The pumping of an engine’s pistons creates a vacuum in the cylinders, and that vacuum is filled by air drawn in through the intake system. If leaks develop in hoses or engine parts downstream from the air intake, that means additional air is going into the engine that’s not being “read” by the engine computer, which determines how much fuel to inject into the cylinders based on the amount of air coming in. The result is an incorrect air-fuel mixture than can create a number of problems, such as hard starting, a rough idle, hesitation, misfiring or a drop in fuel economy — issues that could, without proper diagnosis, seem to have other causes, such as incorrect ignition timing or fouled spark plugs. A vacuum leak can trigger a “check engine” light because the fuel system is running too “lean,” meaning it has a greater mix of air than is ideal. Vacuum leaks can develop in many locations, including the intake manifold, throttle body evaporative emissions system, power brake booster, and several other places, because some vehicles have a multitude of vacuum hoses.
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