GMC 2009 Sierra - Manual (Page 345)

2. Then keep turning the pressure cap slowly,
and remove it.
3. Fill the coolant surge tank with the proper mixture,
to the FULL COLD mark.
4. With the coolant surge tank pressure cap off,
start the engine and let it run until the engine
coolant temperature gage indicates approximately
195°F (90°C).
By this time, the coolant level inside the coolant
surge tank could be lower. If the level is lower, add
more of the proper mixture to the coolant surge
tank until the level reaches the FULL COLD mark.
5. Then replace the pressure cap. Be sure the
pressure cap is hand-tight and fully seated.
6. Verify coolant level after the engine is shut off
and the coolant is cold. If necessary, repeat
coolant fill procedure Steps 1 through 6.
Engine Fan Noise
Your vehicle has electric cooling fans. You might hear
the fans spinning at low speed during most everyday
driving. The fans can turn off if no cooling is required.
Under heavy vehicle loading, trailer towing, and/or
high outside temperatures, or if you are operating
the air conditioning system, the fans can change to
high speed and you might hear an increase in fan noise.
This is normal and indicates that the cooling system
is functioning properly. The fans change to low speed
when additional cooling is no longer required.
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