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Mine closes completely, but I only close it when preheating (if I think of it), or when not in use.
I heard some of the newer smokers don't close completely, but it wouldn't bother me, because mine is nearly always wide open unless it's windy--Then I set it half open.
Yeah, mine lacks about 1/32" and maybe 1/16" max of being totally closed. So slight that a person could call it closed. The only time I do close it is when I am not using it.
Both my MES 40" GEN 1's appear to have a tab on the top vent that points down on the swiveling part of the vent that keeps it from closing completely. Maybe look at the top vent from the inside of the smoker to see if the vent's not closing al the way due to the tab on the moving part of the vent. I don't have any concerns about it. Put a large refrigerator magnet over the closed vent. For what ever reason other than a water tight seal on both chip loader and top vent for bugs etc.