I think the heater core has gone bad, but I don't have coolant on the floorboard...
 

My heater is not really heating up at all, but I don't see any coolant accumulating on the passenger side floorboard like in my last f-body when the heater core on it went out. That was a 95 TA and this one is a 95z28. I can't find any coolant leaks under the hood, but my low coolant light keeps coming on and I keep topping it off. Whats going on here?

The fact the heater core is not producing heat does not mean it sprung a leak. It may simply mean its plugged up - a very common problem. The core can be flushed by pulling the lines at the water pump and pushing water through them with a garden hose. Loss could be from some other point.

Thanks. How can I tell where the leak is coming from?

You could get a pressure tester, pressurize the system and look for leaks. Or just get it hot and look for leaks. Check the bottom radiator support. Coolant might be pooling there. Check all the hoses. Check the water pump weep holes. Check the coolant reservoir to see if it is over-filled. Might indicate a problem with the overflow hose. When it gets a hole or crack in it, coolant flows out of the radiator into the reservoir as the coolant heats up and expands, but when the coolant cools off, is can't pull a vacuum to refill the radiator from the reservoir.

Lots of possibilities.... you have to work at it.

Which way should I flush the system? Which water pump hose?

When I did this on my Caprice a few years back, I flowed both directions several times to clear the crap out.

Thanks. At first I thought there were just the two large diameter hoses at the top of the pump, but I noticed there are a few more below those. Am I just supposed to use the top two hoses for flushing? If the diameter of the hoses are larger than the diameter of the garden hose, how would that create enough pressure to flush the system?

Prestone used to sell a kit that you splice onto one of the heater hoses and attach a garden hose to back flush the system. It comes with a cap so you can close it when not in use, or if you don't like how it looks, just purchase an extra hose and swap it in whenever you want to flush the system.

Most service shops can also do this for you. I believe the last time I had it done it was about $150 and included replacing the coolant.

A lot of answers can be found on Shoebox's site:

http://shbox.com/1/4th_gen_tech2.html#heaterflush

Thanks, but which hoses do I run water through? I see more than 2.

You're trying to flush the heater core, so you have to use the hoses that run to and from the heater. As above, Shoebox has the answers:

http://shbox.com/1/heater_hoses.jpg

I think it worked. I do have more heat than I had before. There is one thing though: I had the radiator cap off with the engine running, and it never started flowing through so I could add more. It just continued rising and started overflowing.

Update: It heated for a little while, but its not working anymore. Is it possible that I didn't flush it with enough pressure?