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When the a/c is in Max setting, the air will sometimes switch from the a/c vents to the defrost vent. I'm guessing it's a vaccum issue somewhere because when you get on the throttle it dose it. It works fine in reg a/c mode. Any tips on where to start looking? Thanks.
First place to check is the "coffee can"......it's been known to rust out on the bottom where it can't be seen. The line going from it through the firewall is the culprit sometimes too. Others would be the lines under the dash.
Hard to find sometimes.....vacuum lines get brittle and crack right at the fitting and the crack isn't always big enough to see. I always move them a little with my fingers. If cracked they will usually break off then.
Look for a broken White vacuum line on top of the A/C box under the hood.
You may not have a vacuum tank if you do it may look like a round ball.
The Coffee Can is for the emission system.