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In an effort to figure out why my 2014 C7 Z51 runs too cool on the highway, I'm starting a new thread here with some hard data. Please see this thread, post #35 for my initial post ...
To summarize, my problem is that at highway speeds in the upper gears (5-7), after my car warms up, it will cool down so much that the yellow "alert max rpm bars" on the tacho come back on. Obviously, once warmed up, the engine should stay at operating temperatures, but mine does not. If I drop down to 4th or so, then the engine will warm back up, and I can crank it to redline.
My C7 Z51 is completely stock except for a larger aluminum radiator, that I inherited from the original owner. I have had two (stock GM) thermostats in the car, and the car behaves exactly the same with either one. Here's a typical event with my car:
A few days ago and around 1,500 miles into owning the car (20,000 miles on it), I switched to Mobil 1 5W-30 (Dexos 2, yadda yadda yadda). The problem seems even worse now -- as an example, from a fully warmed-up engine in town (210-220F on water gauge, mid-gauge on oil temps) on a 39-degree night in Touring mode, I drove home at highway speeds for around 35 miles and lightthrottle in the upper gears, and by the time I got home, the water temp (on the analog gauge) had fallen to around 160F, and the yellow bar had rotated CCW almost to the 12 o'clock position (just a little bit to the right of "4" on the tacho).
In the quote above and in all previous tests, I was referencing only the dedicated (analog) water temperature in the bottom right of the cluster -- I didn't know that there was a digital gauge available in Turing mode. So today, I did some tests with that gauge:
Test 1 (68F outside): Start from cold (68F on everything in the engine compartment, using an IR temp sensor), drive a couple of surface and then freeway miles in Touring mode, observe the digital and analog gauges. I stopped a few times to jump out and feel/check the two big radiator hoses (the lower hose from the block is the outlet to the top of the radiator, the upper hose is the return into the thermostat housing).
The digital gauge reported temperatures from 68F to 215F
The oil temperature often tracked the water temperature pretty well
From this we can deduce:
The analog gauge doesn't track the digital gauge well at all (this is a known issue in the 2014s).
In Touring mode, the yellow warm-up bar goes out around 167F on the digital gauge
The thermostat seems to open around 180F
The analog gauge often doesn't change despite up to nearly 30 degrees of change on the digital gauge
Test 2 (68F outside): Go back out, and do around 15 miles on the freeway in 7th gear after putting around in town:
In town, digital gauge at 195-205F
At the end of the 12-minute highway drive, just before turning off, the temperature reported at the digital gauge has fallen to 177F
Analog gauge is "tracking" digital gauge as usual
From this we can deduce:
The temperature fell to at least the temperature at which the thermostat seems to open/close
In my opinion, had I driven longer, the temperature on the digital gauge would have fallen even more, but my test was cut short.
Test 3 (inside): At home, I tested the old thermostat (which exhibited the same problems):
At rest, the 4mm diameter tip of the thermostat bulb is around 1.5mm proud of the "circular wall" that surrounds it.
I could not clearly see the temperature at which the bulb tip started moving, BUT
Immersed in 204F water for 15 minutes, the bulb tip had extended to around 6-7mm above the "circular wall" that surrounds it.
I'm attaching two pics of the thermostat (the housing body is also in the pot, to hold the two thermocouple probes in the water). In the image of the thermostat, I took the picture at exactly right angles to it, so you can see how much the bulb tip sticks out (I marked it with a black Sharpie, prior to putting the thermostat in the pot). Tip is fully out after 15 minutes of 204F water surrounding it. Looking at the stock thermostat from the side, you can see 6-7mm of protrusion of the bulb's tip (the black part)
From this we can deduce:
It looks like my old thermostat probably works fine. I dunno exactly what temperature it opens and closes at, but being fully (I haven't found the spec yet, but it looks right) open at 204F suggests that it's probably fine.
Conclusions so far:
I have a very reproducible problem -- as long as it's not too hot outside (>90F?), my car will always "go cold" when cruising in the higher gears, for a prolonged (15 minutes or more) period at 70mph or so.
Replacing the thermostat with a new one had no impact on the problem.
Around town the water temperature behaves normally
It looks like the thermostat is opening around 180 degrees. It is clearly opening and closing.
The analog gauge's readings are waaaay off from those of the digital gauge.
Theories so far:
It seems to me like the thermostat does not close properly, when the engine is operating. It certainly closes once I park the car and let it cool down. But when I am driving with plenty of radiator cooling, it's as if the thermostat does not close properly, as the coolant cools down. Today's test did not show this conclusively, but the drive that I mentioned in 38F weather certainly demonstrated this problem.
The analog gauge doesn't track the digital gauge well at all (this is a known issue in the 2014s).
In Touring mode, the yellow warm-up bar goes out around 167F on the digital gauge
The thermostat seems to open around 180F
The analog gauge often doesn't change despite up to nearly 30 degrees of change on the digital gauge
Thanks for doing this and providing all the details Especially confirming the digital vs analog gauge issue on the 2014 as its one of those things that has driven me crazy Since I live in South FL I have never driven in temps below 50, but even on extended highway drives in 7th gear mine has never gone back into yellow zone.
Everything else seems to mirror my observations. The "warm up" temperature I've seen mentioned before is 170, so 167 seems close enough. For example before checking the Z51 dry sump oil level the engine needs to reach that temp (yellow bars go away). Like many cars the analog gauge appears to be just a fancy idiot light - it just shows cold, normal and hot - then moves between those three presets rendering the numbers themselves useless. This explains why the analog gauge basically parks itself at 220 unless the car is warming up OR overheating. The 220 is nonsense, its actually 20-30 cooler then that per the digital gauge. For what its worth a replacement T-stat is listed as 187 degrees at RockAuto. For me, once warmed up, the digital gauges both oil and water in normal mixed driving (city and highway) tend to stay together and hover between 185 to 205.
The larger radiator has a lot of cooling capacity and in those ambient temps may pull coolant temp down to the point the thermostat closes. The thermostat temperature should be stamped into the stock thermostat housing. When the thermostat closes the engine temperature should start to go up shortly after that and the coolant coming in from the radiator will be much cooler thus closing the thermostat again.
I haven't looked at the one in my C7 but the stock thermostats in my two C5s and C6Z were stamped 86 deg C (~186 deg F). With a Ron Davis Racing Radiator installed in my C5s I couldn't drive at 70 mph in 5th or 6th gear in 40 degree ambient temps because the oil temp wouldn't go above 150 degrees. I found just as you did that I had to drive in 4th gear to get the rpms high enough to generate some frictional heating. Even then coolant and oil temps were barely above the point I considered warm.
You may need to do what a lot of C5 owners with RD radiators did, which was to insert a piece of cardboard in front of the radiator to cut down on its surface area.
As for the gauge issue I have a 2015 and only use the digital gauges for coolant and oil temps. They are they are accurate and have great resolution, The analog gauges are driven by a voltage from a digital to analog conversion circuit and may have the same accuracy as the digital gauges but they don't have anywhere near the resolution. The analog gauge lets you know which ball park you are in (between this road and that road) while the digital gauge tells you which base you are on.
I think I have the Ron Davis radiator -- I need to check.
I usually see the fans running.
No tune -- totally stock.
I just discovered last night, by cross-referencing the AC Delco P/Ns for the thermostat (and thermostat housing), that the "CR207" that is stamped on the bottom of the thermostat bulb, indicates a 207F thermostat, and that is how AD Delco describes it (I'm waiting for my other post to be approved, I'll link to it when it is approved). I take this to mean that the thermostat is fully open at 207F (this jives with my stovetop test, at 204-205F), and it probably starts opening 20F or so cooler (187F?).
Engine Coolant Thermostat
Part Number: 12T019F Product Notes:
Engine Coolant Thermostat
All; Temperature: Temperature 207F; Included Components: Includes housing, thermostat, and seal; OEM Recommended Temp; Standard
Per Vehicle: 1; Years: 2014-2014
I'm (semi- ) happy to hear of the need to block the radiator with some cardboard -- I figured that might be my only solution.
If the previous owner was concerned enough with cooling to install an upgraded radiator, it would seem likely that he tuned the fans to run more often/harder. Sounds like the combination of the 2 could be the issue.
I had a similar situation to this with my car, but mine is highly modified. I did the dewitts radiator and dont run a tstat at all in the summer months. This winter my car kind of freaked out a little so I put a 160 stat in it (just a ls3 stat because I have LMR housing) and tuned the fans to not come on until 199. I see 170s-180s when cruising.
You should have the fans dialed back or honestly just leave it alone. It'll be perfect this summer.
Here are the settings in HP tuners if you have access and can pull your tune. I have a tstat in mine now and have the fans come on at 100% at 199 degrees.
Here are the temp to fan % settings:
And here is where you set what degree mechanical stat you're running:
Here are the settings in HP tuners if you have access and can pull your tune. I have a tstat in mine now and have the fans come on at 100% at 199 degrees.
Here are the temp to fan % settings:
And here is where you set what degree mechanical stat you're running:
His problem is at speed Vs put putting around in traffic. Doubtful the fans are on above 35 mph as they would hinder air movement above that speed. The designers would set them to freewheel at 70 mph Vs being electrically driven and slowing down air speed through the radiator.
My completely stock 17 GS A8 runs at approximately 180 when running at 45 mph with an outside temp of 50 to 60. The yellow warning goes away at 180 in my car. So it runs right at the point where the warning comes and goes all the time. People have suggested I have a bad thermostat in a thread in the general section.
If it’s bad I am not going to change it since the car handles 115 temperatures fine, its even been ok at 123 in rush hour traffic. The cooling system in this one is way better than either the C5 or the C6 I have owned.
It seems to me that I should never see a engine temperature that is below the initial opening temperature of the thermostat, assuming the engine warmed up at some point during the drive. The 207F seems to be the full-open temperature, so if we're generous and assign 30 degrees between opening and full open, that means that at 177 degrees the thermostat should be fully closed. Yet on my example driving home on the 39F night, I ended up WAY below 177F (real) degrees -- the reading on the analog gauge was roughly 160F, that means the digital gauge would have read around 145F, and that is way below the thermostat closed temperature. It's as if either the thermostat does not close after it has opened, or something else is going on. I really have no reason to believe that the thermostat is bad, so this is why I'm stumped.
I'll also note that the 2014 thermostat (with its higher full-open / specified temperature of 207) should be going full-closed at a higher temperature, than the later 2015-onwards thermostat (194F). So it would seem to me that a 2014 would be even less likely to have this problem of getting too cool while cruising.
I have a 2019 with 2k miles on it with same issue. If it's below 70 degrees and I coast for a little bit, say a long off ramp, my yellow indicators also come on. I'd say it's kind of normal. Should it be? Probably not.
I believe there is no issue with your temp readings. I as well have the same reading with my 2016 Z, see attached picture. That photo was taken when it was 20 degrees outside. I have seen temp get as low as 169 degrees. The reason I believe there is no issue is because there is no Check Engine Light. When my car was new, the dealer replaced the thermostat & no change in temps, and the dealer also notes no Check Engine Light for low temperature, so I am guessing the car reads temp somewhere else as it is noted in other post that a Check Engine light will come at 160 degrees & below.
Belvin20 -- that's interesting, you're just above the threshold where the yellow rpm bars come (back) on.
In hindsight, the 39F night that I observed that super-low temperature on the analog gauge, I did not get a CEL.
To summarize, I'm not too concerned about the engine temperature getting down to the thermostat opening temp (194F - 20(?) degrees) for 2015 and later cars -- that just means that there is a near-equilibirum between the outside (cold) air, the radiator and the thermostat nearly closing. I am concerned about the 2014 case, where the temperature gets that low (170-ish or lower), despite the 2014s having a higher-temperature thermostat.
I am concerned about the 2014 case, where the temperature gets that low (170-ish or lower), despite the 2014s having a higher-temperature thermostat.
Anyone know why GM switched to a lower t-stat after 2014?
During my drive home tonight I paid close attention to my digital gauge, once warmed up it stays within 5 degrees of 195 regardless of how I drove (hard or easy). The outside temp was 55 which is “cold” for where I live.
The car cooling to the point where the yellow bars come back on can’t be normal, the bars are a warning, so something has to be wrong. Can you switch back to a stock radiator? As Bill said above the aftermarket rad is likely working too well it would seem. Do you have a Z06 grille? If this doesn’t happen around town and only on the highway that rules out the fans to me. Blocking the rad off might be a good test.
So, I need to correct some things I wrote earlier:
The 207F seems to be the full-open temperature
According to Stant, the thermostat's stated temperature is the temperature at which it opens (not the temperature at which it is fully open). Note, however, that this makes the 2014 problems even more odd ... with a thermostat that opens at 207, then my seeing the cold temperature "return at cruise" is even weirder -- I'd expect the thermostat to close enough to regulate the engine temperature close to 207F, not < 170F.
I also confirmed that the thermostat is at the inlet to the water pump / is connected to the (cooler) outlet of the radiator. This is different from "the older way", where the thermostat was at the pump outlet. Apparently this newer way improves overall temperature stability of the thermostat action.
Lastly, I learned (I probably knew this once, but forgot) that there is a bypass in the system, so that the water pump is always circulating some coolant in the engine block, even when the thermostat is fully closed. Once the thermostat opens, because the bypass is a bigger restriction than the thermostat when open, most of the coolant flows through the full circuit (and through the radiator). Apparently one purpose of a bypass is to mix (engine) hot water with (radiator) cooler water, to smooth out temperature gradients as the water from the radiator enters the engine block.
The car cooling to the point where the yellow bars come back on can’t be normal, the bars are a warning, so something has to be wrong. Can you switch back to a stock radiator? As Bill said above the aftermarket rad is likely working too well it would seem. Do you have a Z06 grille? If this doesn’t happen around town and only on the highway that rules out the fans to me. Blocking the rad off might be a good test.
My car is stock, normal grille, no front plate. I don't have the original radiator. I agree the yellow bars coming back on should not be normal. I just don't see how a radiator can work "too well", in a system where the thermostat is working correctly. Reading up on radiator operation in extremely cold (i.e., Siberian) weather, it's seems that in those circumstances, the environment is so damn cold that the block itself is able to radiate enough heat out that you may not be a able to warm up, and any flow through the radiator is excessive cooling. But my situation is not nearly that extreme ...
My current line of thinking is maybe that the LT1 has an amount of thermostat bypass that overwhelms the cooling system when the radiator outlet temperature is really cool ... does anyone know where there is a good diagram of the 6.2l LT1's cooling system? I searched pretty hard but could not find one ...
As in post #17, the cooling system flow is totally different from the old days, the cooling is reverse flow, meaning the cooling radiator water flows to the heads first & then to the rest of the engine, rather in the systems we are use to, where the cooling water went in the bottom of the engine first, & the heads were cooled last. As others have stated, I get the occasional yellow line on the tach as well & my Z is totally stock, so in my estimation what we see is totally normal, even with a lager radiator & 195 to 207 degree stat, it is the nature of the beast. All we should worry about is if a CEL light comes on with a low temperature code, no code, no problem. I think we are trying to over think the engineering that developed the system, and as long as CEL has no low temp code, there is nothing to worry about, and the occasional yellow tach line is no big deal. I feel like the yellow tach line is just a calibration issue from what the sensor reads & what the tach line is calibrated.
As a side note, if you have the PDR, you can download the Cosworth Tool box & play back a drive and it records all the high & low temperatures of the coolant & oil temp & many other things in any drive you may have recorded.
01-27-2019, 05:58 PM
One car, two thermostats
Especially confirming the digital vs analog gauge issue on the 2014 as its one of those things that has driven me crazy 
Since I live in South FL I have never driven in temps below 50, but even on extended highway drives in 7th gear mine has never gone back into yellow zone.
) happy to hear of the need to block the radiator with some cardboard -- I figured that might be my only solution.
