I have an '04 F350 6.0 and have been having a strange problem with the HVAC fan. It started about 2 months ago just about the time my third EGR cooler died. I had the EGR cooler deleted and some other work done and the HVAC has been fine until today.

I started the truck this morning and there was no air coming out of the vents despite the fact the fan was on speed 2 and A/C was also on. I cranked it up to max (4) and still nothing. I drove down the road and hit the highway in a mile or so and within a couple of miles I could feel cool air coming through the vents - but it still did not feel like the fan was working even though it was still on max. I was only going about 15 miles and by the time I reached mile 10 there was air blowing out of the vents but it still did not feel that I had the same force as max usually is. I shut the truck down for about half an hour and on the drive home I experienced the same behaviour which is the same as what was occurring a couple of months ago.

Any idea why the HVAC fan is slow to start blowing?

 

a restricion of air flow around the evap core maybe?

 

Is there flow out of the defrost vents? Possible vacum leak!