Replaced thermostat and water pump engine temperature starts to climb after 6 or 7 miles a/c cools but heater only gets warm

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Originally Posted by birdielou

engine temperature starts to climb after 6 or 7 miles

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Climbs? Does the coolant boil over or the engine overheat, i.e.: temp goes into the HOT range or check gauges message, anything?
What are the ambient conditions when your truck does this?

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a/c cools but heater only gets warm

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You could have a partially clogged heater core. If you can't verify coolant flow, I'd consider replacing the heater core for this concern.