In an emergency I had to add regular (green) coolant to my dex-cool system. Now I have this nasty brown sludge. I assume I need to flush the cooling system. Is there any type of flushing agent that will clean this out better than another?

Well, a 50-50 solution of any good green AF is okay, unless you have the dex-cool, orange colored, GM brand type of AF... If you have that, you want to use orange again. You cannot mix dex-cool with regular green AF (like Prestone or any brand AF that is green colored). A lot of people don't like using dex-cool so they change to regular AF. But to do so, you need to flush out every trace of the dex-cool. If they get mixed, problems like sludge in the coolant may result.

Looks like mud ? What type coolant have you added ? Take the radiator cap off an look into it , do you see the same ? Have you ever had the cooling system flushed ? There was a problem with people adding green coolant to GM - Dex-cool ,orange ! Clog's the passages in radiator ,inside the engine , heater cores etc... Pull the oil fill cap on the valve cover an look at the under side of the cap , do you see brown frothy looking goop ? Coolant getting into the oil ! Why it called DexKILL

The brown sludge you're seeing is the result of GM/dealers/repair shops installing cooling system sealer tabs in the cooling system. This sludge is NOT related to the use of dex-cool coolant. The cooling system sealer tabs are brown in color, and resemble dirt after they are crushed and poured into the cooling system, and pretty much turn into nasty brown mud in the cooling system. I have seen this on many vehicles. The problem you are having with your heater is most likely related to the sludge you are also seeing in the radiator. The sealer finds its way into the heater core and plugs it up, resulting in cool air instead of hot air. The best way to clean it out is to remove the heater hoses from the heater core and back flush the heater core with a garden hose. You will see a lot of nasty brown junk come out of the heater core when you do this. I have found that cooling flushes that dealers and independent shops sell are not effective at completely removing this sealer, and may have to be done multiple years in a row to get most of it out. Even then, there may still be sealer lodged in the system.
Your description of the heating and air system defaulting to the defrost and floor positions tells me that there may be a vacuum leak in the vacuum supply hose between the engine and the interior control (HVAC Programmer, if I recall correctly). The default position is defrost when the system loses vacuum. You may want to have your local shop check that and make sure you're getting vacuum to the HVAC controls.

they say you cant mix different coolants but you can go to auto-zone etc, and buy a cheaper anti-freeze for general motors products. its a 50/50 mix for all gm cars and trucks.you can add this coolant to any brand of anti-freeze. its called universal type anti-freeze. to run a differant anti-freeze in it, you have to flush out complete engine and radiator because the dex-cool and the green stuff do not mix well and cause engine corrosion etc. dex-cool is reccomended for all chevy engines. good-day !

Is your oil level low? Do you have an oil cooler going to the radiator? If there is then its possble the cooler is leaking inside the radiator.

Does the cooling system still use Dex-Cool? The S trucks are one of the models that seem to have problems with the Dex-Cool. People would usually see a dark brown mud basically in their cooling system.
If it still uses Dex-Cool then I would sugest draining and flushing the cooling system (including a good flushing of the heater core. Remove the hoses and force water through with a garden hose) and refilling with another coolant/antifreeze.

check for a bad blend door or actuator in heater. if bad it will not direct the heat were its called for. good-day !

rad/cooling system not serviced.dex cool will sludge up.entire ststem needs to be flushed
adding regular anti freeze to a dexcool system will also do this

If it has dex-cool in it now then yes add dex-cool to it. So if the antifreeze is orange add dex-cool and if its green add the green antifreeze. It won't hurt it to switch the antifreeze but if you are topping it off, add what is already in it.

you are probably thinking of dex-cool. it is what GM uses. if you are having to add coolant on a regular basis you should entertain the idea of having the entire system flushed and switch to regular green anti-freeze cause the dex-cool eats up the gaskets in the engine and they break down over time and you will end up with a slow leak that may end up in your engine. i just had to replace the gaskets on my 02 pontiac and switched coolants as well. hope this helps

Most likely Dex-cool, the nasty orange engine eating stuff of lawsuits (try googling dex cool lawsuit for some background.) If the cap on the overflow / expansion tank has orange lettering, it came from the factory with Dex-cool (it will also tell you so.) If it has green lettering on that cap, or doesn't specify, then the old style green stuff is the order of the day.

To make your life easier, there is coolant available which will mix with every other kind, and it only costs a dollar or so additional at the parts store.

If you have Dex-cool, open your radiator cap - the actual one on the radiator, not the overflow - and look for rust or something that looks like Satan's idea of Jell-o. If you see either of those, have your cooling system flushed by a mechanic - it will be money well spent.

Personally, I use the "mix with everything" or G-05 type coolant in my vehicles. Any that had dex-cool or even the old green stuff got the new stuff at the first opportunity. It is long lasting (good for about five years) and tolerant of crunchy water, and will not harm your engine, no matter who built it.