diff --git a/plugin/syntastic.vim b/plugin/syntastic.vim
index e22eed65..bdc794a8 100644
--- a/plugin/syntastic.vim
+++ b/plugin/syntastic.vim
@@ -19,7 +19,7 @@ if has('reltime')
     lockvar! g:_SYNTASTIC_START
 endif
 
-let g:_SYNTASTIC_VERSION = '3.6.0-27'
+let g:_SYNTASTIC_VERSION = '3.6.0-28'
 lockvar g:_SYNTASTIC_VERSION
 
 " Sanity checks {{{1
@@ -486,8 +486,8 @@ function! SyntasticMake(options) " {{{2
     if has_key(a:options, 'env') && len(a:options['env'])
         for key in keys(a:options['env'])
             if key =~? '\m^[a-z_]\+$'
-                exec 'let env_save[' . string(key) . '] = $' . key
-                exec 'let $' . key . ' = ' . string(a:options['env'][key])
+                execute 'let env_save[' . string(key) . '] = $' . key
+                execute 'let $' . key . ' = ' . string(a:options['env'][key])
             endif
         endfor
     endif
@@ -502,7 +502,7 @@ function! SyntasticMake(options) " {{{2
     let $LC_MESSAGES = old_lc_messages
     if len(env_save)
         for key in keys(env_save)
-            exec 'let $' . key . ' = ' . string(env_save[key])
+            execute 'let $' . key . ' = ' . string(env_save[key])
         endfor
     endif
     " }}}3
diff --git a/plugin/syntastic/checker.vim b/plugin/syntastic/checker.vim
index d8a611e9..f7853c22 100644
--- a/plugin/syntastic/checker.vim
+++ b/plugin/syntastic/checker.vim
@@ -49,9 +49,8 @@ function! g:SyntasticChecker.getName() " {{{2
 endfunction " }}}2
 
 function! g:SyntasticChecker.getExec() " {{{2
-    return
-        \ expand( exists('b:syntastic_' . self._name . '_exec') ? b:syntastic_{self._name}_exec :
-        \ syntastic#util#var(self._filetype . '_' . self._name . '_exec', self._exec), 1 )
+    call self._syncExec()
+    return self._exec
 endfunction " }}}2
 
 function! g:SyntasticChecker.getExecEscaped() " {{{2
@@ -121,6 +120,7 @@ function! g:SyntasticChecker.makeprgBuild(opts) " {{{2
 endfunction " }}}2
 
 function! g:SyntasticChecker.isAvailable() " {{{2
+    call self._syncExec()
     if !has_key(self, '_available')
         let self._available = self._isAvailableFunc()
     endif
@@ -143,6 +143,21 @@ endfunction " }}}2
 
 " Private methods {{{1
 
+" Synchronise _exec with user's setting.  Force re-validation if needed.
+function! g:SyntasticChecker._syncExec() dict " {{{2
+    let user_exec =
+        \ expand( exists('b:syntastic_' . self._name . '_exec') ? b:syntastic_{self._name}_exec :
+        \ syntastic#util#var(self._filetype . '_' . self._name . '_exec'), 1 )
+
+    if user_exec != '' && user_exec !=# self._exec
+        let self._exec = user_exec
+        if has_key(self, '_available')
+            " we have a new _exec on the block, it has to be validated
+            call remove(self, '_available')
+        endif
+    endif
+endfunction " }}}2
+
 function! g:SyntasticChecker._quietMessages(errors) " {{{2
     " wildcard quiet_messages
     let quiet_filters = copy(syntastic#util#var('quiet_messages', {}))