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Answered 2011-09-13 22:24:51
AC blowing through defrosterI think I can point you in the right direction. On my van, this happens when the engine is under a load (such as accelerating) and vacuum drops. It returns to the vents when desired speed is reached and vacuum returns.

However, this problem occured with my van at all times and it turned out to be a vacuum leak. I had a broken line that went to the vacuum motor controlling the blend door for the ventillation system.

There are some vacuum lines that come through the firewall, from inside, and go to vacuum ports on the intake manifold. Follow these lines back to the motor and inspect for breaks or holes. If you are unable to find one, make sure the engine is warm, running, and AC blowing out the defrosters, and spray something flammable like penetrating oil (I've always used CRC brand but any will work) on the lines, connectors, and even where the intake manifold is bolted to the cylinder heads. When you find the vacuum leak, engine idle speed will increase. If that is not successful, you may want to check the vacuum lines under the dashboard. Your problem is definitely vacuum related and is probably near the motor because it seems to happen after the engine warms up.

Hope this helps!Mark

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