Related post by title

related.php

function restrict_search($search,$s) {
  remove_filter('posts_search','restrict_search');
  global $wpdb;
  return $wpdb->prepare(" AND {$wpdb->posts}.post_title LIKE '%%%s%%' ",$s->query['s']);
}
add_filter('posts_search','restrict_search',1,2);
$args = array(
  'post_type' => 'post',
  's' => 'keyword'
);
$query = new WP_Query($args);
var_dump($query->request);