Rough idle and poor gas mileage and cuts out bad between 40 and 45 mph Changed all plugs, coils, throttle position sensor, throttle body, air filter, maf sensor, fuel filter, gas cap, and have tested each fuel injector. Any other suggestions?
My car has 169000 miles.
My car has an automatic transmission.
Hello and thank you for contacting YourMechanic. With what you said about everything that you replaced on your F-150 5.4L, there is only one thing that will cause the problem that you are having. There is a Fuel Pump Delivery Module (FPDM) that delivers fuel from the fuel tank to the fuel rail. This is similar to the fuel pumps in other vehicles. Some Ford vehicles have a secondary FPDM above the spare tire under the bed of the truck. It is possible that the FPDM is not delivering pressure under a load. Try checking to see if the engine cuts out when heavily accelerated from a stop. If the engine cuts out then the FPDM is failing and should be replaced. If the vehicle is only cutting off at 40 to 45 mph, then there may have dirty fuel lines to it and needs cleaned. I recommend replacing the Fuel Pump Delivery Module, due to the mileage your vehicle has. This will cure the rough idle and poor gas mileage along with restoring power. Also, clean out the fuel lines with a small bristle brush. If you need assistance with this issue, a YourMechanic certified technician can come to your location to perform a car idle is rough inspection, confirm the repair needed, and provide you with any further assistance.
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