On my 1983 Trans Am, I can not change the setting on the heater control from heat to defrost to vent, etc. No matter where I but the slide, the air blows out of the defroster vents onto the windshield. I replace the switch that engages that slide control with no results. What now?

 

You've got a vacuum leak. The vents default position is to blow through the defrost. When you slide the controls, a vacuum is applied to the vents, and it pulls them into a different position. (This is why, when the car's off, if you slide the lever you hear a "psssshhhhhhh" sound.) Check the tubing running to the back of the controls, make sure it's connected good, and then check where it connects on the other side of the firewall. Vacuum leaks can be very, VERY annoying to track down so don't get discouraged if you can't find it right away.

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89 iroc-z 305 tbi
k&n filtercharger, open element air filter. nuffin' else