GMC C-Series 2001 Owner's Manual page 208

Water in Fuel
Sometimes, water can be pumped into your fuel tank
along with your diesel fuel. This can happen if a service
station doesn't regularly inspect and clean its fuel tanks,
or if it gets contaminated fuel from its suppliers.
If this happens, water must be drained. Your dealer can
show you how to do this.
CAUTION:
Diesel fuel containing water is still flammable.
You could be burned. If you ever try to drain
water from your fuel, keep sparks, flames and
smoking materials away from the mixture.
6-6
NOTICE:
If there is water in your diesel fuel and the
weather is warm or humid, fungus and bacteria
can grow in the fuel. They can damage your fuel
system. You'll need a diesel fuel biocide to
sterilize your fuel system. Your dealer can advise
you if you ever need this.
If your fuel tank needs to be purged to remove
water, see your dealer or a qualified technician.
Improper purging can damage your fuel system.
Diesel engine vehicles that stand unused for several days
or weeks can develop water in the fuel from
condensation. It helps to fill the fuel tank if the diesel
engine vehicle will not be used for a time.