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Adding to this discussion - after running the HSF and then changing my DCT fluid and filter in July of 2025, I will again run the HSF with a filter change before and after the flush before the end of the year. I will send out for analysis oil samples taken both before and after the HSF and filter change. The accumulated mileage since this last service is 2214 miles. The particle test will show how effective the Tremec filtration setup works at maintaining oil cleanliness.
Focusing mainly on engine life, this addresses the issue of hydraulic system cleanliness - which is just as critical in your DCT:
I agree with your approach. I'm curious on how to reconcile the notion of flushing with a new filter as there is data to suggest that a "used" filter is actually more efficient at grabbing crud than a new one. Lake has a video on this I believe. I change my fluid and filter yearly (track my car) and can't imagine that the filter is close to getting to by-pass territory on that interval.
I agree with your approach. I'm curious on how to reconcile the notion of flushing with a new filter as there is data to suggest that a "used" filter is actually more efficient at grabbing crud than a new one
The best way to measure all this is to sample the fluid as is, pre flush, then after the flush on the existing filter and then again after a flush with a new filter. Three samples. I was going to carry out the HSF on my used filter, but I have heard too many cases where this has ruined valve bodies and I don’t want any part of that. I’ll let the data speak for itself. The initial sample I’ll take with 2200 miles after the last filter change will reveal how efficient the Tremec filtration process works in the GM 9080 DCT. I personally would not run the HSF on any used filter as the possibility of pushing debris already trapped by the filter back into fluid circulation where it finds itself in the valve body is too great a risk.
The best way to measure all this is to sample the fluid as is, pre flush, then after the flush on the existing filter and then again after a flush with a new filter. Three samples. I was going to carry out the HSF on my used filter, but I have heard too many cases where this has ruined valve bodies and I don’t want any part of that. I’ll let the data speak for itself. The initial sample I’ll take with 2200 miles after the last filter change will reveal how efficient the Tremec filtration process works in the GM 9080 DCT. I personally would not run the HSF on any used filter as the possibility of pushing debris already trapped by the filter back into fluid circulation where it finds itself in the valve body is too great a risk.
Are you going to dissect the filter used for the pre-flush to see what the first HSF accomplished? You could probably use cheap Amazon filters for the ones you dissect to save some money.
Are you going to dissect the filter used for the pre-flush to see what the first HSF accomplished? You could probably use cheap Amazon filters for the ones you dissect to save some money.
I am replicating the procedure in my post prior: warm the trans up to temp, sample the fluid as is, install a new Delco filter, run the HSF on this new filter (I ran the HSF twice this time), sample the fluid again, install a new filter again and then run the leak test.
What follows are results from the second round of hydraulic system flushing and oil analysis after 2214 miles of normal driving.
The previous HSF was carried out in July of 2025 - 2214 miles prior. The vehicle mileage currently is 13,123.
The same procedure was carried out both times with the exception that I ran the HSF twice - back to back this last time and I had changed the fluid in July.
The car was run to warm the transmission fluid to about 150 F degrees, at which time, the car was shut down and an oil sample was drawn out of the fluid level check port. The external filter was removed and replaced with a new OEM GM ACDelco filter. The HSF was then run. When complete, the car was shut down and a second oil sample was taken again from the level check port. The external filter was again removed and replaced with a new filter.
It should be noteworthy that when the fluid was changed at the earlier HSF in July, about 9.5 quarts were drained from the unit. So, while the transmission fluid capacity is 12.57 quarts, about 3 quarts remained in the unit.
This converts the ISO 4406 particle count numbers into particle size and number found in 1 mL of sampled fluid.
The latest report shown above appears on the right under 13,123 miles.
The first report shown below appears on the left under 10,909 miles.
This is the lab report from the first HSF in July of 2025. Prior to this, the HSF was run and the fluid changed at 3088 miles prior. In addition, the filter was again changed at 1499 miles prior to this report.
Last edited by Mitchell_B; Dec 31, 2025 at 08:46 AM.
What follows are results from the second round of hydraulic system flushing and oil analysis after 2214 miles of normal driving.
The previous HSF was carried out in July of 2025 - 2214 miles prior. The vehicle mileage currently is 13,123.
The same procedure was carried out both times with the exception that I ran the HSF twice - back to back this last time and I had changed the fluid in July.
The car was run to warm the transmission fluid to about 150 F degrees, at which time, the car was shut down and an oil sample was drawn out of the fluid level check port. The external filter was removed and replaced with a new OEM GM ACDelco filter. The HSF was then run. When complete, the car was shut down and a second oil sample was taken again from the level check port. The external filter was again removed and replaced with a new filter.
It should be noteworthy that when the fluid was changed at the earlier HSF in July, about 9.5 quarts were drained from the unit. So, while the transmission fluid capacity is 12.57 quarts, about 3 quarts remained in the unit.
So in both cases, the column labeled previous sample is before the HSF, correct? When you send the samples in at the same time they know which is which somehow?
So in both cases, the column labeled previous sample is before the HSF, correct? When you send the samples in at the same time they know which is which somehow?
The columns labeled previous sample, in this case, are empty. For any given vehicle, sample results are populated in chronological order from left to right with the most recent populating the left most column. Successively older samples appear to the right. In each of these two reports, the before sample is on the right and the after HSF on the left. Does this make sense?
Samples were carefully labeled and the lab knew what I was doing. An explanation of what the samples represented together with their identifying labels was given to the lab. In addition, I recorded all of this and the sample ID #s appear on the report.
The columns labeled previous sample, in this case, are empty. For any given vehicle, sample results are populated in chronological order from left to right with the most recent populating the left most column. Successively older samples appear to the right. In each of these two reports, the before sample is on the right and the after HSF on the left. Does this make sense?
Samples were carefully labeled and the lab knew what I was doing. An explanation of what the samples represented together with their identifying labels was given to the lab. In addition, I recorded all of this and the sample ID #s appear on the report.
What follows are results from the second round of hydraulic system flushing and oil analysis after 2214 miles of normal driving.
So this shows that the flush does appear to push fluid to the filter and the filter is working... all of which would make sense since gm put this in place and of course has run extensive testing for reliability.
So this shows that the flush does appear to push fluid to the filter and the filter is working... all of which would make sense since gm put this in place and of course has run extensive testing for reliability.
In the service manual, it says the HSF "executes a procedure in the TCM software to actuate the solenoids in a specific manner to flush debris from the system". The normal fluid path is through the pump, then the filter, and then the lines and valves, isn't it? So how does the "debris laden" fluid flushed from the valves make it way back to the filter? Is it dumped into the pan first, or is there a path by which it can re-enter below (in front of) the pump without first being dumped into the pan. Either way, I think it get filtered out.
What follows are results from the second round of hydraulic system flushing and oil analysis after 2214 miles of normal driving.
The previous HSF was carried out in July of 2025 - 2214 miles prior. The vehicle mileage currently is 13,123.
The same procedure was carried out both times with the exception that I ran the HSF twice - back to back this last time and I had changed the fluid in July.
The car was run to warm the transmission fluid to about 150 F degrees, at which time, the car was shut down and an oil sample was drawn out of the fluid level check port. The external filter was removed and replaced with a new OEM GM ACDelco filter. The HSF was then run. When complete, the car was shut down and a second oil sample was taken again from the level check port. The external filter was again removed and replaced with a new filter.
It should be noteworthy that when the fluid was changed at the earlier HSF in July, about 9.5 quarts were drained from the unit. So, while the transmission fluid capacity is 12.57 quarts, about 3 quarts remained in the unit.
This converts the ISO 4406 particle count numbers into particle size and number found in 1 mL of sampled fluid.
The latest report shown above appears on the right under 13,123 miles.
The first report shown below appears on the left under 10,909 miles.
This is the lab report from the first HSF in July of 2025. Prior to this, the HSF was run and the fluid changed at 3088 miles prior. In addition, the filter was again changed at 1499 miles prior to this report.
Isn't the GM procedure to run the HSF before changing the filter?
Am I reading this right that you ran it after changing the filter, then changed it a second time?
In the service manual, it says the HSF "executes a procedure in the TCM software to actuate the solenoids in a specific manner to flush debris from the system". The normal fluid path is through the pump, then the filter, and then the lines and valves, isn't it? So how does the "debris laden" fluid flushed from the valves make it way back to the filter? Is it dumped into the pan first, or is there a path by which it can re-enter below (in front of) the pump without first being dumped into the pan. Either way, I think it get filtered out.
I'm not sure of the normal fluid path. I've run across this video from Tremec, but it's all I've seen (not that there isn't more out there, just only thing I've watched).
My guess, based on running the flush in my car last winter, is each solonoid is opened to flush, and closed in sequence. Then, the fluid is pushed through the filter based on time of pump running perhaps (since flow rate is known). So, you clean out each solonoid, keep it closed while the others clean out, and then push all of that fluid through the filter X number of times. It appears to work.
Unfortunately we don't know (that I've seen) what conditions trigger this sort of cleaning when driving. It would be nice to run that driving condition once, and the next time the flush and compare the two.
I'm not sure of the normal fluid path. I've run across this video from Tremec, but it's all I've seen (not that there isn't more out there, just only thing I've watched). https://youtu.be/ohhZJ1ucYiI?si=HM4yLNkxMC0f13LK&t=125
My guess, based on running the flush in my car last winter, is each solonoid is opened to flush, and closed in sequence. Then, the fluid is pushed through the filter based on time of pump running perhaps (since flow rate is known). So, you clean out each solonoid, keep it closed while the others clean out, and then push all of that fluid through the filter X number of times. It appears to work.
Unfortunately we don't know (that I've seen) what conditions trigger this sort of cleaning when driving. It would be nice to run that driving condition once, and the next time the flush and compare the two.
But for the debris laden fluid that exit the valves, what "pushes" it back through the filter? Doesn't the return path have to be on the suction side of the pump? Attached is my simple minded view of this showing what I think are possible return paths. The normal flow of fluid, with or without the HSF, is from the pan, through the coarse suction filter not shown, into the pump, then out of the pump into the external canister filter, then to the valves and lines. I think in reality the oil cooler is somewhere in the circuit. But filtered fluid goes through the lines and valves and is returned to the pan in my illustration on the left. When the HSF is run, the valves are manipulated to release debris, but where does the debris laden fluid go? The fluid was "forced" through the filter by the pump before it went to the valves - when it exits the valves - how is forced back through the filter? I think it must be via the suction side of the pump either via the pan (shown on the left) or a direct connection somewhere along the the intake path as shown on the right. I'm thinking the short circuit back to the suction side of the pump might be the case, and that would be consistent, to some extent, with Josh Holder's statement about "pushing more fluid through the filter".
But for the debris laden fluid that exit the valves, what "pushes" it back through the filter? Doesn't the return path have to be on the suction side of the pump? Attached is my simple minded view of this showing what I think are possible return paths. The normal flow of fluid, with or without the HSF, is from the pan, through the coarse suction filter not shown, into the pump, then out of the pump into the external canister filter, then to the valves and lines. I think in reality the oil cooler is somewhere in the circuit. But filtered fluid goes through the lines and valves and is returned to the pan in my illustration on the left. When the HSF is run, the valves are manipulated to release debris, but where does the debris laden fluid go? The fluid was "forced" through the filter by the pump before it went to the valves - when it exits the valves - how is forced back through the filter? I think it must be via the suction side of the pump either via the pan (shown on the left) or a direct connection somewhere along the the intake path as shown on the right. I'm thinking the short circuit back to the suction side of the pump might be the case, and that would be consistent, to some extent, with Josh Holder's statement about "pushing more fluid through the filter".
It is quite possible there is a return path to the low pressure side of the pump that bypasses the sump filter and cooler. And we don't know what HSF sequencing does. It is possible that the pump is commanded to full output and the valve are sequenced in such a manner that the fluid velocity in passages is beyond what is seen in normal driving, or even reversed.
