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Starting in 2003, GM discontinued the use of a drain hose attached
to the radiator drain valve. Later, GM discontinued the use of a
radiator drain valve altogether. Some models continued to include a
drain valve, but the valve was permanently sealed during production. Do
NOT try and operate these sealed valves-they will break. GM warns that
if you try to remove and replace the valve, debris from the breakage may
circulate throughout the system and damage water pump, heater core, and
The bottom line is that you must now either remove the lower radiator
hose or use a siphon to remove coolant from the radiator. This bulletin
applies to all 1998-04 GM cars and trucks.
Open the bonnet of your 1998 GMC Sonoma, look under the radiator, you will see a hose/pipe coming out of the radiator going into the engine of the 1998 GMC Sonoma. Remove the hose and the coolant will drain.
You will see a screw holding the hose to the engine, get a screw driver to loosen the screw so that the hose can be easily removed.
After draining, pour more water into the radiator to flush out the coolant entirely from the radiator.
Some models do not have a petcock, pulling the lower radiator hose will suffice in drainage. I suspect you know what you are doing since you have the correct nomenclature. Use extreme caution during this type of draining procedure when coolant is hot. If it is not on the lower left or right corners of radiator side facing engine, it is not equipped with a drain valve.
Sorry but most newer radiators don't have a petcock. I just changed mine by removing the lower radiator hose.Most newer cars only hold a gallon or so of antifreeze anyway. If you have five year coolant (dexcool) DO NOT MIX WITH ANY OTHER BRAND regardless of what the bottles tell you. If you mix red coolant with green it will turn into a radiator and heater core clogging glop.