How go give $value to wp_query

Assuming removeCommonWords() returns a string, you can do the following:

$zoekterm = removeCommonWords('motorkap met'); 

$query2 = new WP_Query( 
    array(
        's' => $zoekterm
    ) 
);

while( $query2->have_posts() ) {
    $query2->the_post();
    echo the_title() . '</br> ';
}

Sometimes the array notating is more useful than the string notation …