Mercedes-Benz 300D Engine idle speed is high Inspection at your home or office.

How do mechanics adjust or repair incorrect idle speed?

The repair procedure depends on the exact cause of the high idle. The below lists common causes of high idle and in each case, the appropriate vehicle system, components, and parts involved would be inspected, removed for repairs, and/or replaced as needed.

Faulty Fuse: In the majority of vehicles, engine idle speed is controlled via an idle control valve (ICV). A faulty or blown fuse in other electronically controlled systems can cause the Idle Air Control (IAC) motor to malfunction, leading to a higher than normal engine idle speed.

Faulty Idle Air Control Valve (ICV): Some cars are equipped with an idle air control valve to maintain idle speed. This part can malfunction due to excessive carbon buildup in the intake manifold.

Vacuum Leak: On any engine, a vacuum leak will cause a high engine idle as the oxygen sensor detects lean operation and the engine computer then tries to compensate, which can result in abnormal idle. Malfunctioning Throttle: A malfunctioning throttle system can cause both a high or low idle as well as an engine stall. A dirty air intake or cracked air intake tube is often the culprit. Computer Malfunction: If the powertrain control module is malfunctioning, idle speed may be affected.