What Causes Engine Idle Fluctuation?
2 Answers
Reasons for engine idle fluctuation: 1. Severe carbon buildup in the engine: Carbon deposits are the main cause of engine shaking. When excessive carbon accumulates inside the engine, the fuel injected by the cold-start injector gets largely absorbed by the deposits, resulting in an overly lean air-fuel mixture during cold starts and making cold starts more difficult. Solution: Regularly clean the fuel system, inspect the idle air control valve, and clean the throttle body. 2. Ignition system malfunction: Spark plugs are wear items that require periodic replacement. If excessive carbon buildup on spark plugs or excessive electrode gap is found, replacement is needed. Additionally, inspect the condition of ignition wires and coils. A vehicle diagnostic computer can be used to assess the ignition system's status. 3. Fuel pressure issues: Check whether fuel supply pressure and intake pressure sensors are functioning properly. Abnormal fuel pump pressure or faulty readings/operation of the intake pressure sensor can both cause engine shaking.
Last time I drove my old car, I noticed the idle speed was jumping quite noticeably, with the RPM needle dancing up and down. This is mostly caused by carbon buildup in the engine clogging the throttle body, or spark plugs aging and failing to ignite properly. A dirty mass airflow sensor can also send incorrect data, causing the fuel delivery system to go haywire. A vacuum leak is the most annoying—when the engine sucks in extra air, it’s bound to shake. Later, I replaced the engine mounts, and it got much better—the old rubber had degraded and couldn’t absorb vibrations anymore. Remember to clean throttle body carbon deposits every 20,000 km, or fuel consumption will spike and damage the engine. New cars nowadays have automatic adjustments, but older cars require more frequent maintenance.
