Really only a couple things it could be. Either a plugged vapor
canister vent, defective vapor canister, Or a defective vapor vent
valve. If there is no check engine light on you can almost rule
out vent valve. unless the mechanical part is broken and the
electrical side is good. A plugged vapor vent hose is the most
likely culprit. Especially in muddy/dusty environments. I have
also seen wasps make homes in them. Or in some parts of the
country, Mud Dawbers. But that would only happen if your truck
had been sitting for at least a couple days.
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