I had a weak heater core, both lines were hot, and it blew luke warm air also. I flushed it out and nothing happened. But I recently flushed it, put some simple green in the core for a few minutes, and then blew it would with compressed air. It puked BIG TIME all over me and the front of the jeep. Nasty brown stuff... I did that process about 2-3 times, and then flushed it out with the hose for 5 mins. Now it works great. Don't forget to use the hose, and air compressor blowing both ways.