Common problem, simple fix. Vacuum goes down when you accelerate, and for safety, these systems are spring-loaded to go to defrost mode when vacuum is lost. That means your feet might be cold but your windshield will be clear.
Look for a storage canister or check valve inline with the vacuum hose going through the firewall. Some cars use just a check valve. There is a larger valve with a small storage area to correct the problem you described if your vehicle only has a check valve now.
Wednesday, May 4th, 2011 AT 4:02 AM