I have replaced four heater cores in my 1998 F-150 in a little over 1 year. The latest replacement is an after market copper heater core. All the previous manufactures aluminum heater cores have failed. I am not holding my breath! Does anyone have experience with issue. Thanks for time and knowlegde.

 

I've seen this a few times with the 4.6 in Police Crown Victoria's and a couple Expeditions and f-150's. Electrolysis
Electrolysis essentially turns your heater core into a battery plate, causing it to break down.
Succesful fixes have been done:
Flush the coolant thouroghly, No Rust! Use descaler.
Install Brass heater core. (may not be entirely neccesary, but I like em better)
Ground heater core with wire directly to battery negative post. (use hose clamp around outlet tube)
Check all body and engine grounds, particularly the braided one at the right rear of intake manifold to the right side firewall.
There are also special kits you can buy that help eliminate Electrolysis. Ask at a parts store.
Make sure any aftermarket electrical products are grounded as well.
Go to a Ford Dealer and install this restrictorF1UZ-18D406-A) according to the directions in the package.

If you have a voltmeter, attach red lead to battery positive post and dip the black lead into your coolant bottle. You see some voltage, maybe .4-.6 volts DC with engine at 2000 rpm.. The trick is to lower that voltage by making sure your cooling system is grounded and not allowing current to cause oxidation of metal parts in your cooling system. Take that reading before and after the above services. You should se it drop to below .1 volt.