it's most ikely a vac line problem, it's very common, there is a vac controlled dashpot against the firewall inline wiht the heater core hoses, and there is another line that goes throught he firewall closer to the battery, it tends to get hard and break on either side of the firewall, if the hoses hceck otu, th eone going throught he firewall leads to a valve body and dashpts ont he trans tunnel just above and to the right of the drivers right foot, the dashpots stock or get brittle cna crack no linger able to move as they shoudl to close the heat door flapper