I don't usually have my DIC set to coolant temp but in the last couple weeks I have. I've noticed the temp climbs very fast sitting still. Steady city travel it sits at 196-198. I come to a red light and in the time it take the light to go from red to green (approx 2 minutes) the temp goes from 198 to 214-217. Soon as I get going it immediately starts dropping back down to 198. I know those are not dangerous temps but it seems like a 20º swing in 2 minutes is a little extreme. Or am I wrong and this is totally normal?

 

Normal. Most cars have a buffered coolant temperature gauge, meaning needle is parked at the same spot between 190ºF and 235ºF or so. On the Vette, you can actually see exactly what it is. Remember the fan doesn't kick in on high (about 90% of max) until 235ºF, I believe.

 

Mine won't usually swing quite that much in 2 minutes - up to about 205 to 208, but yours is not enough to worry about.

 

Ours does the same thing, perhaps not as rapidly but I never timed it.

A more important question would be:
If you stay parked, how high does the temp get?
If it stops below 220, then probably nothing to worry about. But if it gets to 230 and still climbing, then ya gotta wonder...

I don't think our has ever gone above 220, even stuck in traffic at 90'.

 

The highest I've seen is 218º stuck in heavy traffic and over 100º outside. I don't think I've ever sat still for more than 5 minutes.

 

Come to Chicago this summer, that'll give your cooling system a good workout standing still in traffic.

 

In hot weather, sitting in traffic for a long time I could see the temps going into the 230s.

The engine overheating warning will not come on until engine temp hits 256 degrees. Here is the FSM description of how the system works and at what temps the fans turn on and go to high speed. The engine isn't a 1960s engine and does run hotter than ones from 30 to 50 years ago.

Cooling Fan Control - Variable Speed Single Fan System
The engine cooling fan is a variable speed fan. The engine control module (ECM) controls the fan speed by sending a pulse width modulated signal to the cooling fan control module. The cooling fan control module varies the voltage drop across the cooling fan motor in relation to the pulse width modulated signal.

Cooling fan speed is effected by many different conditions and can be adjusted from 10 percent to 90 percent duty cycle (PWM), 90 percent is considered high speed fan. When multiple cooling fan speed requests are received the ECM uses the highest cooling fan speed of all the requests. The ECM commands the cooling fan ON under the following conditions:

• Cooling fan duty cycle starts when engine coolant temperature reaches approximately 95°C (204°F) and reaches high speed at temperatures above 113°C (235°F).

• Cooling fan duty cycle starts when A/C pressure reaches approximately 1100 kPa (160 psi) and reaches high speed at A/C pressures above 2480 kPa (360 psi).

• At engine oil temperatures above approximately 150°C (302°F) the cooling fan duty cycle will be commanded to high speed.

• At transmission oil temperatures above approximately 132°C (270°F) the cooling fan duty cycle will be commanded to high speed.

• After the vehicle is shut OFF if the engine coolant temperature at key-off is greater than 113°C (235°F) or the A/C pressure is greater than 1720 kPa (249 psi) the cooling fan duty cycle is set to 50 percent, low speed. If the coolant temperature drops below 110°C (230°F) and the A/C pressure drops below 1660 kPa (241 psi) the fan will shut OFF. The fans will automatically shut OFF after 2 min. regardless of coolant temperature.

As you can see as the temperature and loads on the engine vary the cooling fan will cycle on and off and the digital gauge which has much more resolution than the analog gauge will show you the temperature transitions in more detail.

Bill

 

Good info Bill, thanks. I'm going to check to make sure my fan is coming on at 204º.

 

Excellent summary as usual Bill! My '06 LS2 is prone to heating up pretty quickly in Houston summers and stop/go traffic. I have a 160 Tstat as well as underdrive crank pulley and Vararam setup, so with A/C on, it it pretty challenged to keep the coolant moving. What I did to help with this issue is to reprogram my fan speed profile in HPTuners to bring on higher duty cycle at lower temps to assist. Helps quite a bit, but won't overcome effects of hot roads and limited cooling circulation. Like yours, mine drops quickly to 185 when I begin driving again. Top temps don't usually exceed 220 so I don't worry about it. Only other way to keep the temp down a bit more would be to install an electric water pump rather than engine driven.. Not worth the expense in my setup.