[manlystanley]

When I have a heater problem, I always flush the engine and change the coolant. Try getting a prestone back flush kit (you'll use a garden hose as well) and get a bottle of radiator flush. One suggestion, don't get the super rust removing flush. I've heard of lots of sad stores of people taking out all the rust and ending up with a radiator leak. So I do a low grade flush twice.

Hope this helps.

Best Regards,
Stan