Originally Posted by 77olds

Blocked/Plugged Heater Core
If coolant can't flow through the heater core, well, then you're not going to get sufficient heating.

Here's how to test it.

Observe this photo.


See the two black rubber hoses that run right by the left side of the valve cover? Those are your heater hoses.

Get your engine up to operating temperature, leave it running but MIND the cooling fan and hot metal parts. Take your hand and touch each hose. Don't worry, it won't burn you. They might be hot, but not enough to burn you.

Is one hot, but the other cold/warm? They should both be almost exactly the same temperature. If they're not, that means there might be a blockage in the heater core. Disconnecting the hoses from the heater core (where they plug into the firewall) and using a garden hose to flush out the heater core will likely fix this problem, if it's the cause of your issue.