I don't know much about the a/c system so if some of you experts out there can help me, I would really appreciate the help. I have an 07 f150 5.4, whenever I am driving, the a/c is cold but as soon as I stop at a traffic light, the air starts coming out hot. I tried to measure the pressure and I got 45psi with the gage I picked up at the auto store. Any ideas?

Thanks again

 

that single gauge on the low port side wont tell you very much of how the system is working or if it has the proper charge of freon in it. you need a proper a/c manifold gauge set or take it somewhere and get it checked out.

how hot is the air comming out of the vents? and how hot is it outside where you are located at?

 

Have you checked the condenser coil in front of the radiator. If it is dirty and full of bugs , can make a big difference. As far as the low pressure side, a good rule of thumb is about 30 to 35 pounds at about 1800 RPM. True enough, it doesn't give you the whole picture. But assuming there is no moisture in the system, it will give you about the coldest temp you can get out of your system.
Another thing is the cooling fan on the water pump. Is it turning up to speed?

 

You also need to know the outside temp. If you live where the outside temp is 100, you may have problems. Regardless, the problem is heat removal from the condensor....if it can't dump the heat because your fan is broke and you don't get adequate airflow over the condensor, the heat will not be removed from inside the cab.