Question about 6L80, cooling line direction & HP Tune

[rav3]

Here's the back story. 2017 LTZ 5.3 6L80 with 63k miles. I did the pill flip when I bought this truck at 32k miles & added the DOD/AFM delete at the same time. I have done 3 'drain & Fills' on ATF. The transmission has aways run about 140-150 F since I did that. Recently, I had the transmission Tuned via HP Tuner, elimination clutch slip, torque management & deleting the DOD. Last weekend as I was driving up the mountain from 2200 ft to 5000 ft, the transmission temp steadily rose up to 197 F and then cold as I descended back to 2200 ft.

 

So, I'm wondering if the Pill Flip is not working for whatever reason or if the cooling line is restricted. So, I will be removing the transmission thermostat & taking it apart to inspect. While the cooling lines are off, I want to blow some air through the lines & flush out the ATF. Can anyone tell me which one of the lines goes to the cooler?

Is this a bad idea, as there won't be any ATF in the lines when I start if back up.

 

Let me know what you guys think

Edited April 1 by rav3

[silveradosid]

higher altitude thinner air and less cooling Efficiency, your engine temp was also probably higher

[rav3]

It's amazing what ChatGPT can figure out...

HP Tuners can completely reshape how your 6L80E shifts—but it’s not just one setting. You’re basically reprogramming how the transmission applies clutches, pressure, and timing.
Here’s a clear breakdown of what it actually does and how it fixes common shift problems
What HP Tuners Changes in a 6L80E
The 6L80E is a fully electronic clutch-to-clutch transmission, so shift quality is controlled by software, not just hydraulics. �
MotorTrend
1. Shift Pressure (Firmness)
Controls how hard or soft it shifts
Higher pressure = firmer, quicker shifts
Lower pressure = softer, smoother shifts
This is the #1 fix for:
Slipping shifts
Lazy or “mushy” gear changes
2. Shift Timing (Speed of the shift)
Controlled mainly by Torque Adder / Shift Timing tables
Adjusts how fast the clutches apply and release
Fixes:
Long drawn-out shifts
Flare between gears (RPM jump)
On newer 6L80s, GM removed “desired shift time” tables and uses torque-based timing instead. �
forum.hptuners.com
3. Shift Points (When it shifts)
MPH or RPM where each gear change happens
Fixes:
Early shifting (bogging engine)
Late shifting (revving too high)
Example:
Raise shift points for performance
Lower for towing or economy
4. Torque Management (VERY important)
Reduces engine torque during shifts to protect clutches
Adjusting this:
Less torque reduction = harder, faster shifts
Too little = can damage transmission
Important:
Turning it off completely is a bad idea (can cause harsh timing issues and wear) �
YouTube
5. TCC (Torque Converter Clutch) Control
Controls lockup timing and pressure
Fixes:
Shudder
Surging at steady speed
Slip under load
Common changes:
Increase TCC pressure
Adjust apply/release speeds
Disable lockup in lower gears (sometimes)
6. Adaptive Learning Reset
The transmission “learns” wear over time
After tuning:
You must reset adapts, or it may shift weird
What’s Actually Happening Inside
Each shift in a 6L80E is:
One clutch turning OFF
Another clutch turning ON
If timing or pressure is off:
Too slow = slip
Too fast = harsh bang
HP Tuners lets you sync that handoff perfectly
Real-World Examples
Fixing a sloppy 2–3 shift:
Increase shift pressure slightly
Reduce shift time (torque adder)
Check torque management
Fixing a harsh bang shift:
Lower pressure a bit
Smooth timing (increase shift time slightly)
Fixing converter shudder:
Increase TCC pressure
Adjust lockup speeds
Sometimes disable lockup in lower gears
Important Reality Check
This is where most people mess up:
The 6L80E is very sensitive
Bad tuning = burnt clutches fast
Good tuning = longer life AND better performance
Even stock power trucks rely heavily on correct calibration for durability. �
MotorTrend
Simple Summary
HP Tuners fixes your shifting by adjusting:
Pressure → how hard it hits
Timing → how fast it shifts
Shift points → when it shifts
Torque management → how controlled it is
TCC settings → how the converter behaves
If you want, tell me exactly what your truck is doing (flare, bang, delayed shift, shudder, etc.), and I can point you to the exact tables and safe starting values for your 6L80

[rav3]

And ChatGPT said this......

Got it—that extra detail actually matters a lot.

Short answer first:

With a fill-flip thermostat mod, 77°F ambient, and a 2,200 → 5,000 ft climb, seeing ~195°F is completely normal

Why your temp hit 195°F

You basically created a perfect storm for heat generation:

1. Elevation climb = heavy load

Going from 2,200 → 5,000 ft means sustained load

The transmission is working harder to maintain speed

2. Torque converter slip (big heat source)

On climbs, unless locked, the converter:

Slips more

Generates a LOT of heat quickly

This is the #1 reason temps rise on hills

3. Fill flip mod changes behavior

With the thermal bypass “flip”:

Fluid is always routed through the cooler

BUT:

At lower temps, flow can be slightly restricted depending on setup

Cooling efficiency depends heavily on airflow

So:

It helps overall cooling

But doesn’t eliminate heat under load

4. Airflow reality

Climbing a mountain = often lower speeds

Less airflow through the cooler

So even with the mod, cooling is reduced right when you need it most

What matters most here

Your temp did NOT run away past 200+

It likely leveled off around 190–200°F

That means:

Your cooler system is working

Your fluid isn’t overheating

No immediate issue

Where HP Tuners can improve THIS exact scenario

1. Earlier TCC lockup (biggest gain)

Reduce converter slip during climbs

This alone can drop temps 10–20°F

2. Increase TCC apply pressure

Prevent partial slip under load

Helps eliminate heat buildup

3. Adjust downshift behavior

Keep engine in a better power band

Reduces load on the converter

4. Slightly firmer shifts

Less clutch slip = less heat

Simple test for next drive

Next time you climb:

Put it in Tow/Haul mode

Forces earlier lockup

Watch temp difference

If temps drop noticeably:

You’ve confirmed converter slip is your main heat source

When this WOULD be a concern

Watch for this pattern:

Temps keep climbing past 210–220°F

Doesn’t stabilize

Or climbs fast even on flat ground

That would point to:

Flow restriction

Cooler inefficiency

Tuning issue

Bottom line

195°F in your exact conditions = totally acceptable

Your system behaved like it should

The climb + converter slip explains everything

If you want, I can help you:

Dial in exact HP Tuners settings for cooler temps

Or confirm your cooler routing after the flip (a lot of people get that slightly wrong)

Just tell me

[rav3]

Can you believe that AI is this quick & accurate about my question? 

This is crazy technology! 

[carkhz316]

TL;DR You're likely fine. The mountain climbing makes it work harder so it ran a bit warmer. Anything under 200* is fine.

[CamGTP]

Also, removing all of the allowed slip in lock up on a stock converter is actually harder on the converter and will make it fail over time. The stock converter must have slip, we can just reduce it by roughly half to help it last longer.

[Pearl2017]

sounds normal to me, uphill your losing some speed/airflow and rad fans set point is much higher stock temp so don't kick in right away. going downhill the load is gone slip is minimal and vehicle speed/airflow is usually increased. no service needed IMO

[truckguy82]

On 3/30/2026 at 7:48 AM, rav3 said:

Here's the back story. 2017 LTZ 5.3 6L89 with 63k miles. I did the pill flip when I bought this truck at 32k miles & added the DOD/AFM delete at the same time. I have done 3 'drain & Fills' on ATF. The transmission has aways run about 140-150 F since I did that. Recently, I had the transmission Tuned via HP Tuner, elimination clutch slip, torque management & deleting the DOD. Last weekend as I was driving up the mountain from 2200 ft to 5000 ft, the transmission temp steadily rose up to 197 F and then cold as I descended back to 2200 ft.

 

So, I'm wondering if the Pill Flip is not working for whatever reason or if the cooling line is restricted. So, I will be removing the transmission thermostat & taking it apart to inspect. While the cooling lines are off, I want to blow some air through the lines & flush out the ATF. Can anyone tell me which one of the lines goes to the cooler?

Is this a bad idea, as there won't be any ATF in the lines when I start if back up.

 

Let me know what you guys think

Yeah your trans temps were at the design temp, it’s what every person that didn’t do a pill flip drives around with all day everyday.

 

I actually bought my truck with a faulty thermostat and it was blocked for 20k miles and it got up to 210-220 on a regular basis. It was never a problem.

[Howie99]

On 3/30/2026 at 6:48 AM, rav3 said:

Here's the back story. 2017 LTZ 5.3 6L80 with 63k miles. I did the pill flip when I bought this truck at 32k miles & added the DOD/AFM delete at the same time. I have done 3 'drain & Fills' on ATF. The transmission has aways run about 140-150 F since I did that. Recently, I had the transmission Tuned via HP Tuner, elimination clutch slip, torque management & deleting the DOD. Last weekend as I was driving up the mountain from 2200 ft to 5000 ft, the transmission temp steadily rose up to 197 F and then cold as I descended back to 2200 ft.

 

So, I'm wondering if the Pill Flip is not working for whatever reason or if the cooling line is restricted. So, I will be removing the transmission thermostat & taking it apart to inspect. While the cooling lines are off, I want to blow some air through the lines & flush out the ATF. Can anyone tell me which one of the lines goes to the cooler?

Is this a bad idea, as there won't be any ATF in the lines when I start if back up.

 

Let me know what you guys think

I was told to keep rpm's up to cool engine. U need to be in a lower gear to cool transmission 

[davester]

4 hours ago, Howie99 said:

I was told to keep rpm's up to cool engine. U need to be in a lower gear to cool transmission 

I find, for both my '04 w 4l80e and '12 w 6l90, the transmission will heat up more/faster by driving around in a lower gear than the transmission would automatically select.  I do this for checking the fluid level, as it needs to be a temp range that takes a long time to get to if I don't do this.

[Howie99]

On 4/5/2026 at 5:11 PM, davester said:

I find, for both my '04 w 4l80e and '12 w 6l90, the transmission will heat up more/faster by driving around in a lower gear than the transmission would automatically select.  I do this for checking the fluid level, as it needs to be a temp range that takes a long time to get to if I don't do this.

does your transmission lines run through radiator. when your engine warms up u have 210 deg coolant heating up transmission fluid.

[mikeyk101]

Just an fyi, even though the gauge in the dash may be pointing at 210, that isn't what the actual temperature is. The temp can be anywhere from 185⁰ to 220⁰. If it drops off the 210 mark, it is already falling below 185⁰. And likewise if it goes above the 210 mark, it is already heading past 220⁰. The only way to know the actual temperature is using one of the OBDII readers that monitor live conditions. 

[davester]

3 hours ago, Howie99 said:

does your transmission lines run through radiator. when your engine warms up u have 210 deg coolant heating up transmission fluid.

Yes, both run the fluid through the heat exchanger in the radiator, then through a separate cooler mounted in front of the cooling stack.  GM's done it like that for a long time, at least since the 90's if not earlier.  It works fine.  The trans fluid always reports as being significantly cooler than engine coolant temp.

 

I need to make a special effort to get it up to the temp range GM wants, to check the fluid level.  During normal driving, it doesn't get to that temp, except on hotter summer days, for me.