Nissan Repair: Heater Core issue with 1998 Quest?, heater control valve, coolant level

Question
Von - Thanks very much for being generous with you mechanical knowledge.

My '98 Nissan Quest's heater no longer works unless at speeds of 40MPH+.  I've studied all of the heater related threads within the Nissan Repair section and noted a similar case to mine.  You suggested that it may have been an air bubble in the heater hose.  I followed your instructions and checked the two hoses that enter/exit the firewall. - both were cold so I loosened the top hose to release the air and nothing came out.  It was completely dry.

(I did this on the passenger side, however on the driver side the top hose was very warm and the bottom hose was cold, if that means anything)

Why would this hose be completely dry?

Do you think it is the heater core?  If so, how do I back flush a heater core?

I've checked the coolant level, the thermostat seems to be circulating and I think the blend door is working - but I'm not absolutely positive.  I've also been reading that it could be a "heater control valve"... but I'm not sure what that is or how to check it.

Again, thanks very much for your time.  I'd like to fix this myself before winter arrives, if possible.

~ eric

Answer
Eric,

Sounds like someone has used stop leak in your car in recent years and the heater core is plugged up.  I would disconnect both hoses at the engine and use a water hose on one of the hoses to get water to wash out the blockage.  If that does not do it use the garden hose on the other one and see if there is a plug of stop leak that comes out the other hose.  Water has to flow for the heater core to work.  Email me back if you have difficulties.