Date registered: Aug 2002
Vehicle: '85 2.3-16 '99 C280 '11 GLK350
Location: Scottsdale, Arizona, USA
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Check the temps of the supply and return lines to the core, particularly the return line. The supply line runs on the exhaust side of the engine bay, aided by a small electric aux pump; is regulated by the heater valve; flows through the heater core, then returns to the engine via a return line on the intake side of the engine.
If the return line isn't carrying warm/hot coolant back to the engine, then you've got some interruption/blockage in the system. Could be at the aux pump; heater valve, or inside the core. Many heater cores were bypassed (blocked at the valve) to cure leaking heater cores rather than spending the money to remove the dash to fix it.