According to About.com, there are several reasons vehicles idle low. Common reasons include vacuum leaks, carburetor problems, ignition issues, damaged power circuits and dirty fuel injectors. When a vehicle idles low, the vehicle stalls or shakes excessively when the brakes are activated, and this is an indication of a serious engine problem. Diagnosing low idle issues requires checking the carburetor, fuel pressure regulator and fuel injectors.
Underhood Service states that low idle issues are only a symptom of a more serious problem. A diagnosis of a low idle issue is achieved by checking various parts of the engine. A common issue associated with the carburetor is air problems. This issue occurs when the carburetor does not open properly to allow air to enter. Underhood Service explains that another issue that causes low idle is vacuum leaks. This type of leak occurs when the vacuum line is not securely attached or there are bends in the line.
Additionally, dirty fuel injectors can cause vehicles to idle low. The fuel injectors are normally set to equal specifications; however, there are factors that directly affect the injectors. About.com notes that, when the fuel injectors test at different levels, they are malfunctioning and require cleaning or replacing. The site recommends repairing low idle symptoms immediately before the issues cause irreparable damage.