For an automobile to operate properly, an adequate amount of vacuum pressure must be present on the air intake side of the engine. This vacuum affects the careful balance of air and fuel that powers the engine. Diagnosing a vacuum leak can be done with a little knowledge of the telltale signs.
Engine Idling to Fast
If an engine without computerised controls is idling too fast despite your attempts to adjust the carburettor idle screw, or adjust the air-bypass control on a fuel-injected car, there could be an air leak past the throttle. Common leak paths include the carburettor and throttle body gaskets, carburettor insulator spacers and the engine's vacuum fittings, hoses and accessories.
Stalling or Rough Idle
A serious air leak can alter the air-fuel mixture so that the engine won't idle at all. There are many other engine issues that can cause this problem, such as an EGR (exhaust gas recirculation) valve that is stuck open and an incorrect PCV (positive crankcase ventilation) valve.
Misfiring When Accelerating
Engine misfiring when the vehicle increases in speed may be the result of a vacuum leak. It could also be a faulty accelerator pump, dirty fuel injectors or even ignition problems.
Finding the Leak
Visually inspect all the vacuum hoses and connections for cracks or loose fittings. A quicker way to find the leak is to attach a rubber hose to a small bottle of propane. Open the valve so there is a slow steady flow of gas. With the car idling, hold the hose near suspected leaking points and listen for a change in the vehicle's idle. If the propane in sucked into the air intake, you have found your leak.