modifying data in a node
This depends ... D6 or D7 ... is the field a reference to something else (term, node, entity)? 

Generally you have to write something that looks up the value of the item in the list, for example for a node reference: 

  $modelcode='Toyota';      

  $query = new EntityFieldQuery();
  $query-&gt;entityCondition('entity_type', 'node')
        -&gt;fieldCondition('field_models_code', 'value', $modelcode, '=');
  $result = $query-&gt;execute();
  $model_nid=array_shift(array_keys($result['node']));

  unset($result);
  unset($query);


Then you would take the $model_nid and update your field in the form state: 

$form_state['values']['field_model'][LANGUAGE_NONE][0]['value'] = $model_nid;


This example assumes D7 and a field set to node reference.