Chevrolet 2011 Volt Owner's Manual page 389

If coolant is needed, add the proper
DEX-COOL coolant mixture at the
coolant surge tank.
The coolant surge tank pressure
cap can be removed when the
cooling system, including the
surge tank pressure cap and upper
radiator hose, is no longer hot.
1. Turn the pressure cap slowly
counterclockwise. If a hiss is
heard, wait for that to stop.
A hiss means there is still
some pressure left.
2. Keep turning the cap and
remove it.
3. Fill the coolant surge tank with
the proper mixture to the cold
fill line.
4. Replace the pressure cap.
Be sure the pressure cap is
hand tight and fully seated.
The vehicle has an indicator to warn
of engine overheating.
If the decision is made not to
lift the hood when this warning
appears, get service help right
away. See
.
If the decision is made to lift the
hood, make sure the vehicle is
parked on a level surface.
Then check to see if the engine
cooling fans are running. If the
engine is overheating, the fans
should be running. If they are not,
do not continue to run the vehicle
and have the vehicle serviced.