Yellow arc over tach vs. operating temps
Thread Starter
Instructor
Joined: Jan 2022
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From: Odessa, TX
Yellow arc over tach vs. operating temps
Hi All;
New to me Z51, 7m, all stock, 20K miles. My question concerns the yellow arc that appears over the tach and diminishes as the car warms up. The few times I've had the car out it's been pretty cold, around freezing. Car has a OE thermostat that starts opening around 185-190ish as indicated on the analog gauge (I am aware of the digital gauge, and that it reads from a different location, the radiator I believe). Oil temps also seem to hover around the 190ish mark. Keep in mind, it's cold out. After some prolonged driving around town, the yellow arc diminishes and eventually goes away. But get out on the freeway and it reappears periodically. I suspect with it being so cold out that something related to either oil or engine temps is triggering it. Again, it's cold out and the heater is cranked, and I'm driving conservatively so there just may not be enough heat overall to satisfy it. What triggers the arc? Is it strictly engine temps, oil temps, a combination, or something else? Car runs fine, so I suspect given the conditions and how I am driving it is right on the edge of "warm enough" and "not warm enough" by just a few degrees.
New to me Z51, 7m, all stock, 20K miles. My question concerns the yellow arc that appears over the tach and diminishes as the car warms up. The few times I've had the car out it's been pretty cold, around freezing. Car has a OE thermostat that starts opening around 185-190ish as indicated on the analog gauge (I am aware of the digital gauge, and that it reads from a different location, the radiator I believe). Oil temps also seem to hover around the 190ish mark. Keep in mind, it's cold out. After some prolonged driving around town, the yellow arc diminishes and eventually goes away. But get out on the freeway and it reappears periodically. I suspect with it being so cold out that something related to either oil or engine temps is triggering it. Again, it's cold out and the heater is cranked, and I'm driving conservatively so there just may not be enough heat overall to satisfy it. What triggers the arc? Is it strictly engine temps, oil temps, a combination, or something else? Car runs fine, so I suspect given the conditions and how I am driving it is right on the edge of "warm enough" and "not warm enough" by just a few degrees.
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From: Norman OK
The warm up lights are related to coolant temps only, so even though you may have a stock OE t-stat it could be stuck open. Typically if that was the case and it had impacted the ECM from seeing 167° within soo many minutes of the drive cycle it would pop the SES code for P0128. the other possibility may be that someone has tuned the car and the fans are running a aggressive tuning profile. That can keep the coolant temps low enough that it triggers the warm up lights as well.
I run a 160° LSx style t-stat on my 19 C7Z, I drive in 20° temps and rarely see the warm up lights come back up on my long drives once the coolant temps have warmed up.
I run a 160° LSx style t-stat on my 19 C7Z, I drive in 20° temps and rarely see the warm up lights come back up on my long drives once the coolant temps have warmed up.
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2015 C7 of the Year Finalist
If you are sure it is fully stock, then I'd say your t-stat needs to be replaced.
My $.02
Elmer
My $.02
Elmer
Thread Starter
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From: Odessa, TX
I believe it is stock. Had concerns about the t-stat. During a warm up in my garage I monitored the water outlet/t-stat housing vs. the temp gauge. Did not detect initial opening until around 185-190 indicated on dash analog gauge. A fairly abrupt "still cold to the touch" to "oh year, it's hot now" span of time about a minute long. A stuck t-stat would (should?) get warm progressively from start. May be the OE t-stat is getting old and opening too soon? Maybe running at freezing temps with the heater cranked up is removing enough heat to cool down the engine against the t-stat?
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From: St. Louis Metro East IL
My GS did that when new. I replaced the thermostat with an OEM AC Delco. (Dealer said GM told them normal, would not pay to replace stat).
Temps now stay around 190 even in cold weather.
My observation was that the yellow band appears at 174 F or below, goes off at 176F or above.
You can watch the digital temp display and tell when the thermostat opens.
Also - oil and coolant temperature should track - but oil will take longer to warm up.
Ron
Temps now stay around 190 even in cold weather.
My observation was that the yellow band appears at 174 F or below, goes off at 176F or above.
You can watch the digital temp display and tell when the thermostat opens.
Also - oil and coolant temperature should track - but oil will take longer to warm up.
Ron
Thread Starter
Instructor
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From: Odessa, TX
Thanks Ron, for sharing. Your experience is paralleling what mine seems to be doing. Yeah, I'm pretty convinced my car is stock and I have a "weak" OE T-Stat. A replacement from AC Delco goes for around $53 on line, so I think I'll just order one... besides, it probably wouldn't hurt to change the coolant at the same time.
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From: Norman OK
Thanks Ron, for sharing. Your experience is paralleling what mine seems to be doing. Yeah, I'm pretty convinced my car is stock and I have a "weak" OE T-Stat. A replacement from AC Delco goes for around $53 on line, so I think I'll just order one... besides, it probably wouldn't hurt to change the coolant at the same time.
if it was me I would find one of the aluminum housings that uses the LSx style t-stat and go that route. They flow better than the LTx t-stat and are less prone to failure like the LTx t-stat. I have used them on both of my previous cars with zero issues,
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From: Charlotte, NC (formerly Endicott, NY)
Thanks Ron, for sharing. Your experience is paralleling what mine seems to be doing. Yeah, I'm pretty convinced my car is stock and I have a "weak" OE T-Stat. A replacement from AC Delco goes for around $53 on line, so I think I'll just order one... besides, it probably wouldn't hurt to change the coolant at the same time.
Bill
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From: Norman OK
Coolant will hold up for 5 years so not really necessary to change it unless getting close to the 5-year point. If you do change make sure you purchase straight DexCool Vs the 50/50 mix as the C7 uses a 40/60 of coolant/water mix instead. If you flush the engine by repeatedly draining and filling it with clear drinkable water until the drained fluid is clear. Once it runs clear add 4.5 quarts of straight DexCool followed by topping off the cooling system with clear drinkable water to reach the 40/60 mix.
Bill
Bill
Distilled water should be used in the cooling system, not just drinkable water from the tap.
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From: St. Louis Metro East IL
The manual calls for drinkable water.
Use a mixture of 40% DEX-COOL
coolant and 60% clean, drinkable
water. If using this mixture, nothing
else needs to be added.
Distilled water by definition has minerals removed and can cause some problems.
Been using tap water for a very long time with no problems.
Your car - your choice.
Ron
Use a mixture of 40% DEX-COOL
coolant and 60% clean, drinkable
water. If using this mixture, nothing
else needs to be added.
Distilled water by definition has minerals removed and can cause some problems.
Been using tap water for a very long time with no problems.
Your car - your choice.
Ron
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Distilled water, while quite pure, seeks ions to chemically balance itself and to get this strip electrons from the metals in a cooling system. That is, distilled water is an acidic solvent. Making things worse, it becomes more acidic as the temperature rises.
As a practical matter, while pure (distilled or de-ionized) water is corrosive in its pure form, once mixed with the full strength anti-freeze its corrosive effects are neutralized by the inhibitors in the anti-freeze.
Tap water is perfectly fine unless you have very hard water, then buy distilled water.
As a practical matter, while pure (distilled or de-ionized) water is corrosive in its pure form, once mixed with the full strength anti-freeze its corrosive effects are neutralized by the inhibitors in the anti-freeze.
Tap water is perfectly fine unless you have very hard water, then buy distilled water.
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From: Austin, TX
I’ve had the reappearing yellow tach bar (randomly). Replaced the t-stat, piece of cake except the bolts are no longer 10mm but T30 instead. I had like 3 of every other size so had to go to the “Zone”. Then, a ton of the old o-ring gasket stuck to the water pump. OMG, spent forever with goof-off? and very fine steel wool and gently scrubbing until super clean/smooth.
Like always should have been quite quick, ended up being a “drill” as my uncle used to say.
Problem fixed. No telling how long the t-stat has been flakey.
Like always should have been quite quick, ended up being a “drill” as my uncle used to say.
Problem fixed. No telling how long the t-stat has been flakey.
Last edited by SnowyATX; Feb 24, 2022 at 11:15 AM.
Thread Starter
Instructor
Joined: Jan 2022
Posts: 105
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From: Odessa, TX
Just to close out the thread. I removed my old t-stat. Indeed, it was stock. On the bulb there is a temp of 90c stamped on it. It was also partially stuck open. Sooo... all indications are of a failed stock thermostat. Replaced with a AC/Delco OE that had the same 90c stamped on it (194f). All is working correctly and it's holding temp once warmed up. Problem solved!
