Just as the subject says. What are the most common symptoms of an vaccum leak?? I recently has a cam installed on my car, and now I think I have a vaccum leak. I have a hissing sound coming from the engine. I don't think it's an exhaust leak either. Any help would be great.

 

rough idle.

 

I have a pretty loopy cam, so it's kind of hard to tell.

 

It can cause increased idle (if not a severe leak) and definitely lower than your normal IAC counts will be seen. The pcm will try to compensate for the leak.