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Discussion Starter #1
Been reading up on this and none of the solutions seem to fit my problem.

'99 F350, 7.3, crew cab, long bed, 299510 mi

On very cold mornings the heater works fine for about 15 minutes. After everything gets warm the air flow slowly slows until I stop getting air movement from the vents. I can hear the fan working full tilt but no air from any vent regardless of switch position - not even the defroster which I'm sure you were all thinking. The fan was replaced in the summer of '10.

With the engine off and the ignition on I can hear the vacuum pump run for about 15 seconds and stop.

If after I lose air flow I turn the system off for a few minutes, then back on I get a burst of air that slowly fades away. Sometimes it comes back on it's own, some times not.

Again, this only seems to be an issue when it's below 45 out (cold for Phoenix) and when I need it most.

Any insight?
 

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Does it change over to defrost when it quits blowing out of the vents? Or does the fan shut off?
 

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Discussion Starter #3
Does it change over to defrost when it quits blowing out of the vents? Or does the fan shut off?
Neither. I can hear the fan running (and the sound of air moving), I can hear it change if I switch through the speeds, but no air comes out any vent (floor, vent or defrost) regardless of the selector setting.

Like I said, it will sometimes return to normal after a time.
 

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The air has to be going somewhere. The most common place for it to go is to the defrost. This happens when you loose vacuum due to either a vacuum leak or a faulty pump.
 

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Discussion Starter #5
The air has to be going somewhere. The most common place for it to go is to the defrost. This happens when you loose vacuum due to either a vacuum leak or a faulty pump.
I agree 100%, but it's not coming out the defroster either. Today while it was doing it I tried switching to AC but the symptoms were the same. I got home and tested the vac system with a vac gauge and regardless of 4WD selector position or vent control selector position I held vacuum.

I'm stumped. I've been trying to locate a diagram of the vent system internals to see if I could spot anything obvious but haven't had any luck. A mechanic buddy on the east coast gave me a website they use to diagnose problems but I haven't had a chance to look at it yet. I'll go look now.
 

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Discussion Starter #6
OK. I found a crude diagram of the vacuum system inside the dash. It did show the different doors within the system in a rough layout to the system operation and their default positions. None of these failing would cause my symptoms EXCEPT the fresh/recirculate door having completely failing - meaning broken off.

Without having taken the dash apart to see what's what I have a theory. The fresh/recirculate door has broken off of it's mount/pivot and is obstructing the fan intake. This would create a slight vacuum and with the truck bouncing around would cause the door to eventually want to migrate to cover the opening completely. When I turn the system off the door is able to bounce around and create an opening so when I turn it back on I get the burst of air which quickly fades.

How am I doing so far?

IF this were the case, how in the hell do I get to the door and where would I get replacement parts? Would Ford even carry that?
 

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If my memory serves me right you should be able to see the inlet behind the glove box and if my I am remembering correctly you should be able to see the door. Should be almost right tight up against the firewall.
 

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Discussion Starter #8
Well that was a bust. Everything checked out ok. There's no way the fresh/recirculate door could obstruct the air flow. The only thing I found that was out of the ordinary was how dirty the AC coil was (using a mirror). Hosed it down real well and it looks better but I don't see how that could be the problem, it's either blocked or it isn't.

Tomorrow I'll take off the glove box and access panel to the heater core, maybe even the heater core and see what I can find in there. The bottom line is that something HAS to be obstructing air flow.
 

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The only thing that I can think of is that there is a piece of debris floating loose in there that every now and then is blocking the air flow.
 
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