Hey everyone,

I have used this forum to help solve countless problems over the years with my 2004 f150 so thank you to all the contributors. I have a weird issue now that is perplexing me. I have a 2004 f150 5.4. When the A/C or heat runs for 45 mins or more the flow of air seriously decreases, at times just to a trickle. You can hear the fan working the same but just about no air coming out. When I turn the temperature down when it is heating or turn it up when it is on A/C, within a 5 -8 minutes the flow is back at full. It doesn't seem to matter what vent setting it is on either.

Any suggestions?

Thanks

 

Evaporator is freezing up and turning to a block of ice.

If you run the system in either VENT (or PANEL) or the HEAT (Floor) setting, that should disable the compressor and then it shouldn't freeze up. All other modes except OFF run the compressor.

 

Thanks for the info. So that I understand correctly, Even when I am running the heat the compressor is still running and freezing up the evaporator if it is on the windshield/floor setting?

 

The temperature control has zero effect on whether or not the compressor is enabled. It is the MODE switch (or setting, if EATC) that determines whether the compressor is enabled or not.

 

Mine has manual AC and the AC button determines whether or not the compressor is on except for defrost mode when it is on regardless.

 

Turn off the A/C then and see if the problem doesn't occur.

Generally, evaporator freeze-up is going to be caused either by a mechanical failure with the clutch (stuck in the engaged position) or by it's control system causing it to remain engaged when it shouldn't, often by a cycling switch that is stuck closed.

Good luck.

 

Or low refrigerant.