Smell raw diesel when heater is on?

[meanjean]

When I turn the heater on I smell diesel fuel, not exhaust fumes. I notice it when I first start the truck. My windshield fogs up for just a second also. I was thinking the heater core is going but the diesel fumes throw me off. Im used to the smell of coolant when a core goes not diesel. Anyone have this happen before. The truck is a 2000 F250 w/ 7.3
How hard is it to change the core?
What else could cause the smell of diesel. I looked and didnt see any leaks. Could the fuel filter drain be leaking? I hear the O rings can go bad in the valve?

[meanjean]

after looking around it may be bad Orings. Ordered some new ones and will let you know.