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.

 

I was suspicious of that. What do you suggest I look at?
Could one of the wire I had to fix be for the temp sensor and cause higher resistance?

 

"Looking" isn't going to tell you what you have wrong but testing will...looks like you didn't replace the ECT sensor so if you have no electrical skills you can start there...best to use a scan tool to see exactly what the PCM is seeing as it relates to your ECT...which wire had to be fixed...did you do a "butt splice" repair perhaps ??...NOT the best method...I ALWAYS solder and heat shrink...with key on the yellow wire has 5 volts on it...that wire runs up along a harness to the sensor...on the rear of the left fuel rail is the "fuel rail stop bracket"...sometimes the harness chaffes on that bracket and you can have issues there...the ECT sensor pulls down that 5 volts through the sensor to ground and that voltage is read as your ECT...I'd stay away from Amazon or EBay sensors !!...the brown wire is called a "low reference" or a computer ground.

 

Buy several ECT sensors and measure the resistance across them when they are all at ambient temp. I have noticed all new sensors are not created equal even though they come from the same factory. Find one that is 50 to 100 ohms different than the one you have in your car and it will likely read closer to ambient temp when the engine is cold. I don't remember if more ohms is cooler or warmer. I assume you can google it or do the test yourself.

With all you have tested, it sure sounds like you just have a reporting issue. You could also use a cheap infrared temp gun to check the outlet of the radiator and a few other places to get a better picture of what is happening.

 

I did replace the sensor again, I used a Standard brand sensor from Oreilly, but it's doing the same thing. I used HPTuners VCM Scanner and the temp shows exactly what is displayed on the dash. I did do butt splices but on 3 other wires that I believe are not related to ECT, one black with white, one purple, and one pink. I don't see any chaffing on the wires at the rear of the fuel rail. I tested the Ohms across the sensor that I took out and the sensor that's in the car. The one out of the car read 1.5 kOhms, with ambient temp around 90F, and the one in the car rear 1.7 kOhms. That resistance seems too high. I used a Fluke 107 and I put it in the Ohm setting and put one lead on one pin of the connector and the other lead on the other pin.

Here are pics of the wires I fixed. Highlighted is the brown wire that goes to the ECT sensor, I tested continuity from this spot to the sensor to verify. You can see the butt connectors I did, and you can see I didn't have to do anything with the brown ECT wire. In the last pic you can see the only damage on the ECT wire is some light damage to the insulation.

 

At 90 degrees you should see around 2,674 ohms...is your Fluke accurate ??...if the sensor reads OK on the "resistance vs temp" chart then you may have a wiring issue...do you see 5 volts on the yellow wire ??...I'd also check pin 80 at the C1 connector for possible corrosion.

 

C5 Diag is on the right path. Ensure all your sensors/signals are reporting accurately first. The wiring already done will allow the wires to corrode again because they are not weather sealed. I do use the same butts, but I buy without the jackets or remove them and then run two layers of shrink wrap.

If the fans turn on at the right point that assures correct fan function and provides a secure baseline. From there, the question is going to shift to proper airflow across the radiator.

I didn't have a cam in mine, but I had a P-1sc car on full factory cooling system which never went over 196*F when setting the fans to come on at 175. It only went to 202 on a 100+ day in stop and go traffic on the highway where the air was clearly hotter coming off the heated pavement. So if you're having issues it is going to be a question of correcting function, not the cooling capacity of your setup which should exceed your needs unless you're living in death valley.

 

Why would you remove the jacket of a heat shrink butt splice to then cover it with heat shrink? The butt splices the OP used are the heat shrink type, he just did not finish the job...so the wires are open to the elements with no strain relief.

 

I've had issues with fluids following the wire in. So I use shrink with glue on it and run it a bit longer length and end up with splices which have not repeated the issue. I then put a second wrap over that but shorter. Needful? Maybe not. But as strong as a butt splice is I like the idea that I've done it so the wire is less likely to bend or break at the butt by supporting the wire in that location.

 

What you described is the exact purpose of a heat shrink splice. The ends of pink splice shown (translucent part) provide the seal and the strain relief. Depending on the brand of splice you choose, some will have a additional heat activated sealant. Sounds like you are making your own environment splices when there are numerous off the shelf solutions available.

 

Yup. But I've had those crappy ones fail. Which is why I do it by hand. None of those have failed. And so my evidence drives me on.

 

Kind of a self-fulfilling prophecy, right? Use crap materials and get a bad out come. It's almost like planning to fail.