Heater core maybe?
 

My 86 Accord has good heat for the first 15 minutes of operation, but then the heat being blown gradually turns to cool. Both inlet and outlet heater core hoses are hot. I hear the core is a huge job. The blend ducts are cable driven. Any input on this? Thanks.

RE: Heater core maybe?
 

is your tstat stuck open? if your heater hose / core was clogged, then one of the hoses would be a lot cooler than the other or even both would be cold due to lack of circulation

RE: Heater core maybe?
 

From my experience with heater cores most of the time one/both of these two condition occur when a heater core needs replacing:

1st: Something is broken/rusted through and there is a leak in the core. Most newer cars have some kind of leak catching pan that will direct this to the outside. I have a 1991 and am not 100% sure on there, but I think it does. When you have a leak you should notice coolant under the car and potentially overheating should you not keep a full cooling system. Most of the time the heater will not work properly or stink of coolant when this happens.

2nd: Your heater core is clogged with something. It this happens you should notice something like Yeu talked about above.

It sounds like something else to me, check your thermostat, I think on that car it is like most other Accord of that period and easy to get to right by the upper radiator hose at the block. You may also be able to notice something without taking anything apart. Check after you warm up and then after the heater gets cold. Has your temperature gauge dropped??

RE: Heater core maybe?
 

does your Torqe converter lock up in 4th gear? if not, it is most likely your t-stat stuck open as the ECU thinks the car is not at operating temp and will not allow the TQ converter to lock up.