Thanks for all the help on this problem, but we're not outta the woods yet.
My buddies 1990 heater problem is still unresolved. He has check all of the things that you guys have mentioned, to no avail. He is looking for the heater water valve that controls the water flow to the core. Any ideas where this thing is located on a C4? I have a C3 ('80) and C5 so I'm no help...
TIA Pappy