how can I be sure it is my radiator leaking and not my heater core? on 1994 Ford E-150

I can't see where it is leaking exacly when I put fluid in it pours out as fast as it goes in

Asked by for the 1994 Ford E-150
Is it leaking out in the front near the bumper onto your feet as you fill the rad. or at the rear near the firewall/windshield area? Got to be able to narrow it down some, IF it leaks that bad!
Let a helper pour some water in while you watch where it comes out at.
2 more answers
We can easily tell by pressure testing the cooling system. The average cost of diagnosis is about $25