2018 6.2L Denali not coming up to temperature

[TwoBigBoys n dog]

2018 Yukon Denali premier (tri coat white with tan interior) package 86K miles/ wifey daily driver. Temperature gauge not showing 'up to temp' @ 210'f. Maxes @ 185'f then drops after 15+ minutes of city driving, which is new. Garaged, but daily outside temp currently in 20s - low 40s. Thermostat replaced in 2024 due to same issue. Not showing any codes and dealer can't duplicate. Partial drain/ refill of coolant on last temp gauge replacement. Dealer is now looking for bubbles in system. Other thoughts if ECT is not throwing codes?

 

[slupie]

It's the thermostat, they fail. I replaced the thermostat in November, 24 and in December, 25 the same symptom as you describe. Replaced it again. Mind you this was AC Delco gold thermostat, got 13 months and 16k miles out of it.

 

[Fless]

2018 Yukon Denali premier (tri coat white with tan interior) package 86K miles/ wifey daily driver. Temperature gauge not showing 'up to temp' @ 210'f. Maxes @ 185'f then drops after 15+ minutes of city driving, which is new. Garaged, but daily outside temp currently in 20s - low 40s. Thermostat replaced in 2024 due to same issue. Not showing any codes and dealer can't duplicate. Partial drain/ refill of coolant on last temp gauge replacement. Dealer is now looking for bubbles in system. Other thoughts if ECT is not throwing codes?

The ECT likely won't throw codes, but there are other symptoms and causes. Need to view live data and verify hose temps to make an educated decision.

 

[Tonyv__]

Thermostat sticking open IMO. I couldn’t see any other reason for it not getting up to temp.

Unless you’re running an oversized aftermarket radiator

 

[TwoBigBoys n dog]

Thermostat sticking open IMO. I couldn’t see any other reason for it not getting up to temp.

Unless you’re running an oversized aftermarket radiator

I have the tow package, but it is OEM, not aftermarket

 

[Tonyv__]

I have the tow package, but it is OEM, not aftermarket

I had same symptoms earlier this year. I only have 91k miles as of right now. Replacing thermostat fixed it. I would guess your replacement stat is faulty.

Air pockets would cause overheating in my experience because those air pockets don’t alow coolant to flow.

 

[pwtr02ss]

Thermostat. I had to replace mine a few months ago and it was on backorder from the dealer. That made me think it's a common thing. My CEL came on and that's when I noticed it wasn't up to temp. After replacement, everything went back to normal.

Now my radiator is leaking...

 

[TwoBigBoys n dog]

Update: dealer was unable to reproduce. Did not charge me for diagnostics. Yay for me and props to Fletcher @ Riverside GMC!

 

[hagar]

You can't assume new or newer parts work properly anymore, the failure rate over the last 10 years on parts is through the roof.

 

[pwtr02ss]

Update: dealer was unable to reproduce. Did not charge me for diagnostics. Yay for me and props to Fletcher @ Riverside GMC!

What was the issue?

 

[TwoBigBoys n dog]

What was the issue?

see top of thread: temp gauge would not show engine coming up to 'normal' after driving

 

[Rygrego]

2018 Yukon Denali premier (tri coat white with tan interior) package 86K miles/ wifey daily driver. Temperature gauge not showing 'up to temp' @ 210'f. Maxes @ 185'f then drops after 15+ minutes of city driving, which is new. Garaged, but daily outside temp currently in 20s - low 40s. Thermostat replaced in 2024 due to same issue. Not showing any codes and dealer can't duplicate. Partial drain/ refill of coolant on last temp gauge replacement. Dealer is now looking for bubbles in system. Other thoughts if ECT is not throwing codes?

Thermostat is the issue and replacing is the remedy. I just had the exact problem and changed mine. Mine would eventually come to temp and then drop back to 185 and stay for a while then rise and drop again. Luckily it's a simple replacement.

 

[pwtr02ss]

see top of thread: temp gauge would not show engine coming up to 'normal' after driving

I'm asking what caused the problem. Was it the thermostat like we suggested or was it something else?

 

[TwoBigBoys n dog]

I'm asking what caused the problem. Was it the thermostat like we suggested or was it something else?

I dont know. The dealer was not able to reproduce (rig heated up for them) and they sent it back 2 days later without replacing any parts. i presume the group is correct and the 'new' thermostat is faulty but it was not replaced.

 

[Fless]

If you have access to a good scanner, graph the ECT over time and save any graphs that show it fluctuating and going too low. That might help you prove the issue.

 

[TwoBigBoys n dog]

If you have access to a good scanner, graph the ECT over time and save any graphs that show it fluctuating and going too low. That might help you prove the issue.

Thank you. I don't have a scanner.

 

[DLSinPV]

2018 Yukon Denali premier (tri coat white with tan interior) package 86K miles/ wifey daily driver. Temperature gauge not showing 'up to temp' @ 210'f. Maxes @ 185'f then drops after 15+ minutes of city driving, which is new. Garaged, but daily outside temp currently in 20s - low 40s. Thermostat replaced in 2024 due to same issue. Not showing any codes and dealer can't duplicate. Partial drain/ refill of coolant on last temp gauge replacement. Dealer is now looking for bubbles in system. Other thoughts if ECT is not throwing codes?

I will give this a shot. If you recently changed radiator read on. I has a 2007 H2 Hummer. Swapped our radiator to aluminum. This was done in the summer months. When winter rolled in it thru a code of not coming up to temp. Dealership told me it was thermostat, had them change out. Nope, not it.After researching on hummer site I found the return line from block to right side of radiator, about 3/4 in size. Inside the radiator nipple is a plug with about a hole about the size of large pistol primer. The new aluminum radiator didn’t have that. I understood some aftermarket plastic radiators didn’t have that flow reduction either. Used a 45 cal shell casing with the primer removed and drilled out placed near radiator and clamped in place. Worked great and temps returned to normal, even in summer. If you pull hose and see the flow reducer, your the problem.

 

[TwoBigBoys n dog]

I will give this a shot. If you recently changed radiator read on. I has a 2007 H2 Hummer. Swapped our radiator to aluminum. This was done in the summer months. When winter rolled in it thru a code of not coming up to temp. Dealership told me it was thermostat, had them change out. Nope, not it.After researching on hummer site I found the return line from block to right side of radiator, about 3/4 in size. Inside the radiator nipple is a plug with about a hole about the size of large pistol primer. The new aluminum radiator didn’t have that. I understood some aftermarket plastic radiators didn’t have that flow reduction either. Used a 45 cal shell casing with the primer removed and drilled out placed near radiator and clamped in place. Worked great and temps returned to normal, even in summer. If you pull hose and see the flow reducer, your the problem.

Thank you DLSinPV; original radiator with about 87K on the rig now.

 

[Fless]

I will give this a shot. If you recently changed radiator read on. I has a 2007 H2 Hummer. Swapped our radiator to aluminum. This was done in the summer months. When winter rolled in it thru a code of not coming up to temp. Dealership told me it was thermostat, had them change out. Nope, not it.After researching on hummer site I found the return line from block to right side of radiator, about 3/4 in size. Inside the radiator nipple is a plug with about a hole about the size of large pistol primer. The new aluminum radiator didn’t have that. I understood some aftermarket plastic radiators didn’t have that flow reduction either. Used a 45 cal shell casing with the primer removed and drilled out placed near radiator and clamped in place. Worked great and temps returned to normal, even in summer. If you pull hose and see the flow reducer, your the problem.

The hose you're attempting to describe goes from the radiator to the surge tank (not the block). This is a well-documented issue with some aftermarket radiators and is discussed many times over in this site.

Not having the proper restriction in the radiator won't cause the OP's issue of it warming up fully, then dropping temp; it will normally cause the engine to not reach operating temp at all.