I live In Massachusetts and it's VERY cold right now. The C5' temp goes past 220° as much as 2 lines past. Not to the danger zone but it so cold here that I'm concerned the first warm day is going to turn into an overheating issue. I'm thinking it's a sticking T-stat but idk if someone here knows differently. Also it's. Six speed if you.hang in a gear longer than normal the temp.will just peak over the second line past 220

 

On a cold winter day and a stock vehicle you will be around 192 degrees. I had my mine out on a 20 degree day and stopped to check the oil after a 10 mile drive. The lower radiator hose was still cool to the touch.

Some diagnostic questions:
What is the temperature reading from your DIC?
Have you checked your coolant level?
When was the last time you thought the cooling system was working correctly? Have you done any work or had any work done since then?

 

Under what conditions does the temperature rise, are you in city traffic or out on the highway? Important to know under what conditions this is happening.

 

If you haven't tried cleaning the radiator and AC, start there. C5s are prone to sucking random crap off the road and into the radiator.

Search for "radiator genie" for a hose attachment that I'm told works well.

 

Very slow driving seems to be the trigger but once I'm cruising the temp doesn't seem to drop all that much. Maybe slightly. One other thing I've noticed there have been a few occasions when the temps will abruptly drop back down to 220. (dead center of the temp gauge) this is partly why I'm thinking it's a sticking T-stat.

 

So I bought this car 2 weeks ago and haven't had much time to drive it or get to know the car too much because we got a blizzard days after I got it.... I have had 2 Trans Ams, and a Firehawk, that were all LS1 powered. As well as a 2001 Corvette that I had bought new. I never remember any of them.running.a temp above centerline of the guage. So 220°.. Now I know the next question. And yes the car does have a warranty. I will be bringing it to them to look at. But I like to diagnose on my own and if it's really that simple I will fix it myself. I don't like dealerships taking my cars after my father had his Viper abused by a dealership back in the late 90's.

 

Simple explanation of the C5 cooling system, below 35mph cooling fans turn on at low speed at a certain temperature, then they switch to high speed at a higher temperature set point. Fans are turned off once the car exceeds 35mph as a powered fan will restrict air flow. You might start by simply replacing the coolant tank pressure cap, and cleaning the radiator with a water wand, C5's are notorious for sucking up road debris and clogging up the air passanges in the radiator, and if at all possible do not take it in especially to a stealership. Good luck

 

Rather than look at the needle on your temperature gauge, what are you reading on your DIC? On a cold winter day you really should see very little change in temperature and it should be around 192 degrees on a stock cooling system. Have you checked the basics - coolant level at the full mark? Squeezed the radiator hoses on a warm engine and see if they feel pressurized? Does your car have the air dam in place?

 

Temp as per DIC 228°, Plenty of coolant, upper radiator does not feel pressurized at all when engine is warmed up. Air dam is gone but I have ordered one.... The air dam would explain why it doesn't cool down at speed but doesn't explain anything else!

 

At 228 degrees, your radiator hoses and heater hoses should feel quite hot and pressurized. Are the heater hoses and radiator hoses hot? You wrote that the radiator hose did not feel pressurized. This suggests a few possibilities:
Air in the system. By any chance do you hear a gurgling sound from the heater?
Bad pressure cap or cracked coolant tank but this would not explain high temperature and temperature swings. It would only explain no pressure.
Blown head gasket but I would think you would see high pressure with your temperature.
Bad coolant temperture sensor but I dont know if they fail in the temperature swing way you describe.

 

The hose was hot, but not fire hot. I literally put a thin Rubbermaid janitor style glove on and grabbed the hose. It would have been uncomfortable to grab barehanded. I could effortlessly squeeze it flat. And I could hear a rush of coolant into the overflow. I haven't noticed any gurgling sounds from the heater. The heat does work like a champ though. Sooo can I open the coolant reservoir while at idle and watch for bubbles to determine a bad head gasket orrrrr is that a no fly zone with the LS motors? I know that's an old school trick!

 

Combustion or overheating bubbles would be easy to see with the cap on and a light illuminating the tank. They would come right up at the "T" connector at the bottom of the tank. There is baffling under the cap that would make it hard to see.

Regarding head gasket, if it were blowing combustion pressure into the cooling system, the pressure would rise fast. You can also purchase an inexpensive chemical tester to check for combustion to cooling. When combustion blows into the coolant you tend to get a lot of air in the heater core and you state your heat works great.

You can use an inexpensive handheld temp gun to test both sides of the thermostat and the other hoses. That will give you a lot of information as to what is really happening in the system. This is how I found a sticking thermostat a few years back. They can be found for under $20 on amazon.

If you are not losing coolant, I would probably put a thermostat in it if you don't know the last time it was changed. It's a quick and inexpensive maintenence item.

If you do not have one you can free rent a cooling system pressure tester. C5s need an adapter to the tank. This will help you find small leaks.

Also - until the air dam is in place, you will not truly be able to verify overheating at speed.