If a car turns over but does not start, it has a problem with compression, ignition or the fuel to air ratio. Determining which of these three areas is responsible for the problem is the first step in troubleshooting, according to the Linn-Benton Community College.
An engine experiencing low compression does not increase the temperature of the air and fuel mixture enough that it burns. There are several reasons cars experience low cranking compression, such as a slow turning starter motor, improper sealing of the cylinders or a broken timing belt or chain. An experienced mechanic is often able to determine the problem by listening to the engine as it attempts to crank.
In order to operate correctly, the ignition system provides a spark to ignite the compressed fuel and air mix. A spark tester helps to determine if the ignition is providing sufficient spark to start the engine. If the engine produces spark but does not start, the problem is often improperly installed plug wires. Following the manufacturer's firing order helps to resolve the problem.
An improper fuel to air mix prevents engines from starting. A bad fuel pump or filter limits the amount of fuel entering the cylinders. A plugged air filter makes the mix too rich for the car to start.