2001 Ex just replaced injector cups, but still smell fuel in the rear heater core

[mws]

Newbie to the forum. Newbie to the 7.3 PSD. Just bought the 2001 7.3L Ex used and it had gallons of fuel in the coolant. Had the cups replaced by a reputable guy and he flushed most of the fuel/coolant from the system. I am working on the remaining fuel scum which shows up in the coolant reservoir after a long drive.

The heated fuel/coolant mix always smells strong, even with the cap screwed on tight. It is pretty amazing how it permeates. No, it's not fuel filter bowl O-rings, those were replaced and not leaking now. The top of the engine was pressure-washed and is dry as a bone.

The question is: have any of you smelled the fuel/coolant mix in the rear compartment where the rear A/C HVAC system is. I do, but only when the engine has been run a long time. It seems to be permeating the smell from the hose ends, or perhaps, might even be weeping at the heater core.

Looking to see if you had any success dumping and cleaning out the rear heater core.