No Heat From Heater When Moving
 

I've got an 84' 380sl. Problem is that when at idle, heater seems to work at least partially, yet when I start moving, it quickly stops providing heat. Is this a valve or pump issue? The car had AC removed when I got it; the fan and defroster work fine (I know the defrost is electrical). How should I proceed in troubleshooting this? Many thanks in advance for any input.
JEFF

erratic heat
 

could be from plugged heater core, low coolant or control issues.

tom w

No Go Heater
 

What is the best way to check the heater core? Also, which direction is the flow of coolant, looking at the engine from the grill. I assume it would enter the core from the passenger side since that is where the pump is...? Do I need to drain the system to check for blockage in the core? Many thanks, Jeff