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.
We have a 1998 Marquis with 4.6L engine. This car has sat for 10 years. Initially the intake cracked and my wife (I didn't know her then) knew enough to shut it down when it began to overheat. Took it to a shop and they confirmed the intake was cracked. Fast-forward to 2009. My wife moved from Houston, Tx here to Missouri so we could start a life together. I found a retrofit-kit on Ebay for $200. We installed it and then it wouldn't get fuel. I found the pump wasn't activating at all so I replaced that sucker. Then I got hurt and couldn't do much on it so it sat. Yesterday, I had the fuel lines cleaned out and finally got it started and it runs a little rough, but no overheating but upon checking the coolant in the reservoir it has the same muddy clay-like residue. I'm thinking the whole thing needs to be flushed and replenished.
I bought a Prestone Flush N Fill kit only to find out that the radiator has no radiator cap, just the coolant bottle. So I pulled the coolant tank and washed it out really well. then I opened the petcock and ran water through just the hose to the radiator until it came out clean.
By the sounds of it here, I need to pull other lower hose and flush it again perhaps before I do anything else. Then remove the thermostat and use a coolant system cleaner through it. Can I just pour it in the coolant tank and same with adding the water for the flush?
Then what? Undo the lower hose again and the two heater hoses? I'm assuming this is so the block has the best chance of emptying without removing the soft plugs?
Then what? Do I need to hook the hoses up and re-flush with clear water?
The hoses themselves are pliable and seem to be free of cracks and not in bad condition.
Next thing is drain the oil and replace it and the filter.
Thanks folks! I thought it best to revive a thread rather than create a new one.