Towing Heavy Transmission Temps

So my trans is getting hotter than I want while towing in the summer weather...
This has been discussed and I've given advice to many on this. My mods are in my sig
I tow a bumper pull camper at around 60-65 mph. It's BIG. 8500 lbs.. 32ft

When ambient temps are under 85* it's fine... but when gets over that my highway trans
temps creep up to 210-218*(highest I've seen) and my coolant temp creeps from about
200* also up to about 215* (not that concerning imo, on coolant temp)...

The sweet spot for speed (avoiding wind/sway/etc) is the 62-65. This generally puts
me between gears 4 & 5. Uphill or slight grade have to do 4th. Downhill or slight down can
shift into 5th gear.
The weird thing is that it wants to heat up more in 4th gear which makes no sense to me.
The 1.15:1 gear puts me at about 2500-2600 rpms in lockup. 5th gear is .85:1 so it's
a slight overdrive rpms are 1920-2050.... 5th gear really 'lugs' and I can watch all my
aftermarket gadgets showing lockup, temps, load, wideband afr, trims, etc etc

From everything I understand, I should run cooler in 4th gear even though the rpms are higher...
It's not really working so hard as far as load or throttle position, just more rpm.

I have my trans setup to stay in lockup so it's not a slip or TC issue. So I'm perplexed.
The only thing I can think of is something is off in my tune to "cruise" at 2600 rpm in 4th gear
decreasing my MPG and creating more heat....
Thanks for looking !!!

Just wanted to add that at suburban/city speeds I have zero issues with
trans getting to warm ( as fans are set to come on)

my only thing would be if the tranny pump running at a higher rpm is causing more heat?
but I know nothing about the subject really.


it's a big ask to get logs and try to adjust timing and fuel a bit in 4th to see if anything helps while towing.
one of the founding fathers of prom tuning that's long gone now, (rip grumpy) from the old grand nationals once said for interstate tuning, add a good oil temp sensor, adjust fueling and timing to the lowest oil temp and you'll have the happiest engine, mpg be dammed lol.

All I can offer is a data point on my 2021 Yukon with 6.2L / 10-speed regular (does not have towing package). We pull a 5300lb 7 x 14 dual axle -> camper and on 107F ambient and 10mile 6% grade the transmission reached 200F. The engine coolant is 210 normally and stayed that way up and over. 210-218F is a little hi but from what I've read on this forum, it's not fatal for occasional occurrences.

What year/model do you have?

tagexpcom said:

All I can offer is a data point on my 2021 Yukon with 6.2L / 10-speed regular (does not have towing package). We pull a 5300lb 7 x 14 dual axle -> camper and on 107F ambient and 10mile 6% grade the transmission reached 200F. The engine coolant is 210 normally and stayed that way up and over. 210-218F is a little hi but from what I've read on this forum, it's not fatal for occasional occurrences.

What year/model do you have?

Click to expand...

2014 denali 6.2. Modded for towing purposes. Mods in signature.
Big radiator. Big trans pan. External trans cooler with fan...
It's strange to me that it runs warmer in 4th gear vs 5th gear lugging.
The newer stuff like your 10 speed is gooood stuff !!

We have the same transmission cooler, I believe (in my 2012 XL Denali, not my suburban 3500). I usually run in 4th on the highway, pulling 7500 lbs at 65 but my transmission runs 15-20° cooler in mid 90’s ambient temps than what you’re describing, except when pulling up a long grade, but it comes back down to 198-200 pretty quickly after cresting the hill. Similar engine temps though, except mine holds 210° except when climbing. Can’t tell if that’s what you’re experiencing or not.

A couple of things come to mind. One is I’ve read that shifting has a significant impact on transmission temperatures. I read a study on it that showed as much as a 5° increase in fluid temperature for every shift under towing loads. I used to let mine shift between 4th and 5th, but I keep it in 4th now unless I’m on a long gentle descent.

The second thing is related to what John said about oil temperatures and the wise old tuner saying run the engine for cooler oil temps, fuel be damned. In gasoline engines, the amount of heat being sent into the cooling system (and oil) from each combustion event is highest when the AFR is very slightly rich because it results in significantly higher peak internal cylinder pressures. Go richer still and the power output is the same, but the combustion event generates less of a pressure spike, sending less heat into the cooling system. Go on the other side to a leaner burn, and EGTs go up and power output decreases slightly, but it will also result in lower peak cylinder pressures and also generate less heat into the cooling system. I could show you this happening in real time in a piston-powered aircraft with controllable mixture and the effect on cylinder head temperatures is dramatic. It’s related to the speed of flame front propagation relative to piston position near TDC. It could be that enriching the AFR just a little under the typical RPM and throttle settings you would see while towing at 62-65 in 4th, would generate considerably less heat.

Because the radiator and transmission cooling system are linked in our trucks, more heat in the coolant means more heat in the transmission fluid. It’s a wild-ass theory but plausible also. My Derale is good, but significantly less effective than the trucool 40K in my ‘07 XL Denali at keeping trans temps down when towing. That said, its position in front of the radiator sends engine temps to the roof when towing (thanks, 6.2), so it’s not a viable solution for us.

I had thought about my ignition timing maps too as well.
Maybe in 4th gear since it's a lighter Throttle Position I've got
a little too much advance (or not enough??).. I'm not having any
detonation issues in either gear. Although my total timing in real time
hangs out around 32-38* in 4th gear steady speeds.. in 5th it's less timing
prob around 20-24* but the throttle position AND load is higher, but it
"tends" to run cooler and use a bit less gas - if I don't hit any type of incline
or wind drag issues...

Maybe I need to experiment with the shift points a bit so it stays in 4th
longer and at lighter throttle and see what happens. Or bring the 5th gear
shift mph down so it stays in 5th.. but I just feel like lugging it along is NOT
the solution.

Along the same lines, perhaps it is the power enrichment setting.
Would be easy to watch if you have a scanner/logger that shows when it's active.
Activation threshold is above 55kpa.

towing sounds like it's own special tuning thing, since you're at wot loads but only part throttle and there for a very long time. the airplane engine is probably a good comparison plus it has the required sensors. those exhaust gas Temps and cylinder head temp would probably tell an interesting story and react faster since water cooling isn't involved. I didn't think about the coolent and atf link. being they are linked on the cold end of the radiator and your cooling temp sensor is in the head. it could be conceivable the temp of the coolent in that end tank changes significantly during those climbs while head temp only goes up a bit as the radiator gets over whelmed by engine Temps.


my only 2 cents is 32-38 deg sound like a lot for an ls. they usually run much lower under load on pump gas. my timing map falls off very quickly at mid rpm under high load., not that I can pull that kinda weight, but I bet stock map would be in the 14s range or less. but again. zero real world experience doing what you're doing. my time has been spent looking at wot pulls and part throttle cruising for mph.

mikez71 said:

Along the same lines, perhaps it is the power enrichment setting.
Would be easy to watch if you have a scanner/logger that shows when it's active.
Activation threshold is above 55kpa.

Click to expand...

I've got my PE coming in at 60 kpa. And it's a bit lower multiple than the stock settings.
I've watched my LT fuel trims and WideBand in real time while towing. ..It's hard to
closely watch both sometimes
while I'm driving slower than the semi traffic !!!!
Like I mentioned, it's only when ambient temps are high.
The shifting adding temp every shift makes sense to me too.
And maybe my VE tables are just off at this load range.
While not towing (this camper) All my stuff is golden !!