Per what I can find in GM manuals, optimal temp range is about 130-198. Mine runs about 165-170 on flat highway but approaches 195 when towing, or even unloaded in the hills of Austin.

From what I've researched, seems there is a built in thermostat bypass valve that opens around 158F to allow fluid to finally circulate through the cooler. Anything under that and the valve doesn't open to send fluid through the cooler.

So would a beefier aftermarket cooler accomplish the goal of maintaining 165-170 at all times if it won't see fluid until 158+ degrees? It appears you can also modify the bypass valve to be open at all times so fluid is always running through the cooler, but in colder climates that isn't advised as it sometimes runs under the optimal temps.

Any advice, or experience with this exact transmission, is appreciated.

Can you change to a lower temp tstat? Maybe a 142ish?

mosdefn14 said:

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Can you change to a lower temp tstat? Maybe a 142ish?

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Haven't seen any indication that is an option.

A lower telp thermostat isn't going to solve the 'overheating problem'. That LT T-stat will cause the coolant to flow sooner rather than later... but the thermostat isn't going to cause your engine to NOT reach operating temperature.

I installed a LT T-stat on my 986... still maintains the ~200 degrees operating temperature. It now opens at 160 instead of 187. I installed he LT T-stat because the M96 engine has a couple of known hotspots.

I did install an S oil cooler, and marginally that has helped lower the temperature.

Best wishes on a positive outcome.

mm98 said:

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would a beefier aftermarket cooler accomplish the goal of maintaining 165-170 at all times if it won't see fluid until 158+ degrees?

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Yes, peep the 6l80e rebuild thread.

mm98 said:

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Per what I can find in GM manuals, optimal temp range is about 130-198. Mine runs about 165-170 on flat highway but approaches 195 when towing, or even unloaded in the hills of Austin.

From what I've researched, seems there is a built in thermostat bypass valve that opens around 158F to allow fluid to finally circulate through the cooler. Anything under that and the valve doesn't open to send fluid through the cooler.

So would a beefier aftermarket cooler accomplish the goal of maintaining 165-170 at all times if it won't see fluid until 158+ degrees? It appears you can also modify the bypass valve to be open at all times so fluid is always running through the cooler, but in colder climates that isn't advised as it sometimes runs under the optimal temps.

Any advice, or experience with this exact transmission, is appreciated.

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If you think that is hot, you should check out 2019-2021 Tundras.

No aux cooler of any kind. They removed it. There are folks on flat ground unloaded running 230-235F.

Any additional loads the temps climb to 275-280F. The high temp warning is 302F.

I put a large cooler on mine and reinstalled factory hoses and thermostat and run around at 170-180 unloaded, and 210 towing.

All this to say it seems like OEM's are intentionally pushing the transmission temps higher. I wouldn't be too concerned about 195F when towing. The transmission fluid is likely ether a group iii or group iv base oil that easily handles 200F temps.

Yes a slightly larger cooler would probably be the easiest ( not the only) way to drop temps a bit.

(Hill country is a ***** on transmission temps because the changing inertia triggers the torque converter to lock and unlock alot. The unlocked operation drives the heat up big time from friction. When I drove through hill country, un loaded, before installing cooler on my 19 Tundra, i'd see sustained pan temps in range of 240-248F.)

silverado_lover said:

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mm98 said:

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would a beefier aftermarket cooler accomplish the goal of maintaining 165-170 at all times if it won't see fluid until 158+ degrees?

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Yes, peep the 6l80e rebuild thread.

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Would a products like this be helpful? The OEM valves seem to be faily ****ty and at semi-frequent contributor to the overheating problem.

https://ppepower.com/products/transmission-fluid-thermal-bypass-valve-8l90-8-speed?variant=42081986576627

https://superiortransmission.com/product/superior-stl010/

Hayden 698 is on my list too. Just wanted your opinion on those smaller devices.

silverado_lover said:

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mm98 said:

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would a beefier aftermarket cooler accomplish the goal of maintaining 165-170 at all times if it won't see fluid until 158+ degrees?

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Yes, peep the 6l80e rebuild thread.

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Yeah I would suggest bypassing that valve. It is essentially the reason my 6L80E burned up while towing a 24' car hauler. Will end up costing me 5-6 grand all said and done.

https://texags.com/forums/46/topics/3291202/


That being said, your temps sound just fine to me. You really don't want to go over 200, but if you get to 220 it's like cutting the life of the ATF in half for every ten degree jump in temp after 220. If you ever get to 220-ish I would suggest a fluid drain and refill asap.