I have a Class A motorhome with a 4.6 liter Ford engine. When driving at any speed, I would be okay for a bit. Would run normal, then start losing power, sputtering, only able to go 5 miles per hour. I would remove the gas cap and it would run fine for a few minutes then start sputtering again. May have to mention that this problem happened while driving last summer when the outside temperature was over 90 degrees. Not sure if it happens at cooler temps, cause had to put rig in storage... in another state. Haven't driven since problem started. What would cause this problem.

 

There is a vent solenoid valve in the Evap system of your vehicle.
It is supposed to be open when not energized , and closed when an Evap monitor is being run to test for air leaks.
The valve could be stuck closed or ,the foam filter on the valve that allows air to get in could be plugged.
.Some are located in the engine compartment while others are located under the rear of the vehicle.

 

Thank you Hank. Will try having evap system checked.