Just fixed a vacuum leak issue with my 97 F150 PCV valve hose now my A/C goes from cold to hot instantly after giving it the gas. Is this a coincidence or do I still have a leak somewhere? I have also noticed that my gas mileage has dropped to around 7 MPG since changing the PCV valve hose so I feel something is not 100% yet. The truck runs and idles good. 5.4L with 138,000 miles.
Any help is greatly appreciated!!

 

Go to the HVAC section and see the FAQs.

 

Thanks for the info. After reading the HVAC posts I think my problem is in the blend door. Any ideas as to why my gas mileage tanked after fixing a vacuum leak?

 

the air/ heat system works off two seperate controls. There are 3-4 different vacuum cylinders that positon the doors inside the duct to direct the air to the defrost, vents, floor or recirc.. The other control is the blend door which has an electric motor. It is variable in postions and mixes the outside air and inside air to get the temp you want. On the dash you have one control for "temp" and one control **** for "location of the air". In reality, vacuum has very little to do with the air temp, just duct location it exits.

 

Shoulda caught the wording, he is right to an extent. There is a blend door that controls hot and cold though. Although the older ones had a cable to control that, pretty sure that none of our trucks do, it is all vacuum. I know I have had mine out, regular temp. controls, not climate control, and it is all vacuum.

 

no, the blend door is electic motor controlled. thats how you get the variable position... the vacuum doors are 100% open or 100% closed. ON--OFF... that directs the air up or down in the ducts. the door is above the transmission hump, under the radio.. to the drivers side.

 

A few of the threads in the HVAC section had a link to Auto replacement part, blend door fix, Heat, AC replacement parts for a fix of the blend door. I am pretty sure this is my problem as the air goes to where it is supposed to I just can't control the temp.

 

that fix assumes your electric motor and gear box are good, and the cam or pivot rod on the door is broken.. You should check the motor and gear box to make use they are functioning.

 

Thanks Steve...gonna check that out tomorrow.

 

can you give me a link to the hvac section in faq please. I have tried to search it but i must not be smart enough to use the search in faq section

im having a issue with my ac going from cold to warm back to cold and back and forth

 

Search in this forum; Cooling, Heating, Ventilation & A/C - Ford Truck Enthusiasts Forums

 

going hot to cold can be the blend door flopping around, or could be somethink like low gas pressure inside the compressor which makes the low pressure switch cut "in and out".. you can check on a hot day in the driveway. hold the throttle at 2000 rpm and see if the compressor cuts in and out too much, or is basically "on".