Recently purchased a 96 F150 XLT. It had been unused for about 4 years. The more I run it the better its overall operations appear to be. The truck is discharging coolant from underneath the air conditioning unit. There appears to be a tube coming from the firewall with an occasional drip, sometimes a small stream. The engine is not running HOT, but the reservoir is low. Mechanic says it is an overflow vent, really? Is this true or do I have a much bigger problem??

I bought the truck on a whim - I am semi-retired. Business mgt - controller. Excel guru

 

Checked reservoir when engine cold - it is empty.
The leak location I discovered when changing the back spark plug (passenger side). I was underneath the truck installing the spark plug (such a nightmare).