Door closer closing too fast or slamming

First work out which half is wrong

Most closers have two separate adjustments and they control different parts of the swing. Fixing the wrong one makes the door worse in a way that looks like the closer is broken.

1. 1Open the door fully and let go. Watch the whole swing without touching it. Note whether the door is too fast all the way, or only at the very end.
2. 2Too fast all the way: the main speed valve. That valve controls the swing from fully open down to roughly fifteen degrees from the frame. Turn it half a turn towards slower, close the door, and watch it again.
3. 3Bangs only at the end: the latch speed valve. The last stretch before the frame is controlled separately. It needs enough speed to push the latch home against the keep, and no more. Slow it half a turn at a time until the bang stops but the door still latches.
4. 4Re-check after the door has been used. Check it again once people have been through it, and again in a different season if you can. Hydraulic fluid is thicker when cold, so a door set on a warm afternoon closes more slowly in winter.

When adjustment is not the answer

A closer sized well above the door produces more force than the door needs in both directions. It slams on closing and it is heavy to push open, and slowing the valves does not remove the force, it only delays it. On a public building that is an accessibility problem as much as a nuisance.

Do not back a valve out far to compensate for the opposite problem. On a hydraulic unit that is how you get a leak rather than a different speed.