I drive my 2022 C8 Z51very fast in the open road races several times a year. This is the fourth Corvette I have used in these races. The C8 is by far the best above 150 mph. But my car has a problem that has shown up three times and cost me finishing well. No DTC codes ever show up. All the fluid and air temperatures are well within limits.

After 30+ miles of 150 mph driving on a warm day, the car slows down. Foot to the floor is 155, then 150, then 145. For the next 30 miles the maximum speed wanders around between 135 and 145 mph. No gear changing or mode changing makes any difference. It just won’t go fast. My dealer and service advisor are very good but they can’t find anything. This is a CND -Can Not Duplicate.



With Jeremy Welborn’s help (thank you very much), I managed to contact Corvette’s Chief Engineer, Josh Holder and Lane Rezek, Corvette Brand Quality Manager. Through several email conversations we tried to isolate the problem and eliminate false paths. I sent them PDR files and other diagnostic files from the road rallies. I use Cosworth’s PI Toolbox and already knew there was nothing in the normal OBD II at the ALDL that would help this problem. Josh and his staff concluded the same thing.

Before my last high-speed event in October, Josh had a field engineer meet me at the dealership. I left the car with Lee Williams for several hours. He installed a custom-made data recorder on the ALDL (OBD socket) and mounted it securely behind my seat. I had a button mounted by my right knee to push when the car started acting up.

Event day was cool and I was afraid that nothing would happen. I drive this car as fast as I can until I reach a finish solution, then I slow down as required. Well, the car decided to slow down first. I was in the middle of nowhere with no cell service or anything else. I had no idea if the recorder captured the slow down event data and whether it was able to upload it.

A few days later Josh emailed that the needed data had been received and his engineers were working on a solution. Today I got an email with the summary of his investigation and a simple solution. All this preamble to the solution is needed. There are going to be some people very surprised by his answer. But remember where this came from and his position. Here is his answer.

On October 27, 2023 Josh Holder wrote:

When you can schedule a visit to your dealer to return the data logger, please ask them to REMOVE 2L of oil from your transmission. The +2L fill was intended for “track” use as in “closed road course tracks” with sustained high G loading. You aren’t doing that in this event. Frankly most people would never see pressure drop outs even on a road course track at the factory fill level.

When the transmission has the extra 2L of oil, the sump level allows 6th gear (which you are in a lot) to spray oil directly onto a temperature sensor. When subjected to high load, like wide open throttle for long durations, the oil spray from 6th gear gets hot enough to cause the sensor to report to the transmission control module that it should begin to limit torque to protect itself. That is exactly what is happening to you. It’s a localized spray of hot oil. The sump average temperature is still quite low.

For the Z06 and all Corvettes built after July of 2023. The case and lube system have been revised so that the extra 2L of oil is no longer required for any situation.

That you very much for your willingness to help us out. We could not recreate the problem in our own vehicles because they were not track prepped for high-speed run (It does not meet our definition of track use), but I can completely understand why it would seem logical to add the 2L for your event. It just was not intended for that use case.

Drain 2L and you should be good to go.

 

Impressive! Thank you for sharing!

 

Post of the Year

Very interesting & informative

 

This is great information to have. This shows that DCT is not designed to have that extra 2 liters of oil and that it really should only be filled with the extra 2 liters to solve a flaw in the case design when used in sustained high G track usage. That people using their C8s on the street should not be adding the extra 2 liters. That adding the 2 liters can have unintended negative consequences that GM has not anticipated. The good news is that, moving forward from August 2023 onward, we don't have to worry about this problem.

 

Interesting that he says that even in road course situations most people wouldn’t experience an issue without adding the 2L. I think that definitively shows it’s not needed for the street as some people have done.

 

Agree its post of the year. It shows once again that the 2 extra liters is not needed except for "track" use. It shows that what pistolpete817 was doing is not considered track use. It shows the there are actually consequence to adding the extra 2 liters of fluid - it is an overfill. But to me, it shows something else as well. I especially like the detail in the explanation - that with the excess fluid, 6th gear is able to "spray" on to a sensor and heat it up - a sensor that with the normal fill would not have that. It just illustrates the complexity of the transmission - and the difficulty one would have in second guessing what is going on inside of it - unless you were actually involved in its design. I mean, who would have come up with this explanation - unless you 1) had the data and 2) understand in detail how the transmission works?

I wish we could get a more detailed explanation about what the hydraulic system flush does, and why it is needed.

 

@pistolpete817 , I wonder if you can change the title of this thread you started? To something like "potential issue with using the Track Fill". This is an important message that is backed up by data and by Josh Holder/GM's review of that data, leading to an interesting conclusion. But folks searching for it may not find it easily - even though it is in your post 1 of this thread. Or, may just another thread with that title that points to this thread?

 

I find this odd, be interesting to see pete's results after removing 2 qts.

 

Great info and great that GM stepped in to garner real data and spend the time to analyze the situation.

I’m curious to understand if the issue is associated with being in 6th gear and with the 2 extra liters and if the slinging of fluid onto the sensor is while only being in 6th gear. Does that issue go away if in other gears?

For most of us that doesn’t happen for sustained periods. Unless you’re in z mode a lot and never get past 6th.

unintended consequences….

Again, great info.

Mark

 

Transmissions are usually designed with the planned fill level form the beginning so it isn't surprising that an overfill to address one deficiency could lead to an unexpected consequence that the engineers didn't test for. The explanation is plausible but I agree that it is too early to declare victory until the additional fluid is removed and the car run is similar conditions without repeating the problem.

Best of luck to the OP.

 

Little details like this are the reason I never dispute those that claim the transmission shifts smoother (not better, but smoother) with 2 extra liters. That is, we just do not know everything about what is going on inside that beast - and with all the sensors and computer controls - there is no telling (unless you understand the design) what kinds of complex interactions might affect transmission and engine behavior. And in this example, the combination of extra fluid, being in 6 gear, a little spray on a sensor, led to torque limiting the engine (and here much of the discussion about what was causing his problem, prior to this, in another thread he started about the issue, focused on the engine sensors and ECM.

 

It also leads back to the often-heard refrain, "read [and follow] the manual." I do not track my Z51 so I did not add the 2 liters of fluid and the car has performed properly going on two years and twenty-five thousand miles.

 

Yes that is always good advice. Although the manual also says if you have added it as part of track preparation, it is not necessary to remove it for street use. For many, this would suggest that while not necessary for street use, there is no harm either. Of course the high speed road course driving is not street use either. Will they now add a statement that if previously added, the 2 liters should be removed prior to sustained high speed driving? Its rhetorical, but not far fetched.

 

I don't disagree that some people may have had smoother shifting after adding the additional two quarts but it might not have been caused by the additional fluid. One person posted they had smoother shifting after an engine oil change.

Perhaps a software update was performed when the car was brought to the dealer for the fluid fill. Many dealers check for updates and install them when a car is brought in for any service. Or perhaps there was an OTA update. There have been several updates for the transmissions.

The transmission shifting can smooth out during break-in. My car shifted smoother after it got several hundred miles on it. If I had added the additional two quarts I might have attributed the smoother shifting to the additional fluid.

There isn't an obvious mechanical reason for smoother shifting due to overfilling a transmission, especially if the design has pressurized lubrication and spray nozzles (I don't know if it does). Correcting an underfilled condition, yes. The software algorithm is the main influencer to how the car shifts. I'm not saying the overfill can't cause smoother shifting but there are more likely reasons.

 

I always wonder what they mean by smoother. Less "snappy"? "slower"? I'm not sure mine could be any smoother. This week I have spent a lot of time with powertrain set to track, but using manual shift mode. The shifts are remarkably quick, but somehow without any jerk - I attribute that to the automatic rev matching.

I agree there are other reasons possible (including confirmation bias). My cars always run better after they have been detailed. I thought a possible reason was that the extra fluid was creating some sort of extra damping. The clutches are wet clutches as we know. As they engage there a stage they go through where the torque is transmitted by viscous coupling only (per a paper that I have posted a few times on the forum). That is coupling via the layer of fluid on the surfaces that are engaging. As they engage more they start to transmit torque through "asperity" contact which is a highfalutin way of saying the contact between the clutch surface peaks. The final step is full engagement. Perhaps the extra fluid results in more fluid between the clutches which would might affect somehow the clutch engagement during that viscous phase of engagement. But, I was shot down for suggesting that idea. And its speculation.

 

Good idea. I have a post in the General Section that points to this post.

 

When I talked to Josh at the bash in BG, he told me that only on instrumented cars did they see a slight pressure drop that lead to the add 2 quarts on track in the early cars. He said the pressure never went to zero, just didn't stay constant in a super high G long corner. He said he doesn't think there are too many corners in America that could even replicate it.

 

I wonder how they came up with the 2 liter number (and not less). They must have done (extensive?) testing with the two liters on both street and track to verify worked and did no harm. But, based on Josh's statement, they did not retest sustained high speed with the extra fluid. And as pointed out, it yet to be verified in the field that removal of the 2 liters solves this problem.

 

They didn't say how they verified this solution. I would have done a high speed drive in a car without the two additional quarts to establish a baseline, added two additional quarts to verify the problem could be replicated, and then removed the two quarts to verify the issue has been resolved. Hopefully they did something like this and not just relied upon telemetry and analysis to establish the cause and corrective action.

 

Unfortunately I can't verify that this is a fix until next April. Not a whole lot of long track 150 mph opportunities between now and then.
I'm working on a way to get the two liters out without making a mess.