Now that you have known what vacuum leak is and what it does to your engine, how can u detect a leak?http://www.automobileglobe.com/what-is-an-intake-vacuum/ One way is to visually check all the vacuum hoses and piping. Look for disconnected, loose or cracked hoses, broken fittings. Vacuum leaks are most times the elusive needle in a haystack. And if it is not a hose leaking vacuum but something else such as a gasket, worn throttle shaft, injector O-rings, etc., you may never discover it with this technique.
A faster method for finding intake manifold vacuum leaks is to get a bottle of propane and attach a length of rubber hose to the gas valve. Open the valve so you have a steady flow of gas. Then hold the hose near suspected leak points while the engine is in idle. If there is a leak, propane will be drawn in through the leak. The resulting discovery and solution in the engine’s air/fuel ratio should cause a noticeable change in idle speed and/or smoothness (N/b: on engines with computerized idle speed control, first disconnect the idle speed control motor first).
http://www.automobileglobe.com/how-to-detect-engine-vacuum-leak/

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