What are the causes of unstable idle speed in diesel engines?
3 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.
In the repair shop, I've encountered many cases of unstable diesel engine idling, usually due to fuel system issues, such as clogged fuel lines with impurities in the injector pipes, or insufficient fuel pump power causing inconsistent supply. Air system leaks leading to unstable intake are also common, like cracked hoses causing the engine to draw in incorrect air. Faulty control components, such as a bad crankshaft sensor, can cause the ECU to receive incorrect signals and improperly adjust the RPM. Mechanical problems like a misfiring cylinder or stuck valves causing uneven compression are also culprits. During diagnosis, I first check if the fuel filter and air filter have been replaced, then test for air leaks, and finally connect an ECU diagnostic tool. It's best to fix this issue early, as prolonged unstable idling can damage other components and increase repair costs. Regular maintenance can prevent such problems.
