1990 Jetta with a 1.8 eight valve dignifant II. All coolant hoses are hot except lower rad hose. Changed thermostat and same problem. Checked both old and new thermostats by the boil water submerge test. Both open and close when removed from hot water. Re-installed new thermostat and filled rad by unhooking top rad hose and filling through it. Took one of the coolant sensors out to burp the system. Still, no heat at bottom rad hose. Rad is not plugged.
When I had the thermostat out the second time, I jammed the water pump impellor with a screw driver and tried to turn the water pump drive pulley. The impellor would not slip on the shaft. Is it possible that the water pump is failing some other way? I don't believe the head gasket is leaking as there doesn't appear to be any pressure building in the cooling system. The temperature guage sits at the halfway mark. It doesn't appear to be running overly hot. Just poor heater output. Seems like the heater fan motor cools the heater core down. Any and all suggestions would be greatly appreciated. Thanks in advance for your input. Luke

 

Hard to say what's wrong..... but if it had a bad pump,
or head gasket it would be over heating badly. From what
you are saying about the cool heater, and lower radiator
hose, it sounds like it has too cool a thermostat. The
coolant goes in at the top of the radiator and flows down
to the lower side, the bottom should be a little cooler than
the topbut not that much. If you had a blockage in the
radiator one end would be hot and the other would be cool
but the engine would be running very hot. Be sure you have a quality thermostat and the correct temp range and
the temp gauge isn't lieing to you. If your new thermostat
is from a discout store like ( A Z ) with out nameing them.
it's very poss you have a new.....piece of junk thermostat.