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K2XX Trans Cooler Thermostat Thoughts and Opinions

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PLEASE.  The floor is open!  Any thoughts, questions, ideas, suggestions, arguments, PLEASE chime in!  Want to hear from all and any on this.  


Had some time to put some thought into this and I've come up with what I think are some good ideas that may be either agreed or disagreed with on the trans cooler thermostat in the K2 pickups/SUVs.


First part:  Thermostat reasonings (in my opinion) and trans temps 


A little background on the thermostat for those who don't know.  Starting with K2XX in 2014, GM decided to throw a Thermostat Bypass Valve on to the K2 1/2 ton pickups and 2015 on the K2 SUVs.  Goal in mind being, to get the transmission up to a desired operational range for the primary target being fuel economy.  The thermostat inside the bypass valve was set to full open at 194F/90C.  These devices aren't new to GM and other OEs, but new to consumers, especially when they find their transmissions burning up.


That said, there is one major flaw that was overlooked to such a system, and that is the high full open temperature.  Performance transmission companies like TCI state that the ideal temps are 175-225F for the best operation, yet even their own fluid chart points to a big flaw with that logic.  Transmission fluid life.  


Transmission Life Expectancy



Even at the 195F range, fluid life it cut considerably, in half, from a cooler 175F standpoint.  


Another straw in the camel back to this equation?  GM's service intervals.  In the owners manual on K2 trucks, the "normal" driving schedule has ZERO requirement for changing transmission fluid in the 150,000 miles the chart covers.  The severe service chart?  Every 45,000mi they recommend replacing the fluid.


So.  We've got trucks running around with trans fluid seeing 190F+up regularly, some daily driving, some towing here and there, some towing non stop.  You've got OEMs marketing the "filled for life" mentality on all these cars and trucks, and owners who follow that logic to a T.   


Edited by newdude
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Part two:  GM sees the light!!!


And it came at the expense of warranty dollars (of course).  194F is TOO HOT.  Too much stress on the fluid, too much stress on the trans internals (clutches and converters especially) and as a result, a HUGE uptick in warranty repairs and replacements.  Lots of $$$ rebuilding, replacing converters, replacing whole transmissions all in the name of "fuel economy". 


GM released bulletin #21-NA-199 Information on Installation of a New Thermal Bypass Valve, Dec 10, 2021.


Thermal Bypass Valve Installation – TechLink (gm-techlink.com)


GM released a new thermostat for 6L80 and now for 6L90 in T1 HDs to lower transmission operation temperatures.  The new thermostat has a full open point of 158F.  Lower temps promote longer fluid life and still allow the transmission to operate in GM's fluid hot check range of 160-200F, which is really their ideal trans temp operating range, and also falls in the lower end of TCI's range.  


Conveniently as a nice bit of CYA (and a cheap expense to lower warranty costs) GM wants the new TBV installed for ANY warranty transmission concerns, even if the dealer can't duplicate your issue.  That's how much they are going out of the way to attempt to lower those warranty costs.  


Those not new to GM's trans thermostat know of two other methods to control transmission heat.  The "Pill Flip" and the Sure Cool thermostat delete kit.  Both are effective ways to bypass the thermostat and allow for much lower transmission temperatures and longer fluid life.  


Valve 86774933 | QuirkParts

Must Have - Superior 6L80 6L90 SURE-COOL Cooler System - Easy-to-Install

Edited by newdude
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Part Three:  My experience and suggestions for those looking to make a move on one of these options


On my 2019 K2 Silverado, I have now experienced both the Sure Cool kit, and now the GM lower temp thermostat.  Now, why would I go and remove the Sure Cool and install the GM thermostat? 


Glad someone asked.  Shift performance, particularly in cold climate.  Full disclosure, I am dyno tuned, both engine and trans.  Nothing aggressive, just some nice driveability and performance gains.  With the Sure Cool installed, it takes a bit longer to hit 68F, where the torque converter is then allowed to lock up (this has always been GM's minimum trans temp for lockup to start).  In the 68-105F range, I'd noticed some occasional shift quirkiness as well.  Knowing how it operated in the spring/summer/fall at warmer fluid temps, I decided to switch to the new GM TBV and am glad I did as all my quibbles of the Sure Cool were elminated with the trans being allowed to run warmer.


With this in mind, I came up with some recommendations for those looking to modify their 6L80 (and T1 6L90) trans bypass valve:


Stock/light tuning but no or occasional light towing (3000lbs or less):  Switch to the new GM 158F TBV. 


Stock/light tuning but frequent heavy towing (over 3000lbs):  You "could" go to the 158F TBV, but the Sure Cool or pill flip would be ideal in this scenario.


Heavy powertrain mods (blowers, turbos, big lifts with massive tires 34+up):  Pill flip or Sure Cool.  There is also the possibility of running the 158F TBV here as well, user dependent decision.


If you see cold temps, so 40F and colder:  GM 158F TBV.  This will allow for good cold weather transmission operation and lower summer temps.


Warm/Hot year round:  All 3 options can be used.  If towing is involved, run the flip or the Sure Cool.


Edited by newdude
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Some notes worth mentioning:


2014 K2 owners.  You NEED to replace the transmission cooler lines to use GM's new thermostat.  The thermostat is designed to fit the second design cooler lines which use a retention plate to hold the lines into the TBV.  Your current TBV (unless the lines have been replaced) uses quick connect fittings in the TBV that the lines snap into.  This might also apply to some early 2015 owners so if you wanted to install it, make sure you look first.


The new lower temp thermostat is NOT for 8 speeds!  This is a 6 speed TBV only!


2014-early 2016 owners.  You guys actually might be OK running the pill flip or Sure Cool options.  Why?  Your transmission cooler lines flow through the radiator before hitting the aux cooler.  This allows for nice temperature regulation from the heat produced from the engine coolant and should give you temps similar to how the old 07-13 trucks did, right in that 160-190F range during the warmer seasons.


2016-2019 K2 owners.  This is where I ran into my cold weather concerns.  These trucks the cooler lines flow straight to their own cooler (which was the aux cooler on 14-15) at the top of the AC condenser.  With the Sure Cool installed, I could literally watch the transmission temps cool down in the cold.  Say it hit 115F, within 10 minutes of driving it would drop back to 98F and would change the shift performance.  This is why I went with the new 158F TBV.  That is how effective the stock cooler is on 16-19 K2, that it can't maintain a better shifting temperature.  This also lead to fluctuations in the summer.  WOT runs I could get it to 160F, and within 10-15 minutes it would be back to 125F.  Transmissions like operating temp consistency from what I've experienced.  


NOTE:  For any T1XX owners, GM has you covered as well on 6 speeds.  2019-2021 GM has released a 158F TBV for installation.

Edited by newdude
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I learn something new every time @newdude post. Love his content. For example I did not know the exact fluid temperature for converter lockup. 68 F. Great!! Thanks!!


Pepper is a March 2015 built 2015. So later in the cycle and has the plate retained cooler lines. I did the pill flip AND use a lower temp (170 F) water thermostat. As he notes this one does not have the second aux cooler AFTER the radiator cooler so....it warms to its 'normal' pretty quick.  With the factory TBV it would routinely hover 225 F in anything but highway driving. Hot soaking took forever to cool back down as happens in stop and go. 205 F on the Interstate. 


After the pill flip? Hot summer day sees 160 F MAX unless I get in bumper to bumper then mid 170's. This time of year? 






Once I get to 100 F I, personally, can tell no difference in shift 'feel' past that point. I've yet to drive in weather that will bring the ATF lower than 104 F. During warm up, that is until past 68 F evidently, converter lock was delayed, yes, and it takes a mile or two to even shift to 6th. Even my Honda does that. All do come to think of it. 


I have bought the new TBV and will install it. This is an experiment for me looking to regain some of the cold weather fuel economy. Set a floor if you will on the minimum ATF temperature and yet keep it at or under 160 F. IF it holds the floor BUT pushes the Interstate temps past 165 F I will install an aux cooler. IF I don't see a steady floor, I will swap back to the pill flip setup. I don't take her out in the salt anyway. 


My 2 cents worth and a really great topic. 

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24 minutes ago, DeePa said:

Did it come with any gaskets? Or do I re-use what’s there?


im really excited about this. My 17 is solid as a rock. Only weak point is trans and converter and I’m working on improving their weaknesses. 


No gasket. New gasket part # 23135703 I have reused them. 


42 minutes ago, DeePa said:

Part number?


id like to do this asap. I’m already 20k into my second trans. Replaced at 107k. Currently at 127k


TBV part # 86774933

Edited by Grumpy Bear
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My '16 is a later year truck that does not route ATF through the radiator.  5k miles ago I flipped the pill, but now in sub-zero temperatures, I was never seeing trans temperatures exceed 104*F on my commute.  So, to experiment, I ordered and installed the updated thermostat (which does NOT come with cooler line o-rings).  My results:


1) the ATF heated up quicker with the pill flipped than either version of GM's thermostat.


2) however, once the fluid started heating with the revised 70*C thermostat, it kept heating; in 16-26*F air temperatures, the ATF kept on heating into the mid-high 120*F's during that same 12-15 mile commute.


That said, no driveability differences were noted; nearly concurrently with the installation of the new thermostat I also installed a better flowing exhaust (my stock muffler valve was freezing shut often), and noticed my fuel economy jump from 12.8mpg to 17.4 over the same roads and winter temperatures.


@Newdude: unrelated, or possibly related: does the 6L50 have a similar ATF thermostat-cooler set-up, and if so, is there a thermostat upgrade for that transmission as well?  Asking because my father has a '16 Colorado that he uses exclusively to drive from NY-ND-NM and back each year.

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2 hours ago, silveradosid said:

has anyone compared the old with the new, could you take the inner parts from the new one and put them in an old one, this way i would not have to change my tranny lines




And the pill can be installed in the 8L and 10L as well. 


 (@11bravoZ71) noted in the "6L80 Trans pill flip maintenance questionthat the pill itself is stamped 135 F and this that pill installed in his 8L90E ran down the road between 140 - 160 F.


Edited by Grumpy Bear
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I installed the new GM lower temperature thermostat and I'm pretty happy with the temperatures. 


40 mile drive today at 36 degrees ambient temperature.

It heated up quickly to 114 then more slowly to 144 on a 75 mph highway drive.

After getting off to a 45mph flat road it bounced between 140 and 144 degree's. 


That's MUCH lower than having it previously pinned at 190.


We will see how it works out once temperatures get back to the mid 80's here in WV.

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OK, here I go with my .02 worth.....I had a '95 Silverado 1500 Z71 with the 5.7l V* & 4 speed transmission without any transmission thermostat & it did just fine. Now on my '17 Silverado with 5.7l & 6 speed transmission, it was running in the 190's or so with the Thermostat Bypass Valve (TBV). I did the "pill flip" & it runs sooo much cooler here in WNC, even driving up in the mountains from 2000 ft up to Mt Michell @ 6600'.  This whole TBV was engineered for EPA/MPG & not transmission fluid life.

This DEXRON VI ATF is much thinner than the old DEXRON !!! ATF. All auto mfg are going to more gears & thinner fluids for Gas mileage per the Gov EPA requitements. (Along with the stupid AFM) Running cooler is always better IMO. Sorry for the rant!!!! 


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