What are the causes of unstable idle speed in diesel engines?
2 Answers
Diesel engine unstable idle speed causes are as follows: Excessive cylinder leakage at idle: Significant energy loss in the cylinder results in lower compression temperature and pressure, poor fuel atomization, and greater combustion fluctuations. Small fuel supply at idle: Minor variations in fuel supply to each cylinder can lead to different working conditions among cylinders. This uneven fuel distribution during idle causes the diesel engine's idle hunting phenomenon. Nozzle inaccuracy in fuel delivery precision: The direct injection trend in diesel engines has led to higher injection pressures, particularly with multi-hole injectors, making small fuel quantity delivery less precise and prone to fluctuations. Extended usage and fuel injection system aging: Creates differences among cylinder injectors and delivery valves, exacerbating fuel supply unevenness and worsening idle speed fluctuations.
I've been driving a diesel car for several years and have encountered unstable idling multiple times, with the RPM fluctuating up and down like dancing, which was quite scary. The main reasons may include clogged fuel injectors, where accumulated dirt affects the injection; aged or blocked fuel filters, causing poor fuel flow like a blocked water pipe; excessively dirty air filters leading to insufficient air intake, making the engine struggle to breathe; sensor issues, such as faulty RPM sensors sending erratic signals; and even unstable fuel pump supply or ECU control module failures. Once, I didn't replace the air filter in time before a long trip, and the car shook violently on the highway, almost stalling. I suggest not procrastinating like I did—regularly replace the three filters and maintain your car. Unstable idling not only wastes fuel but also damages the engine. Address it early to avoid major repairs.
