There is no `setlocal  foldopen` or `...close` as explained in the Vim doc.

To prevent messing up user settings, disable it for good.

To preserve this functionality, how about adding the following to `plugin/ack.vim` ?

```
autocmd BufReadPost quickfix call s:qfEnter()
autocmd WinLeave * call s:qfLeave()
function! s:qfEnter()
  let g:oldfc=&foldclose
  let g:oldfo=&foldopen
  set foldclose=all
  set foldopen=all
endfunction
function! s:qfLeave()
  if &filetype!='qf'
    return
  endif
  set foldclose=g:oldfc
  set foldopen=g:oldfo
endfunction
```
Instead, I'd propose `nnoremap <buffer> x zx` for convenient fold toggling.