The heat is very weak in my 2001 Audi A4 2.8 Quattro. I have read that this can be caused by a partial block in the heater core (I guess that this is a fairly common problem.) Many people flush the core with CLR.
I have tried running distilled water through it with a drill pump and I have used an air compressor with the regulator cranked down to a fairly low pressure. Nothing really came out. I was thinking about using some sort of detergent. Any recommendations?