8L90 Fluid Capacity for Flush

6.2Creedmoor · January 26, 2021

Dumb question.....but why is 20 quarts recommended? If the Total Fill Capacity is 11.4 quarts, does it really take almost another 9 quarts to completely remove the previous fluid using a TransFlow or BG PXT 2? I get maybe a couple more quarts but 9?

My mechanic is recommending only using 13 quarts for a complete flush on the 8L90 using a BG PXT 2 but everywhere else I am finding the recommend 20 quarts based on the GM TSB but I don't have a good argument why you couldn't do it with the 13 he is recommending.

mookdoc6 · January 26, 2021

13 hours ago, 6.2Creedmoor said:

Dumb question.....but why is 20 quarts recommended? If the Total Fill Capacity is 11.4 quarts, does it really take almost another 9 quarts to completely remove the previous fluid using a TransFlow or BG PXT 2? I get maybe a couple more quarts but 9?

My mechanic is recommending only using 13 quarts for a complete flush on the 8L90 using a BG PXT 2 but everywhere else I am finding the recommend 20 quarts based on the GM TSB but I don't have a good argument why you couldn't do it with the 13 he is recommending.

Are you having any of the known fluid causing problems? If not you will be fine with 13 qts imo. The original TSB was written I believe with removing and replacing fluid twice. My services are done with 12qts using TransFlow.

6.2Creedmoor · January 26, 2021

2 hours ago, mookdoc6 said:

Are you having any of the known fluid causing problems? If not you will be fine with 13 qts imo. The original TSB was written I believe with removing and replacing fluid twice. My services are done with 12qts using TransFlow.

Yes, dealer is recommending the Mobil 1 HP LV ATV HP Blue Label flush but they are stating the TSB is not covered under warranty (bumper-to-bumper or powertrain) as it is a maintenance item. It's odd as others have stated this has been done under warranty. And after hearing that it hasn't permanently fixed anyone's issues I am going to go the Amsoil SS ATF route first as I have heard better results. My TCC shudder issues are very minor but still present and I am at 55k with no previous transmission fluid service done by the previous owner that is on record.

Grumpy Bear · January 26, 2021

16 hours ago, 6.2Creedmoor said:

Dumb question.....but why is 20 quarts recommended? If the Total Fill Capacity is 11.4 quarts, does it really take almost another 9 quarts to completely remove the previous fluid using a TransFlow or BG PXT 2? I get maybe a couple more quarts but 9?

My mechanic is recommending only using 13 quarts for a complete flush on the 8L90 using a BG PXT 2 but everywhere else I am finding the recommend 20 quarts based on the GM TSB but I don't have a good argument why you couldn't do it with the 13 he is recommending.

Put a water glass of milk in the kitchen sink. Now with a second glass of clean water pour it into the first. Will it displace all the milk? How many glasses of fresh water will this take to make the first glass clean water? This is what a flush machine does.

Even 20 quarts isn't enough for complete removal. However they are not looking for a complete removal they are looking for a specific dilution...thus 20 quarts some smart guy has figured will reach that point.

Edited January 26, 2021 by Grumpy Bear

newdude · January 26, 2021

20 quarts will swap out 90-95% of the old fluid per GM, hence why they have the dealers use 20 quarts.

Also, you shouldn't be running through a BG machine as most BG machines use BG universal ATF and that is NOT compatible with Dexron LV HP ATF that the 8 speed uses. You should only be doing the GM drain and fill procedure or the GM TransFlow machine that has been converted by the dealer for HP fluid only.

Edited January 26, 2021 by newdude

mookdoc6 · January 26, 2021

Actually, Grumps the ratio is like 5qts or something is enough to bring the ratio inline with GM specs. I would as the previous poster replied only use GM specific fluid and machine. I use both consistently and my 8L90 rips off shifts just as tight as DAY NUMBER 1

This gearbox likes HEAT and lots of INPUT! It enjoys it and can handle it!

LDM · January 26, 2021

2 hours ago, 6.2Creedmoor said:

Yes, dealer is recommending the Mobil 1 HP LV ATV HP Blue Label flush but they are stating the TSB is not covered under warranty (bumper-to-bumper or powertrain) as it is a maintenance item. It's odd as others have stated this has been done under warranty. And after hearing that it hasn't permanently fixed anyone's issues I am going to go the Amsoil SS ATF route first as I have heard better results. My TCC shudder issues are very minor but still present and I am at 55k with no previous transmission fluid service done by the previous owner that is on record.

TSB 18-NA-355 can be done in or out of warranty and has nothing to do with the bumper-to-bumper or powertrain warranty. Your dealer is lying to you if they are telling you otherwise. The TSB involves fully flushing the old fluid and using the new blue label Mobil 1 HP LV fluid. It probably does take around 20 quarts to do a full flush for this procedure.

As far as fixing the issue, I had it done my truck last year around April due to the hard 1-2 shift when the fluid was cold. Fixed the issue immediately and haven't had a problem since.

6.2Creedmoor · January 26, 2021

1 hour ago, Grumpy Bear said:

Put a water glass of milk in the kitchen sink. Now with a second glass of clean water pour it into the first. Will it displace all the milk? How many glasses of fresh water will this take to make the first glass clean water? This is what a flush machine does.

Even 20 quarts isn't enough for complete removal. However they are not looking for a complete removal they are looking for a specific dilution...thus 20 quarts some smart guy has figured will reach that point.

Stupidly, I can't believe I am going to poke the bear. I am here to learn but need to understand the concepts being shared. But I don't think the milk glass analogy applies here.

Unlike the milk glass, the transmission is sealed and fitted with effectively an inlet for filling (by removing the oil cooler lines) and an outlet for draining (by using the level set plug) while using a pressurized medium to exchange the volume of fluid based on a finite capacity of 11.4 quarts. Under Pascal's Law and the concept of a siphon if we are displacing 11.4 quarts under pressure then we should only need 11.4 quarts to replace the volume of fluid lost. If the viscosity is different across certain manufacturer's base oil then other applied concepts of fluid dynamics such as dynamic viscosity would come in to play and that would shed some more light on this question and your point about reaching a specific dilution. What am I missing here?

6.2Creedmoor · January 26, 2021

1 hour ago, newdude said:

20 quarts will swap out 90-95% of the old fluid per GM, hence why they have the dealers use 20 quarts.

Also, you shouldn't be running through a BG machine as most BG machines use BG universal ATF and that is NOT compatible with Dexron LV HP ATF that the 8 speed uses. You should only be doing the GM drain and fill procedure or the GM TransFlow machine that has been converted by the dealer for HP fluid only.

Correct, I will not be using the BG ATF but will be supplying the shop with Amsoil SS ATF.

newdude · January 26, 2021

18 minutes ago, 6.2Creedmoor said:

Correct, I will not be using the BG ATF but will be supplying the shop with Amsoil SS ATF.

I'd be leery on using Amsoil over the proper Mobil 1 LV HP ATF blue label, but that's just me. I'm not knocking Amsoil, they make great products, but Dexron 6 and Dexron HP aren't interchangeable. I wouldn't have much faith in a "universal" fluid that claims to meet both.

Edited January 26, 2021 by newdude

Grumpy Bear · January 26, 2021

2 hours ago, 6.2Creedmoor said:

Stupidly, I can't believe I am going to poke the bear. I am here to learn but need to understand the concepts being shared. But I don't think the milk glass analogy applies here.

Almost 95% of the fluid held in the transmission is held in the torque converter and the pan. The torque converter is a great mixer even when static. I wanted to keep it simple but....so now you have two glasses of milk, one atop the other both full of milk and connected by a straw and the free pour limit to a funnel.

Believe it, don't believe it. It isn't rocket science. You are, in addition, badly misapplying Pascal's Law:

Pascal's law says that pressure applied to an enclosed fluid will be transmitted without a change in magnitude to every point of the fluid and to the walls of the container. The pressure at any point in the fluid is equal in all directions.

Simply it states liquids are not compressible and the reason they work effectively as hydraulic fluids.

You also error in believing the pan is part of the hydraulics. It is neither sealed nor under pressure EVER. GM's use of the word sealed is misleading. It has a vent.

flyingfool · January 27, 2021

better off leaving the fluid in the trans and running a spin on filter or bypass unit, this will scrub clean every drop inside your trans without costing you a fortune for 20 quarts of new fluid, just be sure to remove your thermostat first. if you got one inline.

I'm currently doing testing on a motorguard trans filter system, but had good success with $3 spin on units too. updates to come.. what i love most besides clean oil, is the filter elements actually absorbe water moisure from the oil. this was a problem GM is having with thier transmissions fluid being hydroscopic on certain models

https://www.amazon.com/Motor-Guard-M-45-KIT-Clean-Filter/dp/B000TARQ6C/ref=sr_1_6?dchild=1&keywords=motor+guard+filter&qid=1611718577&sr=8-6

Edited January 27, 2021 by flyingfool

mookdoc6 · January 27, 2021

19 hours ago, 6.2Creedmoor said:

Correct, I will not be using the BG ATF but will be supplying the shop with Amsoil SS ATF.

You will be fine with Amsoil ATF fluid. As for the other nonsense of adding inline filters and deleting this this that or the other thing is simply not necessary.

flyingfool · January 27, 2021

how much money is wasted buying 20 quarts or high end synth fluid. keep the stuff in there that you got. and just change the filter element every 10,000 miles. money in the pocket, and super clean oil feeding the trans..

Grumpy Bear · January 27, 2021

2 hours ago, mookdoc6 said:

As for the other nonsense of adding inline filters and deleting this this that or the other thing is simply not necessary.

As Carlin use to say, "Somewhere between Famous Potatoes and Live Free or DIE is the truth."

Mook, Mook, Mook, you know better that to state an opinion as the truth.

Until you know what the necessity is how can you state a suggestion is without worth?

How long does he plan to keep it? How is it used? What will his conscience allow at trade or push it off a cliff time?

What is his necessity?

I know your not going to attempt to argue that heat and cleanliness have no bearing on longevity...right?