1. The proper level is about 1-2" below the cap. If it's losing a little more through the overflow than you think it should, it could be time for a new 15 lb. pressure cap. They do go bad and they cost less than $5 to replace. A 16 lb. cap might be all you can find easily, but it's 1 lb. more than the system was designed to work with. It will probably be fine, but I'd look for a 15 lb. cap. You can order an AC-Delco 15 lb. OEM cap at www.rockauto.com or www.gmpartsdirect.com or several other discount GM parts sites.CGGage said:
2. The temperature range you described is perfectly normal. The fans don't kick on until 225, so obviously that's not a temperature you need to worry about. If it gets to 240 during normal driving, it's time to get it checked out. In the meantime, I wouldn't worry about it too much.
3. Sounds like a mess. Don't flush the cooling system. Just drain and refill. If the system has had green stuff in it before, you have to change the coolant every 2 years for the rest of its life no matter which coolant you use or how long it's supposed to last. The green stuff leaves silicates in the system which reduce the lifetime of coolant. A flush won't get rid of them. To be safe, have the cooling system drained and refilled with a 50/50 mix of coolant (any color at this point, but DexCool is what's spec'd for your car) and distilled water. Don't use tap water, and don't add any flush chemicals. It's too hard to get that stuff out of the system and it's more trouble than it's worth. Two tubes of Bar's Leaks powder or 6 Bar's Leaks large tablets should be added to the system in the radiator hose, NOT IN THE SURGE TANK. So, while the system is drained, put the Bar's stuff in the hose, close it up, and fill with the mixed coolant.
4. The water pumps seem to have a tendency to leak at the bearing seal. Don't know why, but I've had the same problem. I can't explain why both the pump and radiator went at the same time, but I hope they got that black stuff out of the system. Whatever it is, it doesn't sound good at all.
5. Have you checked for any codes? Does it run roughly at any other time?
If you're worried about a head gasket problem or start to have symptoms that indicate that, have the cylinders pressure tested and/or check for exhaust gases in the coolant. A Northstar won't dump coolant into the oil. I suggest using the Search feature above to look for more information about this stuff. Proper cooling system maintenance is critical on a Northstar and there is lots of information available here about how to do it right. Let us know what questions you have beyond that.