If you are talking about the very small hose that has a one way valve on it, you are refering to the vacuum line that controls the inside vents. On a turbo car it is important to make sure all the vacuum connections have hose clamps, otherwise you will find all sorts of odd drivability and performance problems. I have been through these before and they are a pain to diagnose, and by replacing all the vacuum lines and making sure they were clamped helped a lot.

If you are removing the manifold, now would be a good time to replace the wiring harness if it hasn't been done so already (the original crappy fall apart ones need replacing). I would also suggest cleaning out the manifold and throttle body and replacing all the vacuum hoses. You will be also be able to see the intake valves and how clean\dirty they are, which will help you decide if running some fuel injection cleaner and better fuel through the system is necessary in the coming months.

If needed, it might also be a good time to replace the fuel injector seals, they are another cause of vacuum leaks that mystify one during troubleshooting of idle problems.

Good luck,
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Zack Silver 83' 242Ti (221k)