above were in context only of the debris related issues, if any, and in response to the OP's suggestion that short trips might cause build up of debris. While some have reported that a transmission issue was solved with a Hydraulic System Flush, which suggests a debris issue, there are other reported cases where a valve body replacement was required, and fixed the problem. That could be due to a debris, but could also be due to defective components in the valve body. Some others were not solved by even a valve body replacement. When a transmission is replaced, we typically never know what the issue was. And yet others, more recently , required replacement of the clutch components. I think there was even one about the gasket where the harness enters the transmission.

@RKCRLR mentioned the complexity of the valve bodies. Here is bit of info with illustrations of that. There are two valve bodies. Besides the actual castings with all the plumbing, the valve bodies are populated with Transmission Control Solenoid Valves, Transmission Fluid Pressure Sensors, and in one case a Transmission Fluid Temperature Sensors. Illustrations of those are attached. The Main Valve Body in the non Z car has 2 pressure sensors, 6 control solenoids, and 1 temperature sensor. With the Z, which adds the eLSD, there are 3 pressure sensors, 8 control solenoids, and 1 temperature sensor. The Shift Activation Valve Body has 6 control valves and 1 pressure sensor. Thats a total of 14 control valves, 4 pressure sensors, and 1 temperature sensor. That information comes from list of components in each valve body, and not from looking at the pictures. There are other controls and sensors that are not in the valve bodies as well. And the valves themselves are controlling hydraulic actuators that move the clutches, the shift forks, eLSD clutches, park lock pawl, and likely more stuff I don't know about.

Plenty of failure modes.

 

So how many DCT's have failed? 1%,2%, More than 5%. How many are service ? How many have never failed?

 

But a lot of people discard the numbers because 'forums attract users with problems' which I disagree with.

 

I think only GM know the numbers. As to the question of whether or not the C9 will continue with the Tremec DCT: 100% yes.

 

5 years ago I posted that a Tremec engineer said it was from clutch debris. Are the current problems still from clutch debris? IDK.

 

Yep, my feeling as well. Reminds me of the difference between Recession and Depression. When your neighbor losses their job, it's a Recession. If you lose yours it's a Depression. All relative. I don't perceive it's a significant issue (BUT then I had zero issues for 4 years with my 2020 C8 and my E-Ray has been fine for 11 months.)

My subjective experience with my 2020 C8 related to:

• A number of folks reported DCT CELs relatively early. in 2020/2021 Their Dealer Tech called GM Service with the CEL Code and it told to perform a Flush, no new filter, no new fluid just a Flush. Most reported the CEL went away and didn't come back.
• I found my car where I crawled into the garage watching the driver's mirror Tip to garage door wood molding holding the gap to ~1" so the harder to see passenger mirror easily cleared. For the 1st ~1000 miles, crawling in at ~2 mph it was a bit jerky. After 1000 miles it was smooth. I attribute that to the 1st gear clutch (that must slip below ~4 mph) wearing off the high spots on the multiplate clutch disks.

So end year 1 with ~3000 miles I took advantage of the GM offer and had the DCT filter changed with Flush prior. Of interest don't see those early CELs reported as much in the past 2 years. And my E-Ray was smooth crawling into the garage from Day1. Are the clutch plates better?? But seeing few early CELs and the fact that I now drive my AWD E-Ray rain or shine and will have >4500 miles end February when the engine oil changed is required I'll wait and have the free DCT and engine oil change end year 2 when I'll have 7500 to 8000 miles.

BUT enough of this subjective ideas, including mine. Watch this video from a GM Tech who has worked on 50 DCTs. One key point it has many sensors and can generate >100 CEL Codes. Also IMO these DCTs can all probably be repaired. In fact it's possible Tremec is doing just that and certifying them to go back as replacements!

 

Thanks for the video, I had not seen this before. All excellent information being presented here.