quick question.. have you pressure tested the system? a leak on a suction side would be plausible of you fuel heater circuit is on the suction side of the heater core.. i still think cap.

__________________
1996 s10 zr2 with a 6.5 turbo diesel
1996 c2500 suburban with a 6.5 turbo diesel
1981 300sd with a turbo 617
Monte carlo figure 8 car