2004 f150 5.4 engine. I was waiting and idling foe about half an hour and the heater started blowing warm air even when I had it set for cold. When I turned on the A/C, I stated to get cold air...When I started moving the Air temp became cool as I moved up over 30 MPH. The temp Gauge registered normal thru all this time....What could it be?

 

One of two things...

1. Your blend door actuator needs to be fixed/replaced.

2. Your A/C unit is short cycling.

Or the combination of both.

 

Having had a 2004, they were prone to bad blend doors and actuators. You can spend the big bucks and get it fixed, or, you can do like I did- put a quarter turn valve inline with the hot side heater hose. No hot water gets to the dash, no problems with cold air during the summer. But you have to turn it back on during the winter months. I think I spent less than 10 bucks at Home depot for the valve and 1/2" nipples.

 

Thanks much...Would this still be the problem even though the airflow returnes to normal when traveling at speed?

 

look to see if the compressor is running or running a few seconds and shutting off. It may be just low on refrigerant. If the compressor is cycling on and off check the pressures both low and high side.