Thermostat Recommendations

Blue65 · November 3, 2021

I am going to start towing a 3k # boat regularly. I am looking at getting an aux. transmission cooler as well as a lower temp thermostat. It looks like for my year model truck the thermostat is a combo, of the housing integrated to the actual thermostat. Can anyone tell me which thermostat is currently in an unmodified 2018 Sierra 5.3L Direct Injection?

Which thermostat would you guys recommend? I live in Texas along the gulf coast, so it gets pretty warm down here.

In regards to trans coolers. I was looking at the Mishimoto one. It is super nice especially since it comes with all the clean brackets. However, that comes with a price tag. I was wondering if anyone here had other suggestions on trans coolers. Thank you in advance!

CamGTP · November 3, 2021

A stock engine coolant thermostat is 207 degrees. You can run a 180 degree thermostat but just be aware that once it heat soaks that thermostat doesn't really mean much because the factory fan settings are still set for the stock thermostat. They won't even come on until it's past 225 degrees, which is still fine for these engines.

If you haven't yet, just remove the thermostat on the transmission cooler line going into the transmission. This is a very common mod that people do on the transmission. It will drop the temps an easy 30-40 degrees without even using an aftermarket cooler. See if you need a cooler after doing that first. If you transmission temps stay between 170-200 while towing you are doing great.

pokismoki · November 3, 2021

i would upgrade to the 6.2 Radiator, 175 t-stat from Katec, tune fans for 188-190 degrees, re-tune transmission, TCC lock up points activate at 4th or 5th-6th gear for towning, zero tcc slip, and tighten the clutch pressures 10%.

then do 3000 mile oil changes, and trans flush every 8000 miles...

bolt open the flapper valve in the muffler for full flow and disable the AFM

#1 upgrade is thermostat delete in trans

Edited November 3, 2021 by pokismoki

CamGTP · November 4, 2021

It'll be pretty spendy to tune a 2018, he may not want to go through all that.

pokismoki · November 4, 2021

what does it run about to tune the 2018 and newer stuff?

Blue65 · November 4, 2021

Thanks guys. I think everyone can agree I need to go ahead and remove the thermostat on the transmission line. I am assuming that is on the return line back to the transmission? I guess I need to do a bit of research on where that thermostat is located. Thank you again.

5 hours ago, pokismoki said:

what does it run about to tune the 2018 and newer stuff?

Grumpy Bear · November 4, 2021

4 hours ago, Blue65 said:

Thanks guys. I think everyone can agree I need to go ahead and remove the thermostat on the transmission line. I am assuming that is on the return line back to the transmission? I guess I need to do a bit of research on where that thermostat is located. Thank you again.

It's bolted to the drivers side of the transmission body.

Go to the fourth post on page on of this thread. A new option.

CamGTP · November 5, 2021

22 hours ago, pokismoki said:

what does it run about to tune the 2018 and newer stuff?

This kinda depends on if it's a 6 speed or 8 speed truck too.

I'm using HP Tuners as an example because they offer the TCM tuning for 8 speeds.

It's $200 for the PCM, then it's $200 for the TCM plus the TCM unlock service and I think that is about $200. Then if you had to buy HPT it's another $300.

Trucks with a 6 speed wouldn't require the TCM service so knock that price down some.

Grumpy Bear · November 5, 2021

I'm going to add here than the lower water thermostat only throttles lower IF the radiator is large enough to permit that as it does in my case. In which case the fan setting is mute.....unless stalled in traffic. In which case you are no worse off than before on the worst day and better off all others. Give it some thought. It will come to you.

pokismoki · November 5, 2021

8 minutes ago, Grumpy Bear said:

I'm going to add here than the lower water thermostat only throttles lower IF the radiator is large enough to permit that as it does in my case. In which case the fan setting is mute.....unless stalled in traffic. In which case you are no worse off than before on the worst day and better off all others. Give it some thought. It will come to you.

just keep your airspeed up, and the thermostat will do its job to perfection.