There are several potential causes to look for if an F-150 cranks but will not start. The basic steps involved in troubleshooting the problem are to first check the wiring, then verify that there is a spark to the cylinder, and finally to check whether the engine is getting enough air and fuel. Seek the help of a qualified mechanic if you are not confident making these repairs on your own.
Check the harness connections to the Engine Control Unit, fuel injectors and engine timing sensors to verify that they are secure and free from corrosion, abrasion, damage and incorrect attachment. The connectors should fit tightly into the mating connector and the pin and sockets should be free of damage. If there is a problem in the harness, repair or replace the harness.
Remove the air filter and verify that it is clean. The filter should be white and free from debris or obstructions. Replace if necessary.
Mark each spark plug wire so they can be replaced in the proper order. Diconnect the first wire and measure the resistance through the wire. The resistance should be less than 5 ohms. If the resistance is greater than 5 ohms, replace the wire. Continue with the remaining wires. Remove a spark plug. The end of the plug should be free of carbon build up. Remove any carbon build up with a wire brush. Set the spark plug gap to the value printed on the emissions sticker under the hood. Contine with the other spark plugs.
Inspect the fuel filter. A clogged fuel filter will limit fuel flow to the cylinders causing the engine to run rough or not start at all. Remove the fuel filter and replace it if it is dirty or clogged. Some filters have a spring-loaded bypass that will allow fuel to flow when there is a clog. Replace the filter if needed.
Inspect the fuel pump. A bad fuel pump will also limit the amount of fuel sent to the cylinders. Have a qualified service technician look at the fuel system. The technician may connect an electronic service tool to the engine to monitor the actual fuel delivery.
Inspect the cam speed sensors. The Electronic Control Unit, or ECU, uses the cam speed sensors to monitor the cam speed and position. This enables the ECU to determine the proper time to inject fuel into the cylinder. There are two cam sensors located in the head, one on each side under the valve cover. Have a qualified service technician look at the cam sensors. The technician may connect an electronic service tool to the engine to monitor the output of the cam sensors.
