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).
Aerosol carburetor cleaner can also be used the same method. WARNING: Solvent is extremely flammable, so never smoke or uses it if there are any sparks in the area (Eg.arcing plug wires). Spray the solvent on suspected leak areas while the engine is on idle. If there is a leak, the solvent will be taken into the engine and have the same effect as the propane. The idle speed will suddenly change and smooth out.
RECOMMENDATION: If you have a scan tool around, look at the Short Term Fuel Trim (STFT) value while you are using carb cleaner or propane to check suspected vacuum leak areas. If there is a leak and some of the cleaner or propane is drawn in through the leak, you will see a momentary drop in the STFT reading. This shows you have found a leak (keep inspecting because there may be multiple leaks!).