Vacuum leaks are one of the biggest causes of engine performance problems. This video is pretty straight forward in it's approach to finding a vacuum leak. On thing I might mention is that not all vacuum leaks are at the engine. Some HVAC systems run on engine vacuum and can cause a leak under the dash in some cases. I've even seen vacuum operated door locks. Just be sure to be careful spraying flammable stuff on a hot engine, you don't want to take one problem and make it a whole lot worse. You can also use a spray bottle with water to do this same test. It may not be as effective, but you don't have to worry about spraying a flammable substance on a hot engine.
For more information on diagnosing engine performance issues, check out this article I wrote.
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