DC-3 cowl flaps are hydraulic, and pretty much useless. There's one actuator per engine and when it extends a ring of linkages expands, pushing the cowl flaps open. When it retracts, it pulls the linkages tighter and the cowl flaps close. If there is any play in the linkages, the slop in the system takes up the motion before the cowl flaps move. Or the other way, if there is a load on the cowl flaps, they'll use that play to move to where they want.
I've heard of a mod to just fix panels around the cowl and leave a small gap for airflow but I've never flown one with that mod. I pulled upsome NZ DC-3 photos on airliners, some do have cowl flaps but ZK-DAK has panels riveted in place.