The Cooling System Saga Continues

OffTheAir · May 3, 2022

Hi all!

Thanks for taking the time to check this thread out. I have a general curiosity that I was hoping someone could answer, as I am unsure as to what is 'normal' and 'not normal' at this point.

Long story short, about a year ago I had my radiator, thermostat, and cooling lines all replaced due to the infamous cracking caused by the cycling thermostat. All has basically been well, until I just recently had the small scare with the radiator fans and the engine coolant temperature sensor. Since P0128 (the code that was scanned) is also affiliated with the thermostat, I'm starting to think that I might potentially run into this issue again.

In my Dad's 2017 Sierra his engine temperature gauge usually reads a steady 210, right in the middle, which seems to universally be agreed upon as the appropriate temperature for these trucks. However, ever since I had my cooling system replaced, my thermostat seems to barely reach that point unless I'm sitting in rush hour traffic, and even then once I get moving again it immediately goes down. The picture below shows the range in which my truck is usually running:

I guess my question is: is this normal? Did GM update the thermostat or something? I did double-check the part# and it is the updated design, so did I just get super unlucky and it's already failing again?

I just thought I would be safe rather than sorry and ask since I know the cycling of the thermostat is what led to the previous radiator failures. I am also still covered under this particular repair (2-year warranty) so while I would rather not have to get this fixed again, GM will cover it for free if I have to.

Let me know what you guys think! Truck runs great, no issues thus far, just don't want to replace another radiator, lol. If this is totally normal, then even better!

CamGTP · May 4, 2022

I know of no update to the thermostat. The stock thermostat should still be 207 degrees I believe.

I have a feeling that your thermostat is sticking open. I think it would be pretty easy to point out that to the service writer that the truck isn't operating in the correct range for a new thermostat.

When mine was sticking open I had poor heat in the winter, coolant temps really never got above 170ish. Replaced it and now it runs well over 200 with really hot temps from the vents.

ftwhite · May 4, 2022

Agreed. Not normal. I just had the opposite problem. My thermostat got stuck closed, and the electric fans kept running full speed to try to keep temp at 210. Replaced thermostat, and truck runs pegged at 210 once warm.

OffTheAir · May 5, 2022

Hey guys,

Thanks so much for the responses, I really appreciate it. Huge bummer to hear this, but I guess it is what it is. I never really thought about it up until this point when I was messing with the coolant temperature sensor, my whole system was essentially replaced so I guess I just assumed everything was fine.

I found a picture of my gauges around a little after I had the radiator repair and the needle is also in the 180 range, which basically tells me that the thermostat I got was a total dud.

It's been a little over a year driving it like this, did I cause any long-term complications? I know that with it being cool like this it causes the engine to run a little rich, and with it being a DI possibly a lot of buildup. Let me know what you think! I'll look into trying to get this repaired soon.

CamGTP · May 5, 2022

It's going to be fine. It really wouldn't have ran any richer per say because the computer is more than capable of doing real time corrections with o2 sensors. For what it's worth they sell aftermarket thermostats that are designed to run around 170-180 degrees for performance applications.

A warmer engine is more efficient at cruising speeds but I doubt there was any extra build up of carbon deposits.

WT35002017 · May 7, 2022

On 5/4/2022 at 11:02 PM, CamGTP said:

It's going to be fine. It really wouldn't have ran any richer per say because the computer is more than capable of doing real time corrections with o2 sensors. For what it's worth they sell aftermarket thermostats that are designed to run around 170-180 degrees for performance applications.

A warmer engine is more efficient at cruising speeds but I doubt there was any extra build up of carbon deposits.

So true! I put a 180 degree thermostat in my truck to try to keep temps down a little, it runs at about 200 degrees. Doing a few modifications and heat can be the enemy. Fuel economy is not what I’m after but engine longevity is. Running cooler is not detrimental in any way as far as I know. I am hoping to extend engine life by running a little cooler when it can. If your radiator is allowing the engine to run that cool then it tells you that your cooling is more than adequate. If you want the engine to run at stock temperatures then just replace the thermostat with a stock one. It seems that the thermostat you have in there now is either not the stock part number/ temperature, or is stock but defectively opening early. No damage being done. 180 degrees shouldn’t even give you a CEL.