Recently I had my 1991 RS Camaro in for the 100,000 mile "tune-up" with new spark plugs installed. Car ran great for about two weeks and then started hesitating and idling rough again. However, I also have a problem with the Vent/Heater not blowing air out of the vents, it comes out of the defrost vent in the dashboard. After a while, the air will eventually come out of the correct vents and I should note that the problem is not the lack of hot air - the air is hot (or cold with the Vent), it just comes out of the wrong place for a little while. I think everything together spells vacuum leak but I can't seem to find the vacuum line that runs to the dashboard... where is it? What does it look like?

 

I think a vacuum leak is different than the vent routing.

To check for a vacuum leak.

1) visual, check the hoses and clamps in your engine bay. There are a few.
2) use a propane torch (unlit) and move it around the gaskets when your car is running. If it smooths out, you've found the leak.

There are other problems too.
1) IAC
2) check the fuel vapor stuff, the cannister, and the valve. If those are not working properly, you will have idle problems.
3) check the EGR valve and routing.
4) could be the A.I.R when running closed loop, I'm not sure how to check for it.

There are a ton of things that could be causing the problem. The first you should do is check for the vacuum leak and then adjust your Idle Air Controller, which is connected to the bottom part of your throttle body. You may have to do a reset if your idle speed is unstable. If that doesn't fix the problem, check the other stuff above. There may be other things, but I don't know what they could be.

 

I have the same problem with the vents. Its finally stuck on defrost, no more changing slowly. So im going to try to figure out why. If i figure anything out before someone else posts an answer ill let you know what i find.

 

The heater/defroster routing is powered by vacuum. The default (no vacuum) is defroster.

The vacuum line typically is a small, hard-plastic that comes off the back of the intake manifold and goes through the firewall towards the passenger-side/center. But, I'm not as familiar with TBI cars.

 

Ok heres what I found wrong on my 88 Formula 350. There is a vacuum line coming off the back/side of the plenum on the passenger side. It connects to the back side of some type of vacuum module(egr solenoid) that is bolted on under the external ignition coil. The other side of that module has 2 vacuum lines. 1 small black plastic line goes into the wire harness and runs side by side to a vacuum line that comes out of the harness. That black line that came out of the wire harness goes to some type of bypass valve (has the 1 line of radiator fluid coming from the throttle body which turns into 2 lines which 1 goes to the heater core and the other to passenger fender to a line to the water pump) Now the other line from the egr solenoid goes to a wye which goes to the bottom backside of the tpi and the 2nd line from the wye goes to the cruise control( I dont know if these are suppose to be that way, that is how they are on my car) The Vacuum line from the manifold to egr solenoid had a sharp bend in it from the routing. Because of the bend it didn't seal tight, and the heat also has stretched it which I am guessing why it caused the leak. I replaced the line and then tried the vents. I then heard a vacuum leak inside behind the heater controls. But if I moved the control to cool the noise would stop. Figured out that the lines in the wire harness (1 coming from egr solenoid) were switched. Changed them around and everything worked right which also fixed my rough idle that happend the same time the vents quit.