How do you make the heater warmer in a 1991 Honda Accord?

Make sure your heater control knob is not cracked, or it will just be slipping on the shaft. Also inspect the attachment where the control cable attaches to heater control valve (underhood). You can adjust the tension of the cable by pressing on the tin part that holds the cable sleeve in position.

IF you have done the above, there are a couple of other possible faults:

1) Faulty thermostat (stuck open) 2) low coolant levels or other cooling system problem