I forget what eec stands for. I assume you mean "that thing"attached to the tank with the 4 hoses on it. Btw thats a technical term for those of you who don`t know what to call something.
You will need to vent the tank somehow if you only cap the 4 lines coming out of the tank. If "that thing" isn`t working properly you get a huge rush of vapor when you remove the gas cap,vapor lock and stalling can also occur without the tank properly vented. I`m not sure if all you would need is a vented gas cap maybe others here can give you some more info on your question.
If you have "that thing" and the valve inside it is seized up a good soaking for a couple of days in evap-o-rust may free it up that worked for me.