Why won't car start after sitting a few hours then start OK the next day.

The car needs to be tested in a failed state, it is easy to determine when the car fails to start whether the engine is loosing fuel or spark.If it is loosing spark or fuel then you have to determine if it is a sensor that provides an input the computer needs to process to determine when to trigger spark and then trigger fuel. Check for fuel pressure (you will need a fuel pressure gauge and look up the specified fuel pressure for your vehicle). Remove the fuel hose that goes to the fuel rail and get a friend to crank over the engine to see you have adequate fuel volume (you should have approximately 1 pint of gasoline in ten seconds of cranking). Off course take proper precautions as gasoline is highly flamable. Next "listen" to the fuel injector. Using a mechanic's stethoscope (or use a long screw driver placing the handle end up to your ear place the metal tip end of the screwdriver against the body of the fuel injector), get a friend to crank over the engine while you "listen" to the fuel injector listen to each injector at the same point of the injector body. You should hear "click, click, click" as the injector is electrically opened and closed by the computer. If the injector is not being triggered you will not have fuel getting into the engine's cylinders. Disconnect the electrical connector at the fuel injector with the ignition key in the "on" position, (next position after the accessory position) you should have 12 volts at one of the two wires at the injector. This true for most vehicles but you need to consult a workshop manual for your vehicle to see that this is true for your vehicle. When the engine is running the second wire at the injector gets (triggered) grounded by the computer in your vehicle many times a second controlling exactly when and how much fuel gets injected into the engine. Check for ignition spark. Use caution when checking ignition spark firstly it provides very high voltage (about 15,000 to 40,000 volts) but it is low amperage, it can give an unexpected shock but is unlikely to kill anyone, secondly the spark produced can ignite any flammable gas or liquid (for example gasoline or the gaseous vapors of a lead acid battery). Remove the spark plug wire (or ignition coil if the ignition system on your car is coil over plug type), place an old spark plug with the gap between the electrode and shell of the spark plug "widened" to about .060" or .070" of an inch. Firmly ground the spark plug shell to a good engine ground. Hold the spark plug wire or coil with an insulated pliers and get a friend to crank over the engine while you watch for a pulsing bright spark jumping across the spark plug gap. Look on Youtube for a video on "Check for spark" or Check for ignition spark" I am sure someone has posted a good video clip. Again the tests need to be done when the car is in a failed state. There is a company that independent auto repair shops get their information from, this same information is available inexpensively for people that work on their own cars.. The information is year make and model specific, covering repair procedures, torque specifications, fluid capacities and specifications, service bulletins, component locations, wiring diagrams ect.... Alldata is very easy to navigate http://bit.ly/AllData_Repair_Manuals_Online