GMC 2003 C4C042 Owner's Manual page 264

To drain water, do the following:
1. Stop and park the vehicle in a safe place. Turn off
the engine and apply the parking brake.
2. Remove the fuel cap.
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3. Place a fuel resistant container under the fuel filter.
The filter drain valve is located on the bottom of the
fuel filter. The fuel filter is located in the engine
compartment on the passenger's side of the vehicle,
or on the driver's side frame rail near the engine.
6-10
4. Open the drain valve by
turning two to three
turns. When fuel
empties from the valve,
all the water has been
drained. Close the valve
hand tight.
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5. Remove the fuel resistant container and properly
dispose of the contaminated fuel. To find out how to
properly dispose of contaminated diesel fuel, see
"What to Do with Used Oil" in the "Engine Oil
(Diesel Engines)" part, later in this section.
6. Install the fuel cap.
7. Start the engine and let it run for a few minutes.
If the engine stalls, the fuel system may need to be
primed. While draining the water from the fuel filter,
air may enter the fuel system. If air has entered the
fuel system the fuel system will need to be primed.
Refer to "Fuel Priming" later in this section.
If the water in fuel indicator comes on again after
driving a short distance or the engine runs rough or
stalls, a large amount of water has probably been
pumped into the fuel tank. The fuel tank should
be purged.
Fuel Priming
In order for the DURAMAX diesel engine fuel system
to work properly, the fuel lines must be full of fuel and
contain no air. If air gets into the fuel lines, it will be
necessary to prime the fuel lines to eliminate air before
operating the vehicle.