Chevrolet 2006 HHR Owner's Manual page 275

If no coolant is visible in the coolant recovery tank,
add coolant at the pressure cap as follows:
CAUTION:
{
Steam and scalding liquids from a hot cooling
system can blow out and burn you badly.
They are under pressure, and if you turn the
pressure cap — even a little — they can come
out at high speed. Never turn the cap when
the cooling system, including the pressure
cap, is hot. Wait for the cooling system and
pressure cap to cool if you ever have to turn
the pressure cap.
CAUTION:
{
Adding only plain water to your cooling
system can be dangerous. Plain water, or
some other liquid such as alcohol, can boil
before the proper coolant mixture will. Your
vehicle's coolant warning system is set for the
proper coolant mixture. With plain water or the
wrong mixture, your engine could get too hot
but you would not get the overheat warning.
Your engine could catch fire and you or others
could be burned. Use a 50/50 mixture of clean,
drinkable water and DEX-COOL
Notice: In cold weather, water can freeze and crack
the engine, radiator, heater core and other parts.
Use the recommended coolant and the proper
coolant mixture.
®
coolant.
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