c# Xml syntax error

I have method in order to replace xml

Replace method

 public string EscapeXMLValue(string xmlString)
        {
            if (xmlString == null)
                throw new ArgumentNullException("xmlString");

            return xmlString.Replace("&", "\"");
        }

My code below and i get exception below code

Code:

   if (!Directory.Exists(sPath))
   {
   Directory.CreateDirectory(sPath);
     }

    document_type = System.IO.Path.GetExtension(flPolicyList.FileName).Replace(".", "");
      document_name = uf.GenerateGUID(false) + "." + document_type;
      flPolicyList.PostedFile.SaveAs(sPath + document_name);

                        StreamReader objStreamReader = default(StreamReader);

                        objStreamReader = File.OpenText(sPath + document_name);
                        Encoding enc = Encoding.Default;
                        StreamReader reader = new StreamReader(sPath + document_name,enc, true);

                        XmlTextReader Xmlreader = new XmlTextReader(sPath + document_name);
                        int i = 1;
                        while (Xmlreader.Read())
                        {
 if (i == 2)
  {
  enc = Xmlreader.Encoding;
  break;
 }
 i++;
 }



 XML = objStreamReader.ReadToEnd();
 XML = System.Web.HttpUtility.HtmlDecode(File.ReadAllText(sPath + document_name, enc));
 objStreamReader.Close();

  XML = EscapeXMLValue(XML);

  xmlDoc.LoadXml(XML); // I get error here

Error Value :

<Vehicle>HB ICON 1.5 DCI 90 S&amp;S</Vehicle>

Error Name:

An error occurred while parsing EntityName. Line 22503, position 40.

Even i replace & value i get exception . how can i solve problem , Where i miss ?