6.6L L8T coolant temp

cbaker879 · June 11

I'm new to the L8T 2500hd crew. We just took our first trip with our 10k 295RL. With my previous truck (halfton) I could pull the trailer in 5th (6spd trans) gear without engine coolant temps rising. Hauled this camper 9hours at a time with that halfton in 5th gear with no issues

With the L8T it runs at 210 unloaded and loaded running in 4th. But when I let it shift into 5th also a (6spd trans) the temp immediately jumps up to about 230 or so after about 1-2 miles of shifting into 5th. If I manually shift it back to 4th it immediately drops back to the 210 mark. Is this normal? Or should I be having my dealership take a look at it.

I am just stumped that being an HD truck does this, while the halfton I had didn't have any cooling issues.

Epsilon Plus · June 16

What are you pulling?

64BAwagon · June 16

My 2021 pulling a 12k fifth wheel usually ran in the 190's with 6th gear locked out and around 64 MPH. I never once saw 230 on mine after 2 trips from MI to FL and back and one out to Nebraska and back in the summer.

Is this truck new to you or have you had it since new ? .

Edited June 24 by 64BAwagon

Another JR · June 17

9 hours ago, 64BAwagon said:

My 2021 pulling a 12k fifth wheel usually ran in the 190's with 6th gear locked out and around 64 MPH. I never once saw 230 on mine after 2 trips from MI to FL and back and one out to Nebraska and back in the summer.

Is this truck new to you or have you had it since new ? I would start by servicing the trans and making sure it has DexVI in it.

Maybe he’s out West doing big climbs and/or in hot conditions? If his truck was built before May 2021 and has the 195F transmission thermostat, that may make it run a bit hotter than your 2021 if yours has the 153F thermostat.

Edited June 17 by Another JR
Typo

newdude · June 17

On 6/10/2024 at 8:25 PM, cbaker879 said:

I'm new to the L8T 2500hd crew. We just took our first trip with our 10k 295RL. With my previous truck (halfton) I could pull the trailer in 5th (6spd trans) gear without engine coolant temps rising. Hauled this camper 9hours at a time with that halfton in 5th gear with no issues

With the L8T it runs at 210 unloaded and loaded running in 4th. But when I let it shift into 5th also a (6spd trans) the temp immediately jumps up to about 230 or so after about 1-2 miles of shifting into 5th. If I manually shift it back to 4th it immediately drops back to the 210 mark. Is this normal? Or should I be having my dealership take a look at it.

I am just stumped that being an HD truck does this, while the halfton I had didn't have any cooling issues.

Do you have tow/haul on?

5th is lowering the RPMs and lugging, which lower RPMs is lowering water pump speed and seems to be generating more heat.

cbaker879 · June 24

I do have tow haul on, I'm pulling a 32ft 5th wheel, 9500lbs

cbaker879 · June 24

On 6/16/2024 at 8:08 PM, Another JR said:

Maybe he’s out West doing big climbs and/or in hot conditions? If his truck was built before May 2021 and has the 195F transmission thermostat, that may make it run a bit hotter than your 2021 if yours has the 153F thermostat.

In Missouri. This was on a 1hr trip. Had a halfton that pulled the same trailer never once saw that temp.

This is engine coolant temp. Not transmission. It stays solid at 195

Edited June 24 by cbaker879

Another JR · June 24

1 hour ago, cbaker879 said:

In Missouri. This was on a 1hr trip. Had a halfton that pulled the same trailer never once saw that temp.

This is engine coolant temp. Not transmission. It stays solid at 195

Thanks. I understood you were talking engine coolant temp. I was suggesting that if your transmission fluid was running hotter it could be contributing to a higher engine coolant temperature due to the primary transmission cooler being in the radiator.

However I suspect newdude has identified the biggest contributor to the high coolant temps, which is the lower water pump speed and flow when you are in fifth gear.

cbaker879 · June 24

But 25 degree change in 1 mile? And a 5.3l halfton could do it with out that jump?

Not sure how it's supposed to do it on barely half the weight it's supposedly able to haul

Kjduvall · June 26

sounds like something weird going on, wonder if your temp sensor is just not right. or maybe you have a air pocket in the cooling system some how, just for the heck of it next time you pull in to your driveway after shuting engine down (yet still hot) slowly unscrew coolant reserve and let the pressure out all the way then screw back down as normal. next day or so see if any thing changes. ive had an air pocket that was semi hard to get worked out before (not on one of these trucks) but on an older pontiac. you could also use a temp gun on the thermo housing to see what actual temp is getting up to.