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Q: why does my heat blow hot in the rear.but blows out cold in the front on 1991 GMC Rally Wagon 2500

the vents in the front will blow warm air for a couple seconds then it get cool.while the rear vents blow heat all the time
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the hose going to the fire wall engine it get warm.but when i put presure to the hose with my hand the heat works for a while.but when i turn it off for a while then cut it back on it dosent get warm
simple, the front and rear heaters and completely seperatly controlled separate heater hoses water controllers etc so therefore there is some error with your front control valve, check your heater hoses going into the fire wall in engine compartment if one of the hoses is not getting hot while running then there is no water flow to front some actuator valves are vacuum some are electrical when you turn the hot cold knob on dash it should activate it
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