Pontiac 1999 Montana Owner's Manual page 285

If the coolant inside the coolant recovery tank is boiling,
don't do anything else until it cools down.
The coolant level should be at or above the FULL
COLD mark. If it isn't, you may have a leak in the
radiator hoses, heater hoses, radiator, water pump or
somewhere else in the cooling system.
CAUTION:
Heater and radiator hoses, and other engine
parts, can be very hot. Don't touch them. If you
do, you can be burned.
Don't run the engine if there is a leak. If you run
the engine, it could lose all coolant. That could
cause an engine fire, and you could be burned.
Get any leak fixed before you drive the vehicle.
NOTICE:
Engine damage from running your engine
without coolant isn't covered by your warranty.
See "Overheated Engine Protection Operating
Mode" in the Index.
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