<?php
if ( isset( $_POST['submit'] ) )
    { //if nonce check succeeds.
        global $post;
        $postid = $post->ID;
        $data = $_POST['doors'];
        update_post_meta($postid, 'doors', '2' );
        echo self::$auto_id. 'doors'.'2';
        echo $postid;

}


$doors = get_post_meta($auto->ID, 'doors', true);
?>

<form method="post" action="">
   <input type='text' name='doors' value='<?php echo $doors ?>' />
   <input type='submit' value='save' />
</form>