Eu acredito que esse comportamento é regido por este parâmetro:
metadata_expire
Duration in seconds after which the metadata will expire. So if the current metadata downloaded is some seconds younger, then yum will not update the repository metadata. If you think that yum is not downloading the update information as often as you wish, reduce the value of this parameter. You can also change the default format in seconds to days, hours or minutes adding d,h or m to the specified value. The default is 1.5 hours, for running yum-updatesd every hour. You can also use the word never meaning that the metadata will never expire.
Então, eu tentaria um 0d, -1 ou definir o valor para algo realmente longo, 1000d (1000 dias).
Exemplo
metadata_expire=99999999
Você pode atualizar o cache quando tiver acesso à Internet:
$ sudo yum makecache