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Heater Core Problems?

I have a 2006 Toyota Sienna Van that has 111,000 miles on it. I have a problem with both the heat and a/c. If you have the heat on but are just idling, it begins to blow cool air. Likewise, if you run the a/c but are just idling the air begins to blow warm. In either case, if you rev the engine (either in park or nuetral say at a red light), the air gets warmer if the heat is on, or colder if the a/c is on. In other words, the temperature only works correctly if the rpm’s are up. Could this be a stopped up heater core. Thanks for any insight.

That sounds like blend door problem. The blend door moves around to control where the cabin air comes from in terms of the rest of the HVAC system. Many are vacuum controlled and the vacuum comes from the engine. So since yours changes with what the engine is doing it sounds like you have a problem with one or more of the vacuum lines that controls the blend door.

This is often one of those things where the problem is relatively simple, but requires a ton of labor to deal with it.