Hello. Have a 98 F150 w/4.? Triton V8. Just recently, the air only comes out of the defrost vents. It is still cold, but blowing on a 100 degree windshield here in FL. does nothing for interior comfort. In reading some other posts, It appears I may have a vacuum leak. Where do I start looking ? Any help would be appreciated.
Thanks
Tom

 

look under the dash behind the controls and near the door assy. by the evap

 

Ok, Thanks. I started taking things apart and noticed that the big 'door' that directs air into the cabin vents is closed. What actuates this door? It is spring loaded and is closed for the dash vents thus sending all air to the defrost vents. Can I just prop it open somehow? Rarely use defrost here in Fla.
Tom

 

im not to familiar with the 98 but there should be a vacuum switch attached to a motor to open and close the door

 

And, I'm not getting any air to the floor vents either way. ('door' open or closed) This would only become an issue when needing heat. Again, rarely needed here.
Tom

 

Before screwing around under the dash, pop the hood and inspect the small vacuum line that feeds through the firewall.

It is located between the battery and the firewall and is notorious for developing leaks.

When vacuum to the HVAC control head is lost, all airflow automatically defaults to the defroster.

Airflow through defroster only is almost always a loss of vacuum. This applies to all Fords (and probably other makes as well).

Steve

 

Thanks Steve, as it turns out you are correct. There is a cluster of vacuum lines just behind the battery. Battery acid had destroyed ALL! Bypassed all of Ford's plastic multi connectors and just linked all lines with vacuum hose. All is now functioning properly.
Many Thanks
Tom

 

Now, if someone could please tell me what each of those lines controls? There are 2 small black lines, 1 large black line, 2 white small lines, and 1 really small black line which I'm assuming is for the AC. Most of these exit the engine compartment through the inner fender wall on the passenger side.
Thanks
Tom

 

03 F150 4x4 Screw

I pressure tested that line and it checks out fine. I'll test again to verify.

Where does the line terminate in the truck side of the firewall??

 

Did you ever get this fixed?

What else is vacuum used for other than A/C doors?

 

can anyone tell me how to identify this vacuum line and do I need to pull the battery to find/change it? I cant afford to take my truck to a mechanic so I am it

btw: I have a 1998 Ford F150 Lariat, Triton 5.4L V8 and need to get this fixed asap.