Coolant Loss

There are several places it could be leaking. The sneakiest place is the pipe under the intake. Another is the thermostat housing. Heater core could be leaking inside the car.

Take a look at this link:


It refers to your AC Evap drain that should be on your firewall. It’s suppose to drain the condensed water, but it appears from the thread, that a leaky heater core can also drain there. Just wondering if it’s got a leaky heater core, and it’s coming out that drain while you’re driving? Also, how much over what time period for refilling the coolant? It could be coming out the exhaust, if it’s mostly just water, you might not see it. Also, do 98’s have a heater valve that only opens when you use the heater? I ask this because if it was in the off position when you did the pressure test, you could have iso the heater core from your test. I’m not that familiar with 98 Mustangs.

The windows would be steaming up and you will smell burnt and drying coolant if the heater core leaks
More likely head gaskets leaking slightly and not showing up on a pressure test
Dye in the coolant and the hydrocarbons in the coolant tests are next
Buy yourself the little bulb tester kit for the fluorescent coolant dye
Good luck
A shop that does I/M testing can sniff the coolant for hydrocarbons for you