Many BMW 318i owners face a myriad of idle issues, especially on older models with questionable maintenance history. In normal operation when you start a cold vehicle, the 318i on-board ECU raises the idle RPM to an increased value until the engine reaches a preset operating temperature. At this point the idle begins to decrease until it finally reaches and stays at an assigned value. Several engine components look after and monitor this process. If even one of them malfunctions, a poor idle condition may occur.
Symptoms of Poor Idle
The BMW 318i suffers from a variety of idle problems, including rough/low idle, and erratic or surging idle. Associated with ongoing engine rpm increases and decreases, a "hunting" or wandering idle condition frequently occurs when engine rpm is unable to stabilise at any value. The rpm typically wanders from 700rpm to 1500rpm or higher. However, a rough idle results from an engine struggling to stay running due to a low rpm, often giving off strong vibrations felt throughout the passenger cabin.
These issues occur due to engine components which regulate idle functions. If the idle rpm is unusually high, a stuck open thermostat is the likely culprit, not allowing the engine to properly warm up to full operating temperature. In addition, check vehicle timing. A too-aggressive setting contributes to raised idle levels, causing the engine to "run rich", which in turn burns excessive fuel. You should check the MAF, (mass air flow) O2 sensors, and the ICV (idle control valve) for problems. They may need replacement. These components are prone to wear and fail over an extended period of time. In many cases a "check engine" light appears on the driver's side dash console, providing diagnostic information. It can quickly identify any malfunctioning components.
Be sure to perform annual engine tune-ups on the 318i including replacement of the air filter, spark plugs/wires and even the O2 sensor. In an effort to reduce deposits in the gas tank, fuel lines and engine, pour a bottle of fuel system cleaner into the gas tank when performing an oil change. This often cures hesitation and rough idle.