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Cleaning the trails
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Loss of vacuum under acceleration closes/opens the heater doors that are controlled by vacuum. Common occurrence. Nothing is wrong. You may check to make sure the vacuum lines under hte hood have not dried and cracked. The most common one is by the firewall, passenger side. I think it is white....
 

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vacuum check valve, if you had a leak all the time it would never come out of the vents, the check valve holds vacuum from leaking back into the manifold when under load, when it goes bad under load(while hitting the throttle) it take the vacuum away from your control panel causing it to default in the defrost, one of the vacuum lines coming from the manifold will have a check valve in line, probably the one going to the vacuum resivor... check it out, you can test it by applyin vacuum on both sides, one side should hold vacuum, one shouldnt...
 
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