My theory (not an engineer) is the system was designed knowing how much cooling the stock radiator provides. You've got a larger more efficient radiator. So as Bill Dearborn pointed out it is possible that you are now over-cooling the system.

If others are reporting the same thing I guess you shouldn't worry too much. It just seems odd the system is calibrated this way. At outside temps under 40 with a light engine load I would semi-except the engine to run on the cool side... but at nearly 70? That is like the definition of a "normal" ambient temp - not too hot, nor too cold.

 

I can see two possible reasons (or a combination of both) for the coolant temp to move below thermostat opening temps (other than a defect):

1. The cabin heating is turned on and the heat transfer in the cabin heater is high enough to move temps below thermostat opening temp.

2. Most thermostats have a little hole drilled into the plate that always allows somme coolant being pumped through the Radiator. I am not sure this is the case with the C7, but if, together with the more efficient radiator might be enough to move coolant temps below thermostat opening temp.

Regards

Götz

 

Small updates, and another correction :-(

• I had written "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." Nope. As per the factory service manual, the hose that connects the thermostat housing to the upper port of the radiator on the passenger side is clearly identified as the "Radiator Inlet Hose" in Fig. 27 of the manual. Similarly, the hose that goes from the port a bit lower on the engine block, to the lower port on the driver's side of the radiator is clearly identified as the "Radiator Outlet Hose" in Fig. 28 of the manual. This makes sense on the radiator side, since you would always push hotter coolant into the top of the radiator, since heat rises. But it also tells us that the coolant flows from the block into the thermostat housing, and out of the housing to the radiator. This also jives with my observation that before the thermostat opened, the lower hose was noticeably hotter than the upper hose. That's because the closed thermostat was preventing heat transfer through the coolant in the upper hose, but there was no such blocking in the lower hose. TL, DR: I now understand that the LT1 coolant flows out of the thermostat, to the top of the radiator, cools, and then flows out of the bottom of the radiator back to the engine.
• It turns out I have a Ron Davis aluminum radiator P/N 1-16CVC7. Compared to the stock and DeWitts radiator (1139114M), all three have their inlet pipe on the upper passenger corner of the radiator. However, unlike the stock and DeWitts radiator, the Ron Davis radiator has its outlet on the passenger side. The net result is that the return line of cooler coolant to the engine block is much shorter than stock when the Ron Davis radiator is used. I don't know what hose is in use in my car, since it appears to be around 24" shorter than the stock hose -- I suspect it's the stock hose, that was cut to fit on the radiator end of the hose.
• I bought a thermostat housing with the later, cooler, 2015-onwards thermostat, that I may try later.

 

The plot thickens ...

It turns out that this radiator had a leak, was repaired (with some kind of epoxy), and still has a (minor) leak. So right now, my plan is to revert to a stock radiator, re-do various measurements, and then assess whether to return the old radiator (hopefully, fixed) to service. I'll do a leakdown test on the cooling system first, with the old radiator, to prove / get objective numbers first.Fun Fun Fun.

 

Update:

My aluminum radiator definitely has a leak, somewhere where I cannot see it -- i.e. it's not visible from the rear, but with a 20psi leaekdown tester in place, I'm getting multiple drips of coolant after 10-20 minutes (when cold). So the radiator has to come out :-(

The paper towel down under the radiator serves as a tell-tale that it is leaking. Sadly, they are not staying dry. There is also an epoxy-type fix on the right middle edge of the radiator, it does not appear to be leaking there ...

As an aside, I made a leakdown tester adapter for the coolant tank cap:

Leakdown tester and coolant pressure cap adapter used to pressurize the coolant system to 20psi. The adapter goes where the coolant pressure cap went, and is held in place by a gutted original cap with a hole in the center of it.

The coolant pressure cap is kind of interesting -- it has two O-ring seals (lower and upper). The lower one seals the overall system. There is a pressure valve (and atmospheric pressure) above this, and the valve will open if 20psi is exceeded. If this happens, coolant passes upwards where it is stopped by a second O-ring. "In between" these two O-rings is a kind of exhaust vent that is in the inside front of the mouth of the radiator surge tank, down low in the bore. I'm not sure where the passage leads, should be to a hose somewhere so that overpressure coolant will dump to the ground. To make an adapter, I machined some Delrin with the right O-ring grooves, and took the O-rings from an old radiator cap. Technically, I only needed the lower / smaller-diameter O-ring. I drilled a 3/4" hole in the center of an old cap (after I popped off the valve assembly), and that secures this adapter plug in place.

On the top is my adapter and a complete cap coolant pressure cap. Below is a complete coolant pressure cap, disassembled. The O-rings from the disassembled cap have been moved to the adapter.

 

What a difference a thermostat radiator makes!

To recap: 2014 C7 Z51, no tune, no engine mods other than a Tony Mamo ported throttle body (PTB) (irrelevant to this discussion) and a Ron Davis aluminum radiator. Car runs much too cool when cruising on the highway -- it dips below (!) the thermostat temperature when cruising in 6th and 7th gears, to the point where I often cannot drive the car "in anger" because the orange and yellow "too cool notifications" on the tach are showing ...

So, after confirming that the aluminum radiator was leaking (via a pressure test), I ordered a replacement stock radiator and a new lower radiator hose (the Ron Davis radiator's large outlet is on the passenger side, the stock setup has it on the driver's side) from RockAuto. Then, I took the plunge and swapped out the radiator in a few hours. A couple of notes below (dealing with the A/C condenser and how it was attached to the radiator was the biggest hassle), and a bunch of pictures.

Disassembly sequence:

1. Disconnect and remove intake hose between air filter and TB (two screw clamps)
2. Remove air filter lid (two screws)
3. Remove air filter
4. Remove air filter housing (two screws, the third attach point is actually a ball end that pops into a rubber grommet. Lowering the hood makes for more clearance when removing)
5. Remove air duct at top of radiator (4 screws)
6. Drain radiator (easiest to use the drain plug at the lower driver's side)
7. Remove radiator plumbing (The two large hoses benefit greatly from having a radiator clamp special tool. The extra coolant outlet that feeds the engine oil cooler requires a screwdriver to remove its clip. The surge tank inlet hose has a special clip that can be released by hand).
8. Remove radiator surge tank inlet hose (at surge tank and at thermostat housing)
9. Disconnect radiator fan connector
10. Remove radiator fan (2 screws)
11. Remove 2 (large, long) screws that hold radiator to radiator shroud
12. At this point, the radiator and the A/C condenser in front of it are clipped together, and the radiator can be both lifted off of its locating grommets, and it can be tilted to be much more upright. Separate the A/C condenser from the radiator This is the hardest part of the jib -- GM specified that you disconnect the pipes for the A/C condenser, which means that you'll have to refill it with Freon, etc. But that is not really necessary. But it helps to have to sets of extra hands, to release the small catches that are on the radiator, that hold the A/C condenser in place and prevent it from moving up and out of the clips on the radiator that hold it in place.

Installation is the reverse of the disassembly. One key and perhaps non-obvious thing to look out for is to ensure that the A/C condenser centered laterally relative to the radiator, when you are guiding its tabs into the slots/clips on the radiator. The driver's-side tabs have no stop on them, but the passenger-side ones do (this allows the A/C condenser to expand and contract relative to the radiator, without there being a build-up of stress in the tabs/clips).

Now to some notes and pictures.

The Ron Davis radiator is much thicker (and noticeably heavier) than the stock radiator.

Stock plastic tanks + aluminum fins radiator on the left, Ron Davis all-aluminum on the right. The stock one costs 15% of the other.


The Ron Davis radiator (on the right) is all-aluminum (tanks and fins), and is much thicker than the stock radiator (on the left)


The fan side of the Ron Davis radiator. This is a very heavy-duty, very nicely TIG-welded radiator. You can see the 2014 build date stenciled to the bottom right corner. Note that the main outlet is on the passenger side (stock is on the driver side), and the fan and shroud mounts are via threaded (aluminum, of course) bosses. Dunno why there is black discoloration on the output to the oil cooler. You can see the repaired area on the right side, mid-height -- that repair appears on both sides of the radiator, and is where it's still leaking, as best I can tell.


An upper A/C condenser slot + clamp on the Ron Davis radiator. Water-jet (?) cut relief for the A/C condenser tab tensioner.


Here's the same feature (how the A/C condenser tabs are secured) on the stock radiator.


Here's the upper passenger-side tab from the A/C condenser. Notice that it is scratched and somewhat bent in the middle. The scratching is probably from Al-on-Al contact with the Ron Davis radiator (and is not a big deal). The bend, OTOH, made it much harder to disconnect from the Ron Davis radiator, because it was "sticking" in the tab slot. I straightened this out, and it popped right into the stock radiator.

I am the second owner of this C7, and it was under the previous owner's watch that the Ron Davis radiator was installed, to solve the usual on-track overheating problem. Unfortunately this particular radiator had a leak; a repair was clearly attempted at one time, but it still leaks under pressure. I will contact Ron Davis and see what can be done to get it fixed properly.

I also made a small mod that I find very useful -- I drilled two 0.250"-size holes in the body of the air filter housing, to accommodate tie-wraps that will hold the surge tank hoses in place (mine were always popping off).


Holes in air filter housing enable me to keep the surge tank hoses in place. Yes, I cut the extras off after this picture was taken.

OK, now with that out of the way, back to the original problem -- did the new radiator fix the problem? Did it ever! A couple of test drives in 50F weather show the engine heating up nicely, and then moving between around 205 to 222 degrees, in 6th & 7th gear cruise, some spirited driving, around town (that was never a problem), etc. Once the thermostat opens, the oil temperature tracks the water temperature to within a few degrees. The difference between the digital and analog temperature gauges remain, and the the analog gauge pretty much stays pegged at 220F through all of this.

To summarize: The engine warms up quickly and monotonically with the new, stock radiator (I re-used the existing coolant, BTW), I am no longer seeing severe drops in temperature at cruise or in any other condition. The digital temperature reported sits at right around the thermostat temperature of 205F, and increases to around 222F under hard (street) driving. The analog gauge stays pegged right at 220F.

So now, I can actually enjoy the car.

My conclusions thus far are that there is something about the much greater cooling ability of the Ron Davis radiator that prevents the thermostat from actively regulating engine temperature ... With the Ron Davis radiator in place, the system is so overcooled in Northern California weather when driven on the street that I never even observed the thermostat regulating the engine temperature like I do now -- it seems to have opened, and then never closed, even though coolant temperature dropped by 40-70F (!). Of course on track with a much greater thermal load, I'm certain that the bigger radiator would be a godsend and would be necessary to prevent overheating.

There is one fly in the ointment, and that is the fact that the radiator was leaking, and this will have an impact on cooling system pressure, and possible on the boiling point of the coolant. I've experienced rapid swings in reported engine temperature on at least one car (not a Corvette) where there was a pinhole leak in the radiator -- temps would go to mid-range, then to really high, and then collapse down to very cool, and then repeat-- the lack of consistent coolant pressure was causing a localized boil-off where the sensor was, and when it was no longer immersed in liquid coolant, it reported very low temperatures. With this C7, I never ran too low on coolant (but I was often topping it off), and so I'd have more confidence in my diagnosis if the Ron Davis radiator had not been leaking, during these tests. BTW, I did a BlackStone Labs oil analysis and everything is A-OK, no coolant in the oil, etc. so I have no reason to believe that the radiator leak had any far-reaching effects on engine health.

I'll drive the car for a while this way (it's nice to be able to drive and it no longer worry). I'm interested in whether swapping out the thermostat for the 2015- and later cars (about 20F cooler) will introduce any issues in terms of fuel mapping, etc.

 

Glad to hear you finally fixed the issue as some of us guessed the aftermarket radiator was working a bit too well causing the temp to drop.

 

Isn't it nice when a car works as intended!

 

JMII:

Re-reading your posts in this thread, it looks like you have a 2014,. but your cruise temperature is in the 190F's. Did you have the 2015- and later T-stat installed in your C7?

 

Nope - dead stock as far as I know but I have never pulled it to check / verify. My temps runs from 190 to 210 during my daily commute which is a 40/60 city / highway mix in south Florida. Within that range it spends most (80-90%?) of its time between 195 and 205.

Autozone lists the factory t-stats as:
2014 = 207 F (97C)
2015 = 194 F (90C)

I just wonder why GM changed the spec since the engine and all other cooling aspects are the same between '14 & '15 models. Was the '14 t-stat a mistake that was corrected later? The '14 seems to have this 220 analog gauge error so maybe that got GM's attention and caused them to revisit the temps leading to an updated t-state in '15? The analog vs digital gauge error is what got my interest in this problem in the first place.

 

Weird -- it really looks to me like your car has the cooler thermostat, as its running temps are basically 13F cooler than mine, which is the difference between the two T-stat temps. I am certain mine has the 207F T-stat.

Maybe your car is a really, really late 2014

 

I guess its possible... if so I'm not complaining.

 

After all that talk and research. No one mentioned the oil cooler. Your engine is constantly flowing coolant through the oil cooler and back into the radiator. This is the outlet of the radiator but radiator nonetheless. If your thermostat fails closed, you still get some cooling through here.

​​​Then other thing is you're forgetting is the overflow tank. You're getting constant flow from the top of the radiator to the overflow tank. Well, that tank has to then flow somewhere else right?

Finally, there is the heater core someone mentioned. It flows to and from the water pump and Ys from the overflow tank.

It's not much flow. But on a near freezing day, all of it is enough to keep the car cool without opening the tstat. This is all taken into account when setting up a tstat temp during design.

Your car was probably on the edge needing to open the tstat and the better radiator would then supercool the car as soon as it opened and then the tsat would close. Repeat.
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