I talk about vacuum leaks extensively in the Solving Automotive Performance Issues article. That’s because they are a very common cause of engine performance issues. They are also a common cause of poor fuel economy. If you haven’t figured it out by now, anything that causes your engine to run poorly can cause poor MPG.
Vacuum leaks can have many sources, but basically a vacuum leak is an air leak anywhere between the back of the throttle plate and the back of the intake valve on your engine. Vacuum leaks can be caused by a hose that came off or rotted, a leaking intake gasket, or a failed vacuum-controlled component.
A tip: Some HVAC systems use engine vacuum to operate their components. This means that you can possibly have a vacuum leak under your dash. If you have one of these systems, be sure to check it for vacuum leaks if you have a performance issue or poor MPG. Vacuum leaks are at the top of my list of things to check when I have a performance problem. Checking for them is simple enough.
This video shows how you can check for vacuum leaks with a can of carburetor cleaner. You can also use a spray bottle with a little water in it to do the same test to help eliminate the possibility of fire when doing the test. You can also use smoke to help you find vacuum leaks. More information on vacuum leaks and how to find them can be found in the Solving Automotive Performance Issues article.