A vacuum leak tester is a device that provides evidence of leaks in an automobile engine's vacuum lines. These testers introduce smoke or some other sort of vaporized substance into the vehicle's vacuum system. Automotive technicians detect a leak by looking for any location where smoke or vapor comes out of the engine.
Detecting engine vacuum leaks is important because leaks in the vacuum lines cause the engine to take in too much air, worsening performance. When there is a vacuum leak in an engine, the oxygen sensor at the intake manifold causes the vehicle to take in more air than it requires. While the other oxygen sensor at the exhaust end of the vehicle can detect the extra air and force the engine to use a greater amount of fuel, the intake sensor accounts for this, and this cycle continues indefinitely. This cycle of competing sensors causes the vehicle to run poorly while also reducing its fuel economy.
Having a leak-free engine vacuum system is important for more than just preventing lean running. The engine uses the vacuum to ease the effort required in applying the brakes, and it also uses the vacuum to prevent exhaust fumes from escaping the engine before going through the catalytic converter. Numerous other small accessories and systems also either rely directly or indirectly on a strong engine vacuum.