jim spitzer

In central fl this morning it was 46 now thats cold for us. I warm up my car then the wife and I take off for breakfast. As I'm driving,( well first the car went to operating temp) the temp went down to where the yellow came on around the RPMs gauge.
I always thought the thermostat kept the car at the operating temp.
I put the car in manual mode the temp went up the light went off like normal. Then I put it back in auto and within a few minutes the yellow light came on.
I don'r recall that happening last winter. I have 22,7k on my car could I possibly have a thermostat going bad
maybe someone that has their car out in the winter will chime in to see if their car does that.

C5_Tom

It is normal my 17 GS does that all the time when it is cold outside 50 F or below.

robert miller

Good to see someone else is like that also I wouldn't have though it would do what the OP is saying. Robert

KenHorse

Can't say I've noticed that with my '19 GS M7

Woodson

z06inVB

JerryU

Yep, last year I had to drive into town when it was 24F! Engine temp was a bit less than usual but still >185. That is what a properly operating thermostat is supposed to do! I have never had the yellow ring come back on after my early 2014 Z51 or now my 2017 Grand Sport warmed up.

There are higher quality thermostats that the OEM. In my street rod with a 502 cid BB I went through a few. Have one made by Robert Shaw. More precise opening/closing range.

eboggs_jkvl

If the car cools while driving and the tach stripes reappear, I'd get a new proper temperature t-stat.

Elmer

z06inVB

NSC5

Last month I took my 2016 Z06 in for an oil change 30 miles away in 38 degree weather and it warmed up as expected and stayed warm. I can assure you I wasn't driving it hard on summer tires in that weather. 90% of the trip was steady cruising at 62 MPH on rural roads with no stop signs, lights, and almost no traffic which provides optimal engine cooling conditions. If the engine isn't staying within the normal operating band once warmed up it is time for a new thermostat.

It would be safest to stick with the same AC Delco unit that came in the car. I remember from years ago owning a GM pickup with the older 6.5L diesel that the only thermostat that performed reliably in that application was the original Delco branded unit.

And if you are a do it yourself type, I would be very careful of places like Amazon. I had run into issues with them in the past getting counterfeit SD cards and lithium coin cells but the last episode and the one that is making me really rethink my Amazon purchasing was the simple emergency garage door release I bought via Amazon. I have a separate garage where the Corvette and tractor live along with some other tractor implements and it doesn't have a separate door but I put openers on all three stalls so if there is a power failure it would be a problem. I ordered a Genie locking release from Amazon and what I received was a very poorly made Chinese knockoff with only one key that isn't made like the regular Genie keys. I sent it back to Amazon and got the real Genie model from a local big box store. Counterfeit rant mode temporarily off.

eboggs_jkvl

I'm recalling 195 for the t-stat. I'm old!

RCz51

Jaylars

Happens to my 17 GS A8 when cruising in temps below 50. Seems like 178 is the trigger temp for the yellow bands to appear. A quick exhaust check and the band goes away. When in Manual mode it rarely occurs since the RPMs are typically higher and temps climb more often.
Quite a few posts about this occurrence. Seems like the OE t-stats are not exactly the same quality wise, lowest bidder wins the contract.

Is there really any harm in the car running cooler? Doubt it...

NSC5

Running a little cooler once the car has warmed the oil up shouldn't hurt anything except maybe fuel economy and perhaps emissions and the difference probably isn't that significant.

Thermostats aren't on/off devices and they start to open below their marked temperature and they won't be fully open until the temperature is above the marked temperature, there is a range of flow restriction over the temperature range and it is desirable not to have temperature shocks and sudden temperature swings. GM used dual thermostats with different characteristics for their Duramax diesel engines to provide the desired warm up and temperature change conditions. I didn't have to replace them in my later models but I did a coolant change and replacement of the original thermostats with my 2001 LB7 gen engine; they were sold by AC Delco as a matched pair with one rated at 180 degrees and the other at 185 but they wouldn't be fully open until around 210 degrees.

It sounds like quality control and variance is an issue with the current LT series thermostats. Not surprising when even simple things like rear axle seals that have been made reliably for years are a problem item on several GM RWD platforms. You can have good or cheap but not both at the same time and we have far too much cheap.

eboggs_jkvl

Not sure on the harm. I'd say gas mileage would go down. The damn yellow bar would irritate the **** out of me. Cars don't get cold once they warm up unless it's 50 below!

JerryU

With GM buying from low cost supplier! I would NOT operate the LT1 engine if it was at 175 degrees! I'd put in a higher quality thermostat! As mentioned, in 5 years driving a C7 even at 24F and often at 40 F have never had the yellow (too cold) halo come back on!)

Looked in my 2014 Service Manual. As usual it's not perfect but this is what it says for checking the operation of the thermostat that includes temps.

It says to use Tempilstik! (We had used them in welding to measure preheat and interpass temperature BUT today we do have Infrared optical pyrometers ~$15 at Amazon!)

They say place a 188F Tempilstik mark on the thermostat housing. It says at 188F (when the Tempilstik mark starts to melt) the thermostat should start to open. Great, perhaps Superman with X-Ray eyes can see that! It says using a 206F Tempilstik it should be fully open. Again need to get Superman.

They do show watching the thermostat in hot coolent method BUT their temps are listed as the delta from what it says on the thermostat! No help in defining what it is or should be. It says at 10F below the reading of the marking on the thermostat it should be fully closed.

Like the C7 OEM engine air filter made in Thailand and some wheels made in China GM no doubt used a low cost supplier. When I checked thermostats for the 502 cid BB in my Street Rod found the range was tighter (<20 degrees) with a quality thermostat made by Robertshaw.

PS: Decided to see what the current info is about Robertshaw thermostats. Don't know if they are better BUT likely they have a high probability of being in a narrow range:

Features of a Robertshaw Thermostat:
1) Stress-free construction. Robertshaw makes the thermostat's strut assembly from brass, not stainless steel, because brass can be bradded to the housing with heat instead of sledge hammer-like stress. This technique permits two brads per strut. Other manufacturers forcefully attach the struts to the housing with intense, metal-weakening pressure on the struts, binding each strut in only one spot.

2) The Power Pill®. This is the heart of a Robertshaw thermostat. It detects minute changes in the temperature of the coolant, and quickly activates a precision-engineered stem or piston to open and close the thermostat valve.

3) One-piece bypass poppet. With other thermostats, the bypass valve stem is welded on. The weld tends to fail under stress. To eliminate this problem, Robertshaw manufactures the entire copper cup and bypass stem from a single piece of metal.

4) Short-stroke piston. In conventional thermostats, the piston must make a large stroke to open the thermostat far enough for adequate coolant flow. But the long stroke compromises durability. Robertshaw engineers achieved a shorter stroke with a uniquely-designed flange and poppet. This design increases the longevity of the thermostat, yet still allows adequate coolant circulation.

5) Contoured strut. Making the strut assembly from brass instead of steel provides another benefit: brass, being more malleable than steel, can be precisely formed to maximize coolant flow.

6) Uniform quality. Robertshaw builds all thermostats to OEM standards -- so your replacement thermostat is identical to those used in new Fords, Chryslers, and other cars.The Power PillAt the heart of a Robertshaw thermostat is the power pill. Its job: to detect minute changes in the temperature of the coolant, and to quickly activate a precision-engineered piston that opens and closes the thermostat valve. Compared to the thermal elements in some thermostats, Robertshaw's Power Pill® has major advantages:1) Rapid response. The Power Pill's piston is activated by a temperature-sensitive mixture of metallic powder and wax. Some thermostats use an all-wax charge, which responds slowly to temperature changes. Other brands mix copper powder with the wax for faster response, but the copper quickly separates from the wax. Robertshaw developed a process to maintain suspension of the copper powder in the wax, so the fast response doesn't deteriorate and the thermostat will not "stick-open" to cause the engine to run cool.

2) Reliable performance. Most manufacturers use a one-piece rubber diaphragm to seal the charge and drive the piston. If the rubber part ruptures, the thermostat fails. Robertshaw uses two separate parts: a diaphragm to seal the wax, and a stem seat or plug that drives the piston. Rubber material for each part is formulated especially to meet each part's unique requirements. Wear or damage to the stem seat still allows the thermostat to operate satisfactorily.

3) Precise performance. Every Robertshaw Power Pill is calibrated to bring the thermostat's start-to-open temperature within strict tolerances.

Bill Dearborn

Don't warm engine to operating temperature before moving the car. Get in the car, start engine, fasten belts and then drive away. Do this even when temp is at 20 below. Engine warms faster when producing power and when it is idling it isn't producing any power. Doing this may change the way you see the temperature go up and down after starting out.

Bill

NSC5

As Bill noted the best warm up occurs by running the engine at moderate load and low to moderate RPM. You are heavily fuel diluting the oil and washing down the cylinder walls with an extending idling warm up and the OLM will greatly penalize estimated oil life with this type of operation. You don't want to go full throttle interstate merge 1 mile from your house but reasonable driving is a great way to warm up the engine and minimizes wear and fuel/moisture dilution of the oil.

Do avoid high RPM operation until the oil begins to warm up. In addition to flow and clearance concerns, the bypass valve across the oil filter responds to drop across the filter element and higher RPM operation with cold oil increases the likelihood of bypassing unfiltered oil around the element.

I use remote start when approaching my ATS and Z06 and in the few seconds it takes me to enter the car and belt in the engine has dropped off of fast idle and is ready to go. GM provides a nice warm up feature with their diesel pickups when the ambient temperature is 32F or lower where the transmission torque converter is used to load the engine as the idle ramps up and holds at around 1,200 RPM and the turbocharger exhaust vanes are shut to restrict exhaust flow. It isn't as good of a method as driving but it is far better than unloaded idle. Of course the Allision auto used in the heavy duty pickups doesn't have a frail torque converter like the 8L series used in their cars and light duty pickups.

iclick

Nor have I ('17 GS). Our weather may not be cold enough to trigger it, though, as it is rare for me to get out when <50°.

JerryU

^^^
As I posted, I have driven my Z51 and Grand Sport often when 35 to 40 for 5 1/2 years. Last year had an early meeting in town when it was 24F! Wife needed her SUV, my usual ride when that cold (which in Eastern SC is very seldom when I need to drive!) No issue warmed fine and quickly to keep me warm! Grand Sport handled fine set at Weather Mode and driving with logic at those temps.

Have never had the yellow halo come back after it warms up. Have never seen coolant temp below ~190 F. IMO folks reporting the issue have thermostats out of tolerance.