Abnormal idling of your car's engine may indicate a vacuum leak in the engine's intake manifold. You can use a propane torch to determine if that's the case. Although the thought of using propane to find a vacuum leak may sound dangerous, it poses no danger as long as you don't ignite the propane. By allowing propane gas to escape the tip of a propane torch, you can check for leaks along the intake manifold and listen for changes in your engine's idling. A change in the idling means you've found a leak.
Turn on your engine and lift the hood of your vehicle. Locate the intake manifold. The location of the manifold varies, but it's easy to spot. It's a single metal component that looks like a series of straight or spiral metal tubes. Hoses are connected to this manifold.
Open the valve of a propane torch. Do not ignite it; just open it so that a bit of propane gas can escape the tip.
Run the tip of the propane torch along your intake manifold, checking for vacuum leaks in various locations such as the tubes and gaskets. Go along the entire manifold.
Listen for a change in your engine's idling. The idling should change immediately after you pass over a leak with the tip of the propane torch. The reason the idling changes is because the engine will suck in the propane through the leak, providing the engine with more fuel.