to detect a possible vacuum leak, check the intake manifold from the throttle plate and back with the engine at idle speed.
1- with a vacuum gauge connected to a non ported vacuum port (e.g. the brake booster or the pcv valve) check that you have between 17-21 inches of mercury vacuum (inHg), if it is lower, it is either a retarded ignition timing or late valve timing or a vacuum leak
2-you can use propane with regulated pressure and run a small diameter hose and search for leak by tracing the manifold gasket areas with the propane, if the idle speed goes a little higher, the vicinity of the vacuum leak had been found
3-if you have a smoke machine or the shop you take the car to has a smoke machine, you introduce the smoke into the intake manifold and find the smoking gun
4-with a small diameter hose or a stetoscope, place the hose around the possible leak areas and listen for hissing noise






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