Vacuum leaks can cause a variety of engine problems, and pros charge money to find them. Here’s how to track them down yourself with a spray bottle.
Scope out all the vacuum lines under the hood. Then start the engine and spray each connection with a light stream of water. If a connection sucks in the water, you've found your leak.
A vacuum leak can cause a rough idle, high rpm, poor gas mileage and even trigger a check-engine light. Pros find leaks by filling the engine with smoke and looking for wisps of it. But you can search for leaks on your own with an ordinary spray bottle using this technique.