Just got my 99 F150 4X4 Xtracab 5.4L out after 3 1/2 months of storage - the heater is just blowing cold air. Also noticed in the summer that the A/C would blow through the defroster vents instead of out of the dash at me unless I shut off the A/C and blower - then turned them back on. It would blow normally until it decided to start blowing through the defroster vents again, sometimes after 6 hrs of driving.

Any advice would be appreciated.

 

Vacuum check valve, on firewall, right side, I think. When you accelerate, whithout the check valve working, damper doors go to default position, which is defrost. Change it, not too expensive, and it should solve your problem.

 

I have heard of this problem before and, like Dvds said, change the VCV and you should be back in business...

Good luck!

RP

 

I'm having a hard time getting a part number to order a new vacuum check valve - is this located on the firewall in the engine compartment?? Any pics or brief description of it would help.

Thanks

 

I'm having a similar problem. I can't get the air to blow out the floor vent. Is the check valve inside the cab? I found an air intake valve behind the glove box on my 97 f150. Had a friend tell me I had probabely dropped something down the defroster vent, he's a mechanic, that was stuck in that valve. I couldn't see anything in it.

Where is the check valve and how do you replace it?

stephen