Our 2017 Z51 A8 started to develop a problem at about 20k miles; after driving for many miles and fully warmed up on a cool day, the coolant temp would sag down to about 170'F and the yellow "Caution" arc would appear re-appear on the tach. After a while the coolant would slowly increase to over 180' and the yellow would disappear. This happened a couple of times but not always. The dealer replaced the thermostat under warranty and everything was ok.

Everything was normal until recently at about 55k miles, the problem has reappeared. After being fully warmed up and driving steady 60 mph, coolant temp sometimes sags to 170' and the yellow comes back on. After several minutes the temp slowly gets back above 180' and the yellow disappears. The needle movement is slow and steady, taking several minutes for a "cycle". Nothing jerky or erratic. Dealer Tech found a possible loose connection on the temperature sensor, but fixing that did not fix the problem. Recently, the Tech used an infrared thermometer gun on the temperature sensor, when the cockpit gauge read 180' then the temperature gun also read `180'.

I suspect that the thermostat is sticking partially open like the first instance, but our GM extended warranty doesn't want to pay for anything more until the Tech can see the problem with his own eyes (and probably take a video of it). Now that our outside temps are routinely above 90'F, the engine isn't getting cool enough during driving to replicate the problem.

1. What do you think is causing the problem? (I suspect it's another sticking thermostat). Any other ideas?
2. What could we do to satisfy the GM warranty that something is really happening?
3. The car is totally stock, I think the thermostat is supposed to keep the temp at 190'F. Correct?

 

Jim , I agree with you , you have a stuck thermostat,

If they won't fix it , Take it somewhere that u trust as it's a 15 minute job at best .....

Good luck !
Dave

 

Sounds like you need a new thermostat again...

Ron

 

T-stat.

Elmer

 

It's almost certainly a t-stat, Jim. I keep a spare now just because it's pretty common and I've been through two as well. Easy job at home!

 

Just got the car back today, Dealer replaced both the thermostat and coolant temperature sensor. Both under warranty.
Now the coolant temp jumps up to about 190' in just a few minutes, and stays there. Tech says the stock thermostat is marked as 187'.

The Tech and I both believe the problem was with the thermostat, but since the warranty would cover the temp sensor also, just in case we did that too.
Thanks to all for your comments.

Edited to correct thermostat temperature (bold).

 

Glad you got it fixed Jim !

Dave

 

Thx for the feedback!

 

Great that you were able to get the temp issue fixed and under warranty coverage.

 

both parts cheap and easy to replace.

Correct move.

 

Please see post #6, my post from yesterday originally had typos for the temperatures.