1995 GMC Yukon Very little heat in cab when driving.

1995 GMC YUKON
170,000 MILES • V8 • 4WD • AUTOMATIC
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TRUCKERDEE
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We just bought a 1995 yukon the heater works great when we drive in town or just idle around, but when we take it out on the road it cools down. we changed the thermostat, heater core, temperature control valve. what else can it be? the large line at the heater core gets hot and the other one that comes out of the heater core is cold. we put in a new heater contol valve also.
Jan 2, 2009 at 10:14 AM
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JAMES W.
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Check vacuum at the heater control valve. You will have more vacuum to the valve cruising around town than you will on the highway. If the vacuum diaphram has a pin hole in it. it might not opening all the way.
Jan 2, 2009 at 12:00 PM
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TRUCKERDEE
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[quote:7c6aacbcaa="James R. Weizel"]Check vacuum at the heater control valve. You will have more vacuum to the valve cruising around town than you will on the highway. If the vacuum diaphram has a pin hole in it. it might not opening all the way.[/quote:7c6aacbcaa] we replaced the heater control valve.
Jan 2, 2009 at 12:33 PM
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JAMES W.
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Did you have vacuum to the new valve? Also, is the blend door in the heater box under the dash moving from cool to heat and back all the way?
Jan 2, 2009 at 8:39 PM