I have a 2001 C5 Z06 with 41k miles. It has a P1SC Procharger supercharger with the twin intercoolers, 2 small ones that sit on either side of the front bumper. It's cammed with headers and exhaust, and has a Dewitts radiator with stock fans, and stock thermostat.

2 years ago the car got some lower front end damage from a speed bump, which led to having the AC condenser, Dewitts radiator, and the Procharger radiator support mount bracket to be replaced. Before this the car ran fine with temps peaking around 214, but after this the car would slowly keep warming up, reaching 230 degrees. And all the mods are exactly the same.

I got the car with some wiring damage from a rats nest. The rats chewed up 4 wires in the loom that sits just behind the passenger side head, they also chewed one wire on splice pack 122, which was keeping the fuel pump from turning on. I fixed the wires and got the car running.

Initially I noticed the fans weren't turning on at all, I checked all the fan wiring and relays, and ended up replacing the water temp sensor which fixed the fans.

The current symptom is, with the AC OFF, after driving around the city for maybe 20-30 minutes, (some stop lights, pretty light traffic, some long stretched of cruising at 40-50mph) the car will reach 220s and eventually 230. With the AC on (the AC blew cold by the way) the temps would quickly jump up. Let's say the car is at 212 and I turn on the AC, within 30 seconds it will jump to 217 just continue to rise, and would get to 240 if I didn't turn it off. Also, once I turn the AC off the car struggles to cool down, and would pretty much just stay around whatever temp it got up to.
What I've noticed is the car will cool down slightly at idle. If I come up to a stop light at 210 degrees, the car will cool down about 2-4 degrees. However once I get going from the green light, driving normally shifting at 3k rpm not driving hard at all, the temps would rise 3-5 degrees. And once I'm going 50 or even 60mph in 5th gear the temps would just stay where they are, maybe drop 1 or 2 degrees but not more. Basically the car can't dissipate more heat as it creates.

Another thing I noticed just recently is the water temp sensor, with the key on engine off, will read 97 degrees when the car is completely cold while the ambient temp is about 85-87. And when I start the car it will get to 135 degrees within a minute of idling. That seems pretty fast to me, I compared to another C5, this one was completely stock, and within a minute the temp got to 1004 degrees. Not sure what this means but something I noticed.

Here's the list of things I tried:
Made sure the radiator and condenser were clean and not blocked at all
Made sure the system is properly bled (did 3 different methods)
Replaced water pump with another stock one
Replaced thermostat and housing with another stock one (187 degree)
Flushed radiator with BlueDevil radiator flush
I happen to have another exact DeWitt's radiator laying around so I replaced that
Checked upper and lower radiator hoses for blockage
I replaced the surge tank cap (it had an 18psi cap and I replaced it with another 18psi cap)
I cut holes in the fan shrouds to help with airflow at speed (similar to the DeWitt's fans)
I verified the fans come on in both speeds/stages, with AC on and off
I set the fans to come on in stage 1 at 195F and stage 2 at 205F
I ran Redline water wetter in the cooling system
I did a cooling system pressure test (multiple times, set pressure to 16psi left for hours never lost any pressure)
I did a block test to check for a bad head gasket (tested good)
I replaced the AC pressure sensor
Removed supercharger piping (the piping for this kit sits right in front of the condenser and radiator, so I removed it to allow better airflow, didn't make a difference)

So the car had this splitter on it: https://c7carbon.com/products/corvet...55389376414077
Early into this I noticed the splitter seemed to direct air up and behind the radiator. Keep in mind the Procharger radiator support bracket for this supercharger kit is angled more than the stock one, so the splitter probably wasn't design for it. I took the splitter off and made an air dam out of cardboard and drove the car, and I noticed the temps didn't rise as quickly as they did. Then I made a better air dam that would work with the splitter, when I drove the car with this is when I first noticed my current symptom, the car wasn't getting hot as fast as before but still getting hot. After this I just kept the splitter off the car and made an air dam that bolts directly to the bottom of the DeWitt's radiator.

The last thing I tried that actually made a difference is I removed the AC condenser entirely. When I drove the car with no condenser the temps were much better. On that drive I initially hit a bit of traffic and hit a lot of red lights so the car got to 217. At this time I'm only running water through the cooling system by the way. Once I got on a main road and started actually cruising at 40-50 mph the temps dropped to 199. It would get warmer when I took off from lights and put the engine under load, but once I got up to speed it dropped pretty quickly like it should. So after this I got a new AC condenser and put that in (thought maybe the condenser had a blockage of some sort) but when I drove it (without charging the AC system) I was back at square one.

Here are some pics of the car. I know the air dam I made is pretty shitty and it even broke at one point so I had to patch it up lol, but when I drove the car with no AC condenser it definitely did it's job.





I've tested so much stuff on this car and nothing seems to make any difference. I'm thinking that the problem is related to airflow through the radiator when driving, since it can't cool down once driving at speed. It could also be some sort of water flow restriction but I don't see how. The symptoms and the tests I've done make me think it's not a head gasket issue. I am truly at a loss and would appreciate any help, thanks.

 

KOEO the ECT and ambient temp should only be off by a degree or two after the car sits overnight...I'd look at that first.