Need a little help out of the box thinking
Only when running heat or defrost, coolant leak looks like it's coming out of the bell housing. I have a supercharger so very difficult to see behind motor, but with inspection camera, heater hose lines look dry. firewall looks dry. No coolant in cab. No steam on windows or smell. Pretty sure not casting/ freeze plugs because absolutely no leaking with heat off. Can run for days with no loss, then turn on heat and within minutes/ seconds coolant puddling in driveway. Is the hose that runs through valley pressurized only when heat is on? I saw a few older threads but as far as I could tell none specified heat on/ heat off conditions. Just started problem after doing some other work, hoses were off radiator and intercooler, replaced radiator. None of those connections are the culprit.
Thanks--

98 F150 Lariat 1900 TVS S/C Chiller Killer I/C 19PSI Snow performance WMI

 

I cant think of any coolant lines that only have pressure when the heater is on... Most have continuous flow thru heater even when not in use... the damper door shuts off the air flow.

 

That's what I thought, so other possibilities?

 

You need to find out for sure where it's leaking from for anyone to be able to diagnose how to fix it. With the custom work done to your truck there is no way for anyone else to know what has been changed.

 

I agree I should find the leak before fixing, but where to look, does coolant run through the heat exchanger only when heat is on or does it always flow?

 

Coolant always flows through the heater core.

 

let it idle in the driveway with the heater on and look.......... there is no obvious reason for what you describe.