A/C blows hot when at lower speeds...any ideas why?

[pixiesix]

I have a 1991 MX5 Miata. I put stop leak in followed by freon as directed. The a/c runs cold when I am driving at a constants speed, but when I am in traffic going less than 45mph and stop and go at lights it starts to blow hot. Does anyone know why this is?

 

[CX-5_MAZDA]

Just a few guess but maybe your compressor is worn out and needs some speed behind it to get working (try revving the engine up to normal cruise RPM in park for 30sec or so and see if it comes good for that period). Your condenser fan may not be working and the gas uses the ram air to work effectively at speed (when you turn on your aircon, a fan near the condenser, usually in the engine bay should come on. Or your gas pressure could be low.