Coolant temp flucuation
Thread Starter
Melting Slicks
Joined: Jan 2017
Posts: 2,696
Likes: 888
From: CONCORD CA.
I was finally able to get a new thermostat installed this Saturday. I took it for a couple hour drive. The same issue is still happening, maybe not as frequent though. When it does happen, it seems to happen more so in stop & go traffic. Maybe I'm not watching as carefully on the freeway as when I'm driving around town. So far, the normal temp has been 220°F. Around town, commute or freeway, 220° is where it sat. When it does rise, it goes to 230° - 250°, then it drops back to 220°. This cycle takes only about 10-15 seconds. The coolant level appears to be correct and the fans are working too. For now, I'm just driving as normal.
Le Mans Master
Joined: Jun 2019
Posts: 5,385
Likes: 2,124
From: Loveland
WOW, OUCH! When I drive and the temps are at 220* I don't think my fans run. When I shut it off they are not running. Are your fans only running when it gets above the 220*? The only thing I can think of now is an electrical glitch in the gauge itself. It doesn't make sense the real temp would change that fast when it drops. Don't feel bad about the parts, they can be an issue on these cars. It would be interesting what the dealer mechanic would say. Probably, replace the sending unit, "OH, you did that",,,, "replace the thermostat,,, Oh, you did that". Wonder what's next?
Thread Starter
Melting Slicks
Joined: Jan 2017
Posts: 2,696
Likes: 888
From: CONCORD CA.
Stll getting used to the 'bugs' on this car. So far this is only one. Well, I take that back. I had an issue pop-up last friday. When driving around town, I noticed that speedometer seemed to be stuck on its reading (it wasn't changing). I tried to flip to another screen, but it was locked up. Then after a few more seconds, the whole dash cluster went black. The car seemed to be driving normally though. Then a few more seconds, and it all came back. Did a few long drives over the weekend with no repeat of this. This car only has 30k miles (2014). I'm in it for the long haul. At least for now.
Thread Starter
Melting Slicks
Joined: Jan 2017
Posts: 2,696
Likes: 888
From: CONCORD CA.
Nothing battery related yet. The volt gauge is showing in the 14v range (when running). I haven't checked the battery voltage with the car off yet. I'll look at the battery date and see how old it is.
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Race Director
Joined: Apr 2000
Posts: 15,344
Likes: 4,008
From: Texas
Couple other points...
- Did you use an AC Delco sensor when you replaced the coolant sensor? These cars hate aftermarket sensors. Very hit or miss
- Where are you getting the temp from? Sounds like you're eyeballing one of the gauges vs looking at the digital reading so you have a more precise value
- What code(s) did you clear?
- What thermostat did you use? Need to use GM and replace the entire housing. Like sensors, the aftermarket thermostats run about 50/50 regarding success
- Lastly....fan. Yes it runs sometimes but do you know it's running when it's 250* or are you assuming?
Thread Starter
Melting Slicks
Joined: Jan 2017
Posts: 2,696
Likes: 888
From: CONCORD CA.
I did not use the AC Delco thermostat. I think it was the Murray brand (194°F). Chaning this did not change the readings I'm getting. I also changed the ECT sensor prior this. Regarding the fan operation, I can't tell if it's running when the issue happens. So far, the only times that I've noticed this, is when the car is in motion and, also only when at lower speeds. I haven't noticed at freeway speeds. Now, I'm not watching the gauge 100% of the time. This combined with how quickly this happens, (about 10-15 seconds for the whole rising and falling of the temp.), means that I may not have noticed all of the times it has happened. I know that fan at least works, as I do hear on when I shut the car off it (it will run for about 5 seconds). I haven't burped the system (yet). I still have a few more things to check.
Thanks for the 'other points" too. Any ammunition helps.
Thanks for the 'other points" too. Any ammunition helps.
Thread Starter
Melting Slicks
Joined: Jan 2017
Posts: 2,696
Likes: 888
From: CONCORD CA.
If the one I replaced was 204°, replacing it with one for 194°, so far hasn't changed a thing. For about 98% of the time, this runs at 220°F. Whether in commute, freeway or around town, it sits at 220°. Other than occasional temp swing, all is good. It's been a little over 3 weeks. We'll see what happens when as it hits spring / summertime.
currently it's been in the high 30's in the morning, into the high 50's / low 60's in the day.
currently it's been in the high 30's in the morning, into the high 50's / low 60's in the day.
Le Mans Master
Joined: Jun 2019
Posts: 5,385
Likes: 2,124
From: Loveland
That makes sense. Once the thermostat is open, and the engine reaches it's operating temp, it's temp rating shouldn't make any difference. Even when it's 90* out, mine is still solid at 220*. I have been trying to figure out the difference in temp sensor location. On the 2014 it's next to the thermostat, on later years it's about 10 inches from the thermostat.
.
I just had a revelation! I keep hearing of people putting a 180* thermostat in these engines. I think because they want it to run cooler,
. But once the thermostat opens fully, the car doesn't know or care what the full open temp is, the temp will still reach equilibrium, depending on flow and radiator cooling. The only difference would be, a full open thermostat at 180* would cause it to take a little longer to reach 220*.
.
I just had a revelation! I keep hearing of people putting a 180* thermostat in these engines. I think because they want it to run cooler,
. But once the thermostat opens fully, the car doesn't know or care what the full open temp is, the temp will still reach equilibrium, depending on flow and radiator cooling. The only difference would be, a full open thermostat at 180* would cause it to take a little longer to reach 220*.
Last edited by kodpkd; Feb 4, 2025 at 11:57 AM.
Thread Starter
Melting Slicks
Joined: Jan 2017
Posts: 2,696
Likes: 888
From: CONCORD CA.
I agree with all you said (or at least, I have the same understanding). I can see using a higher temp thermostat in colder climes. My previous C6 would run about 195° on the freeway and about 200° - 210° around town. I'm hoping that the C7 runs the same temp in the summer.
Thread Starter
Melting Slicks
Joined: Jan 2017
Posts: 2,696
Likes: 888
From: CONCORD CA.
I think that the opening is the same size, It's the temp range that determines when it opens. Don't quote me on this, it's just my basic understanding. Whatever issue I'm having, I don't think is related to the thermostat or temp sensor (as changing both of these has made no difference).
Team Owner
Joined: Sep 2000
Posts: 24,977
Likes: 2,690
From: Far NW 'burbs of Chicago
St. Jude Donor '13
That makes sense. Once the thermostat is open, and the engine reaches it's operating temp, it's temp rating shouldn't make any difference. Even when it's 90* out, mine is still solid at 220*. I have been trying to figure out the difference in temp sensor location. On the 2014 it's next to the thermostat, on later years it's about 10 inches from the thermostat.
.
I just had a revelation! I keep hearing of people putting a 180* thermostat in these engines. I think because they want it to run cooler,. But once the thermostat opens fully, the car doesn't know or care what the full open temp is, the temp will still reach equilibrium, depending on flow and radiator cooling. The only difference would be, a full open thermostat at 180* would cause it to take a little longer to reach 220*.
.
I just had a revelation! I keep hearing of people putting a 180* thermostat in these engines. I think because they want it to run cooler,
. But once the thermostat opens fully, the car doesn't know or care what the full open temp is, the temp will still reach equilibrium, depending on flow and radiator cooling. The only difference would be, a full open thermostat at 180* would cause it to take a little longer to reach 220*.A properly working thermostat will not be full open during normal operation.
In a properly operating car, the thermostat stays closed until the coolant approaches desired temp, then modulates more open/more closed to maintain that desired temp. Coolant only goes above the rated temp of the thermostat if you are working the engine "too" hard or something is wrong with the cooling system.
Le Mans Master
Joined: Jun 2019
Posts: 5,385
Likes: 2,124
From: Loveland
If the thermostat is 180* rated and the engine temp is at 220* I think it would be fully open. My temp sensor is right next to the thermostat. My thermostat is 204* and the engine runs at 220*, very constant. I do agree these numbers are all dependent on many things, and maybe one temp isn't the same as the other.
Tech Contributor
Joined: Oct 1999
Posts: 41,013
Likes: 9,773
From: Charlotte, NC (formerly Endicott, NY)
Did you check your thermostat temperature before installing it. I haven't touched the one in 15Z but all of the other thermostats I have replaced had the opening temperature marked on them in both fahrenheit and celsius. The thermostats in my C5s were marked 186F/86C.
Bill