What is a vacuum line diagram?


Quick Answer

A vacuum line diagram traces the various hoses and other devices hooked up to a vehicle engine that help create that vacuum. Every make and model of vehicle has a different diagram, making it an essential tool for troubleshooting vacuum leaks. Diagrams are found in vehicle service manuals.

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Full Answer

The vacuum is produced by the crankshaft when it pulls the pistons down into the cylinders. Air is sucked in with the motion. The throttle butterfly controls how much vacuum is produced and is one of the most important items on that diagram. The needed vacuum is only found behind the butterfly. Other items usually included in the diagram include the vacuum accumulator, sometimes called a vacuum box, and the vacuum outlet ports on the intake manifold.

The vacuum line diagram also shows different layouts within the same make and model of car. Using a 1968 Mustang as an example, that car has a different diagram for each engine type, such as the 289 and the 302, as well as for manual and automatic transmission set-ups. Mustangs with air conditioning have a different diagram than those without.

Looking at a 302 engine with manual transmission and air conditioning, compared with the 302 with automatic transmission, the diagram shows nearly identical hose routings. The manual transmission engine has an additional air bypass valve.

The vacuum generated by a vehicle engine controls almost all systems on a vintage car. As of April 2015, most of the work on today's cars is done electronically. Items still controlled by engine vacuum include brake boosters, pop-up headlights, cruise control, and heating and air conditioning vent controls.

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