Why does my 99 honda civic stall out will driving on hot days?

Replace the main relay. Located on drivers side under dashboard(small black box. . $55-65 from dealer. & easy to install. Plugs right in. I have replaced 2 on two diff civics.

It's actually the fact that most any car will do that on a hot day, to some extent. Heat does unnatural things to a car where it's not intended to be hot, and that stresses the car out and makes it run worse when it's under the weather, so to speak. A car was not designed to be operated in extreme heat. It was designed to be operated in temperatures around 70 degrees Farenheit. So yeah. I suppose that doesn't really answer your question, but I thought I'd chime that in, since it is important to understand that when someone else decides to omit that information.