Physical damage, leaks, and missing items are the major answers when diagnosing the evaporative emission control system. Visually inspect the vapor and vacuum lines and connections for looseness, pinching, leakage, or other damage. Remember to include the lines at the top of the canister during your inspection. If vacuum hoses or fuel vapor lines are hard, cracked or deteriorated, replace the lines or hoses. Inspect the carbon canister for cracks and replace if damage is present.
The fuel cap contains an integral pressure and vacuum relief valve. The vacuum valve acts to allow air into the fuel tank to replace the fuel as it is used, while preventing vapors from escaping the tank through the atmosphere. The vacuum relief valve opens after a vacuum of -0.5 psi. The pressure valve acts as a backup pressure relief valve in the event the normal venting system is overcome by excessive generation of internal pressure or restriction of the normal venting system. The pressure relief range is 1.6-2.1 psi over atmospheric pressure. Fill cap damage or contamination that stops the pressure vacuum valve from working may result in deformation of the fuel tank.
Use of an aftermarket fuel filler cap other than an authorized Ford/Motorcraft or equivalent service part could result in damage to the fuel system or improper system operation if not properly designed for pressure and vacuum relief.