Car starts running hot

[pabaker2000]

I have a problem with my 91 5.0 sarting to run hot when it is sitting still, but only with the a.c. on. The temp. is steady if the a.c. is not on, but rises only with the a.c. on and sitting still. Does anyone have any ideas to what may be causing this. Thanks for the help.

 

[HISSIN50]

The two largest possibilities are that the cooling fan cannot keep up with the extra load of the condensor, or that the radiator itself cannot reject the additional heat from the condensor.

If you hold the RPM at 1500-1800 RPM, does it cool back down? Alternatively, if you put a big box fan in front of the car, does that allow it to cool down?

Good luck.