Are you talking about that black plastic fitting on the left end of the engine with 4 hoses and 2 sensors connected to it? If it is that, it is fairly easy to replace. I replaced mine in about half an hour. There is a large diameter rubber hose you need to disconnect at the firewall end, and you need to remove the reservoir.
Dexcool gumming up is not a myth. I have personally replaced the intake manifold gaskets on a couple dozen cars with Dexkill in them, and I saw what was in there. I have also pulled the heads off a few engines that overheated no matter what, and found the coolant jackets around the cylinders plugged up with solidified Dexkill. Dex anything is bad for your car. There was a time when following the owners manual was a good idea. But not anymore. The owners manual is written by the EPA, not the manufacturer. I just changed the oil in my 1.4 Sonic today after 3000 miles. It was black. I don't mean just dark, but black. And I had used full synthetic. I poured some in a clear glass jar, and held a light behind it. I could not see the light through the oil. I have never seen almost clear oil turn black in 3000 miles before. The oil in my '64 Fairlane looks fairly clean after 3000 miles, and it burns a qt of oil in 200 miles.
I would suggest flushing the system really good (it took me almost a week and 10-12 gallons of distilled water) I replaced the Dexkill with Prestone OAT, the same type of coolant that Dexkill is, but it doesn't gum up.
I think I may have finally found the reason why GM is using this horrible stuff. It seems they cannot make an engine that holds coolant anymore, and they are adding whatever it is to the Dexkill that makes it gum up to help stop leaks. It gums up immediately on contact with air. I found huge globs of what looked like sticky way on the oil filter cover on my Sonic. Turned out the outlet pipe had been leaking, and dripping down on the cover, where it solidified. It wasn't until I put the Prestone in it that I found a pinhole leak, and replaced the whole assembly. There is no way to keep air out of the cooling system. It sucks in air when the engine cools off, the pressure cap opens, and it sucks coolant back into the engine. It also sucks in air to replace the missing coolant.
I bought my Sonic used, from a family member that needed to sell it. I will never again buy another GM car. And I used to be a Chevy guy through and through. I still drag race a Chevy, but it is an old school cast iron V8 and I use Engine Ice in it. No leaks.
