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Heater not working aftre replacing heater control valve


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I have a 75 620, the fan blows strong and heater was working until I changed a leaky heater control valve.   I am sure the valve is open and both hoses connect to it get warm once the truck heats up.  However the heater never blows any warm air.  Thanks in advance



Also, a big thanks to everyone in this community.  This is such a great resources. 



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My apologies and thank you for your response.  I will get some pics together and make an introduction post over the next week.  


In the mean time:


The thermostat is brand new.  I will clean the heater core but is there something else that could be going on since the heater worked great right before the valve change?

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You are going to want to flush the core anyway so pull both hoses off the motor. The one on the side of the head and the other down by the lower rad hose. Use a garden hose and flush water through in both directions, several times. If the heater isn't working the valve may be closed, in which case you won't be able to flush water through but at least you will have found the problem.


IOf you can flush water through then the hot water can circulate. Look at the other flap door linkages... air has to flow through the core to get hot.

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Are you actually seeing the valve lever move when you go from hot to cold on the control panel, and does it move freely or with moderate resistance  ?  (i.e. the control cable may not be anchored properly or broken inside ).


Pics of your install would be a big plus here.

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