Im workinf on 04 e350 its a bus rear heat is wroking ok all the rear is hooked with is power ground and heater hose not ford unit the front blows out luke warm if i rev it up ot geting hotter im thinking maybe heater core is clogged up? anyone else have seen this before the upper hose is really hot the lower hose and rad are ice cold

 

That would be my first guess. Disconnect the hoses and try flushing the core to see what comes out. jim

 

Has this condition existed all along or just newly discovered? Does the factory cabin heater core work as it should?

I'd find a way to measure line temps at the rear core both in and out. Flushing the core isn't a bad idea, can't hurt making sure its clean.

Increasing temps that follow engine RPM's could be restricted flow, low coolant or simply the way the lines have been connected to and routed to the engine cooling system.

With the proper thermostat my '03 with after market rear heater tends to put out enough heat even at an idle, both heaters working at the same time.