2017 GMC sierrra overheats at 80

[hdmotorsports]

This is a fun one. Truck runs at normal temp until I try to pass of highway at 80 to 100 mph. Then the temp skyrockets to 260, shut off the ignition and coast for about 3 minutes. Turn on the truck temp drops to normal and runs at 80 for hours with zero temp fluctuations. New thermostat, fluid. No issues at low speed or in traffic. Hose collapsing? 

[mikeyk101]

Did you install an OEM or aftermarket thermostat? And why did you change original thermostat?

Edited December 27, 2023 by mikeyk101

[Earl Cordova]

Had that happen to one of my suburbans,

the front of the radiatior was covered with bugs after driving through a swarm of them.

 

And coasting with the engine off sounds dangerous.

Edited December 27, 2023 by Earl Cordova

[xranger90]

Happened to a friend. He took his truck to the shop for the mounting of new tires from 4Wheelonline and new radiator with an aftermarket thermostat. He ordered a new set of oem thermostat and never overheats at 100mph.

[asilverblazer]

Curious why the t-stat change would cure it. 

[mikeyk101]

17 minutes ago, asilverblazer said:

Curious why the t-stat change would cure it. 

I know on mine, when my thermostat went bad, I put in a Motorad one that was a 194⁰. It worked fine at first but after several months, it began to fail and wasnt opening when it should. I'm guessing it was stuck closed. With the thermostat stuck closed, the coolant never circulated through the radiator so this caused the higher coolant temperatures. Not only did it stop opening at around it's rated 194⁰, but also wasn't opening at the OEM 207⁰. I replaced it with a AC Delco 194⁰ and it has worked fine ever since.

[asilverblazer]

Just now, mikeyk101 said:

I know on mine, when my thermostat went bad, I put in a Motorad one that was a 194⁰. It worked fine at first but after several months, it began to fail and wasnt opening when it should. I'm guessing it was stuck closed. With the thermostat stuck closed, the coolant never circulated through the radiator so this caused the higher coolant temperatures. Not only did it stop opening at around it's rated 194⁰, but also wasn't opening at the OEM 207⁰. I replaced it with a AC Delco 194⁰ and it has worked fine ever since.

Sure, but in the cases above only at high speeds? Plenty of airflow through the radiator, that should lessen the effects of a poorly operating t-stat. (If the t-stat isn't opening, why symptoms only at high speed? It should have symptoms at other times too where engine load is high.)

[mikeyk101]

4 minutes ago, asilverblazer said:

Sure, but in the cases above only at high speeds? Plenty of airflow through the radiator, that should lessen the effects of a poorly operating t-stat. (If the t-stat isn't opening, why symptoms only at high speed? It should have symptoms at other times too where engine load is high.)

I don't know but that the exact symptoms as I experienced. If a thermostat is stuck closed, it won't allow coolant to flow into the radiator so no additional cooling. I was able to check mine by grabbing the upper hose from thermostat to radiator while motor running and could tell that hose wasn't heating up so no flow. I verified coolant temps by using an OBDII reader and not by going off the highly inaccurate dash gauge. Dash gauge will read steady 210⁰ whether the coolant is at 185⁰ or up to 225⁰. I do know at under load (higher speeds), obviously more heat is generated by the motor. 

[mikeyk101]

Just to add, under idle or mild local driving, I didn't see dash gauge move at all but once I got on the accelerator and got up to highway speeds, I would see the dash gauge rise so I knew that it was in excess of 225⁰. 

[asilverblazer]

3 hours ago, mikeyk101 said:

Just to add, under idle or mild local driving, I didn't see dash gauge move at all but once I got on the accelerator and got up to highway speeds, I would see the dash gauge rise so I knew that it was in excess of 225⁰. 

Huh, I guess that's it then. I would expect overheating symptoms (high gauge reading) of a stuck t-stat to exhibit more often than just on the highway above x mph. 

 

Make me think that maybe the aluminum block can dissipate its own heat effectively enough... given that generally speaking there aren't many fundamental differences to significantly alter the cooling system performance.

 

I'm not arguing, just discussing the seemingly odd/specific/isolated symptoms of a stuck t-stat.

[mikeyk101]

I'm guessing with the stuck closed thermostat, it was still running hotter than normal at idle and local driving but wasn't apparant by just watching the dash gauge. As I mentioned, the dash gauge goes right to 210⁰ whether the coolant temp is 185⁰ or as hot as 225⁰. It's also possible that my Motorad thermostat might have been only partially opening and not allowing full flow above 194⁰ as it was supposedly designed for. And as a result, when running at higher rpms and harder throttle situations, the problem showed up.

 

I first noticed the issue while traveling down the highway when my dash gauge rose just a bit every now and then which wasn't normal. On checking using the OBDII reader instead, I saw that coolant would occasionally get up to 230⁰ while rolling down highway but then drop down to 215-220⁰ after slowing down. Sometimes a little lower but not by much. Once I saw that, I looked further into it. That's when I discovered while investigating in my driveway that the coolant was not heating up in the upper radiator hose from t-stat to radiator. Once I changed to a GM brand thermostat, (although it was a 194⁰ one for 6.2l Camaro, Corvette, and Caddy CTS motors), coolant temps would stay in the 190⁰ +/- a few degrees.

 

I originally went with the colder Motorad t-stat and then changed to the GM one because on occasion I tow a toyhauler. I wanted to get coolant flowing through the radiator sooner to help with keeping overall coolant temps lower. 

[mikeyk101]

3 hours ago, asilverblazer said:

Make me think that maybe the aluminum block can dissipate its own heat effectively enough... given that generally speaking there aren't many fundamental differences to significantly alter the cooling system performance.

 

Interesting you bring this up. I did notice that when I had the coolant issue, my oil temperature was also affected. Normally oil temps run very similar to the coolant temps. When the coolant temps were running in the upper 220⁰ range, the oil temp was also in the 225-230⁰ range. It never ran that hot before except when hauling the toy hauler up grades. After changing the coolant t-stat back to a GM, as coolant temp stayed in the 190⁰ range, so did the oil temperature as well.

 

Another thing that came up was the transmission temperature. I had changed over to the updated 154⁰ t-stat some time ago. When normally running down highway, it will stay in the upper 140⁰. In stop and go traffic, it does sometimes rise to upper 160⁰ range or even slightly higher in occasion but then drop back down once rolling again. When coolant problem showed up, the transmission temp started regularly running in the 160-170⁰ range and even went up to upper 180⁰ a couple times. Once the defective Motorad was replaced, transmission temp dropped back down to where it was supposed to be again.

[asilverblazer]

16 hours ago, mikeyk101 said:

 

Interesting you bring this up. I did notice that when I had the coolant issue, my oil temperature was also affected. Normally oil temps run very similar to the coolant temps. When the coolant temps were running in the upper 220⁰ range, the oil temp was also in the 225-230⁰ range. It never ran that hot before except when hauling the toy hauler up grades. After changing the coolant t-stat back to a GM, as coolant temp stayed in the 190⁰ range, so did the oil temperature as well.

 

Another thing that came up was the transmission temperature. I had changed over to the updated 154⁰ t-stat some time ago. When normally running down highway, it will stay in the upper 140⁰. In stop and go traffic, it does sometimes rise to upper 160⁰ range or even slightly higher in occasion but then drop back down once rolling again. When coolant problem showed up, the transmission temp started regularly running in the 160-170⁰ range and even went up to upper 180⁰ a couple times. Once the defective Motorad was replaced, transmission temp dropped back down to where it was supposed to be again.

Makes sense, since the t-stat opening and closing is the primary means of regulating the engine temperature, all other temperature readings would track with the performance of it. 

 

The temperature of the engine determines the oil temp, and a hotter engine can also transfer additional heat to the transmission.