Cleaning heater core
Hybrad has an excellent reply to the checking the thermostat in the car. My car had the cold heater hose/no heat problem, so I finally figured out it meant a clogged heater core. I looked at a core in the parts store and saw that the passages are very narrow, thus easily clogged.
So last weekend I tried to clean the core in situ. I got two lengths of 5/8" heater hose about three feet long and stuck them on the inlet and outlet of the heater core. Then I jammed the garden hose on the outlet then the inlet and alternated back and forth several times. I tried to collect the water in a pail and saw there was a large amount of rusty dirt.
It would be much easier with five foot lengths of heater hose. 3/4" will work too, it just needs to be clamped tighter. I had a pressure regulator on the hose I set to 25 psi, but it's unlikely full pressure would hurt the core, and it's hard to make a good seal when you jam the hose ends together anyway.
It worked! I now have very warm air coming out of the heater. Not as hot as with a new core, but quite good. Plus, now that the hot water circulates through the heater hoses, the temperature sensor sees hot water much quicker, getting the engine out of cold, rich running with fast idle, sooner.