My C5 has a supercharger with a bigger radiator. When the outside temp is below 80, the coolant temp is about 190~200, and I think that it is normal. When the outside temp is above 90 with stop & go traffic, the coolant temp goes up to 260, and I get a coolant overheating warning. What do I need to check? Do I need to check the fans, thermostat, water pump? Or do I need to add some coolant addictive, or oil addictive to lower the temp?

 

Which SC?

 

Be sure the fans come on at the appropriate temps. Not sure what they are, but someone here will. Also probably a good idea to blow out the fins in your radiator with compressed air. This should remove the years of tiny gunk that gets caught in there, and reduces airflow.

 

The supercharger should not even running when you are stuck in traffic, so there is no added pressure to your engine at all.
My guess is that your fans have either gone on the fritz or the programming for them has gone sideways and has them coming on at too high a temperature. Can you hear the fans coming on about 215 to 220 on the DIC? Correct programming should have them turn on then. If this is not the case they can be programmed to come on then or earlier depending on your particular needs.
You could also try adding the "water wetter" product that is supposed to carry heat away from the engine more efficiently.

 

Do the easiest first, check under the front bumper behind the radiator shroud for debris/bags/roadkill, etc that's been sucked up against the radiator and/or condenser.
Is your car an a/t or m/t? An a/t could be adding heat into your "bigger" radiator. A separate a/t transmission cooler might help.
What type of radiator? "Bigger" might not be sufficient if it's older and in need of internal cleaning. The fins may have debris blocking air flow.
Your fan(s) may need to be reprogrammed with a lower temperature opening thermostat as optional.
An engine oil cooler might help pull some heat away from the motor.
"Water or oil addictive" (sic)...you probably meant additives. You might consider using a lower viscosity full synthetic oil in the summer (5w30 is oem) if you're using anything different...changing out your antifreeze, or using a combination of distilled water and WATER WETTER instead of antifreeze might bring some summer relief.
You might consider having your tune checked for timing/fuel curves.

 

Fans are a likely suspect, based on what you've said. If the temp stays okay - below about 210F - when you're moving (should probably be below 200 unless it's a really hot day), the fans are probably it.

They are programmed to come on at something around 228F, then kick into high speed at about 235F. My car, driving in Los Angeles last summer in traffic, ran about 235 all the way up the San Diego Freeway through the San Fernando Valley, then dropped right back to 205 or so as I climbed the Grapevine out toward Bakersfield. A/C on and 70 MPH, up a 6% grade.

Listen for the fans to kick in. They should run when the A/C is on, at least if the car is stopped or moving slowly. I'd start out with that - listen for them with the A/C off at idle, then turn the A/C on. That will give you an idea what you're listening for. Then either drive it or cycle the RPMs through from idle to about 3000 RPM and back, with about 10-20 seconds at each step, until the DIC is showing in the 230 range (and the A/C is off). Did the fans kick on? If so, probably time to look at the rad itself. Start by using a soft brush to knock the surface bugs, leaves, stuff off the A/C condensor. Then use compressed air from the back side to blow debris out of the fins. You may be surprised - I sure was. Be sure to get all the way to the top.

If it still doesn't cool like it should, probably time to go see the radiator guy.

Oh, and do check the coolant level. As the air temperature rises, having the rad and surge tank full is critical. Open the cap on the surge tank, idle the car for a bit to circulate the coolant, fill to half an inch to an inch above the COLD FULL line, and see what happens.

I agree with the other voices - S/C shouldn't have an effect at low/idle speeds. Good luck.

 

So you have added a supercharger - what intercooler do you have? Are you now blocking airflow by placing the intercooler in front of the radiator? Overheating is a typical problem with front mount intercoolers. You may have to look into a water-to-air cooler that can be placed off to the side (behind the headlights).

 

Doug asked what supercharger since a positive displacement SC could affect engine temps even at idle. Also, I have a 25% underdrive pulley and with a low idle (850rpm) I have noticed some cooling issues since the water pump is spinning so slowly.

Fan temps and clogged radiator are good things to start with, but if those aren't the culprits...check if a higher idle (if you have underdrive) solves the issue.

 

Have you changed to a 160 deg thermostat? That will also aid in cooling. Like the above, the fans are suspect...check the temps when both fans kick on. I suspect like others the second fan is not activating.

 

It is procharger installed by A&A Corvette. Today outside temp was 88 and the coolant temp was about 190~195 in local street. I guess it is normal today. I checked the radiator fans and both fans are working. I think that the thermometer went bad. I bought 160 thermostat today, and I am going to install it tommorrow. I will post again after the installation.

A few days ago, I was on the freeway and the coolant temp was going up and stayed at around 245 for 20 minutes, then came down to 210 all of sudden on the freeway. So I am thinking that thermostat is not working properly.

 

Rapid fluctuations in coolant temperatures can also be attributed to air in the cooling system. Air pockets/steam within the motor/cylinder heads do not cool well.
In carburetor cars, we raise the front of the car up a few degrees and fill the cooling system with coolant, allowing the air bubbles to escape before pressurizing the system with the cap.

 

Today I installed 160 thermostat and add some water wetter to the water. The main issue was the thermostat. After the installation, I checked the old thermostat, and there was a lot of scratches. And the mechanic told me that my old thermostat was bad. Now my car is running around 185~195 range in coolant with the outside temp about 85. Before the installation, the range was about 225~240 and sometimes it was hitting 260. I definately recommand to change the thermostat.

 

Glad that you solved the problem. A bad thermostat can really mess up your engine! Remember that a 160 can go bad just as easily as a 180 or 195, so keep an eye on the temp gauge.