Won't idle has miss

[Trm1978]

My ranger has 4.0l. Been having some issues with it lately. It has new motorcraft spark plugs. New wires. When I start it is has a miss then when it takes off it gets worse. Put this is part I don't understand if I take the gas cap off the truck it runs perfect. Put it back on it starts missing an check engine light will flash. Any help would be nice

 

[Andy D]

the EVAP system which feeds gas fumes from the tank to the engine is plugged up. This is creating a vacuum in the tank which you relieve when you open the cap.

 

[adsm08]

I think Andy is partly right. It is almost definately something with the Evap system. Usually an obstruction won't cause running issues since the system is supposed to be closed most of the time though. It is more likely that the purge valve is stuck open. I'd unhook the vapor line from the purge vavle (black plastic line hooked to the intake) and block the valve off, see if it runs any better.

The purge valve being stuck open will cause a rich condition. Opening the fuel cap allows air to mix in with what is being drawn from the tank and even out the mix.