When I got my '67 Newport back in 1981, I suspected the heater core might have some accumulation in it. It worked decently well, but that's all. I could feel different temps in the two heater hoses.
So I removed the hoses from the valve on the outside of the firewall, then using lower water pressure put water into the core, the correct path. I got some accumulation out of that, then it ran clean. So, I decided to do the "reverse ffffflllush" deal. Same low pressure. According to a fictional book I read in 8th grade, the old radiator shop guy said that was the best way to get things clean. Initially, some more accumulation came out . . . then it slowed to a drizzle. Put the water into the input nipple. Similar drizzle. Yikes! So I put things back together, put coolant in it, and that's still how it is. I figured that time might get it opened back up, but it didn't. That was when I was driving it pretty often, too.
If I'd stopped with just the first "input" water, things would probably have been better, but not as good as they might have been. Now, not so much. Someday, it'll probably get the core re-done or replaced. That's in the future, somewhere.
The matter of "clogging" can be easily determined. When fully operational, the two heater hoses should be about the same temp with the valve fully opened. You can tell that just by grabbing them individually. Of course, the thermostat needs to be operating correctly, opening at 180 degrees F.
I know the reverse flush is the best way to get things cleaned out, but it can also be the best way to force things "backward" and clog things up, too, as I found out. Better to have a partial clog than "more clogged".
When we replaced the heater core on the '66 Newport (w/factory air cond), back in the middle '70s, the original '66 C-body part number superceded into the part number for a '70 Charger item. That's the part number the dealer ordered and it worked. Still in there. FWIW
CBODY67
