Starting Problem

When I attempt to start my car (when hot), It does not start. When I turn on the A/C or Vent or Heater before attempting to start, it fires right up. I am guessing that there is a vacuum line problem but do not have the vacuum diagrams showing any lines leading to the vent controls. Anybody know if this is the problem or have vacuum line diagrams that may help?

a) it cranks and does not fire up or it does not crank at all
b) what engine and model year

a) it cranks and does not fire up or it does not crank at all
b) what engine and model year
The starter fires and cranks but does not stay on. It's a 1990 3.1L.