Re: How do I flush the radiator in a 1999 Grand Marquis?
That's common if the radiator and block are not flushed every 4-5 years.
It's probably time to also replace all hoses.
Remove the lower radiator hose to drain most of the coolant. Next, remove the thermostat housing and thermostat, but save the "O"ring. As dirty as the system is, you will need to use a radiator flush.
Reinstall the lower hose temporarily, pour the flush into the block and fill the block with tap water. Then temporarily install the thermostat housing using the old "O"ring. Start the engine and maintain the proper water level. Let the flush circulate per the instructions. Watch the engine temp in case you have air in the cooling system.
Now remove all hoses, including heater hoses and drain the block. I would also remove the expansion tank and clean it as well as possible.
Reinstall all hoses, but fill the block and tank with distilled water and coolant, or pre-mixed coolant, prior to installing the new thermostat and "O" ring. Carefully check the coolant level as the engine warms and the thermostat opens, and fill as necessary. Drive a short distance while monitoring the temp gauge, then recheck the coolant level.