01 LS with no heat

Hello all, I am new to this site. I have read alot of posts but havent been able to find anything that sounds like my problem yet so I figured I'd give it a try. I have an 01 LS V8 with no heat. I have warm air that comes out of the vents when the car is at idle but when you start driving within a mile or so it goes back to blowing out cold air. All 3 of the lines to and from the heater core are hot, the bottom one is hotter than the other 2 but the other 2 are hot. I recently had to replace my degas bottle because it was cracked (I did'nt have heat before I had to replace the bottle) and have bled the system numerous times hoping this could be the problem. I have tried to run a scan on the control panel but I do'nt get any errors. I've tried doing it two ways that I've read on this site. Off and defrost together and then the auto button within two seconds and off and floor button followed by the auto button. One way makes everything on the panel ight up and the other way does a count down from 1-25 on the drivers display and 25-1 on the passengers display and at the end everything lights up. Also I have noticed that it takes a while for the temp gauge to get to operating temp, like somtimes close to 30 miles and after it gets to operating temp if I try to turn the heat on it will drop down to like 1/4 on the temp gauge and takes forever to get back up to operating temp again and has trouble maintaining operating temp. Sorry for the book but I wanted to list as much as possible to help you guys help me. Thanks in advance for any and all help!!!

Do you live in an extremely cold region? If won't reach normal operating temp then check the thermostat. What is the operating temp in degrees F? Another thought is to check the cylinder head temp sensor/coolant temp sensor.

To enter diagnostics my info say interior temp between 32 and 90 degrees F. hit,Off and floor then auto to access codes. If no codes, diagnose by symptom.

Some of the national brand auto stores may check codes for free.

I dont live in an extremely cold area. I live in Maryland. It was like 34-36 out this morning on my way to work. I thought about trying the thermostat but my coolant lines have a lot of pressure on them so it seems to be working, also if you leave the degas cap off you can see when it opens because you can see the coolant flow inside of it. Ive been outside for the last 2 hrs going through the bleeding process and it seems to have more heat coming out of it but I guess the test will be when I go home this evening. I will post if this has fixed or not.

Your thermostat is bad (if it weren't, the gauge wouldn't drop when you turn the heat on), and probably your plastic thermostat housing is cracked/cracking. Having pressure is no indication of a correctly working thermostat.