I have a 89 civic and it only starts when it wants. Starts good in the mouring or when it sits 15 min or more but if the chack engion light stays on won't start.real good motor

It's a sensor that adjust and senses fuel /air and spark to ignition. Heat for air/fuel can effect start IE "check engine" Sensor malfunction that adjust the different conditions.
It could also be a bad head gasket. They are notorious for that Steal Block motor and aluminum heads.( BAD MIX) Head expands and mess up the combustion.
Clogged or bad injectors can cause the same situation. Try Higher octane of fuel to see if its better and this will help confirm Sensor/injectors air fuel mixture.
Signed 21 boat.

Not to be critical of 21boat answers,and maybe I don't have all the details.Here are some Facts: Higher octane fuel won't cure clogged or bad injecters. Bad head gasket won't create a "starts when it whats condition". There isn't"t a Check engine sensor, the Computer reads a few different sensors to turn on the "Check Engine Lite". This "89 Civic probably have a "weak" fuel pump relay : Symptoms-cold start good, usually no "hot restart, definite no start after sitting on asphalt all day in 85 sun!! Relay is located far left up under dash.

Higher octane was to see if the injectors would be effected not "clean/cure injectors" basic on fuel A dragster uses a cooling coil to cool fuel and burns better in combustion chamber. Injectors can only be of 8 to 10% for bad running/starting An absolutely a cracked head or head gasket can produce the exact symptoms. Been through the crack scenario already. So if injector was borderline on atomizing higher octane would possibly change a 1% diff. for warmer fuel into HOT cylinder. A sensor can kick on "check engine" Or why is it there. Johnny 58 also has a good Idea. Is it hot there "macadam heat fuel" If not still check fuel system. If good back to a sensor or injectors / combustion again.