What Causes Low Idle Speed in a Car?

[Ella]

Low idle speed in a car may be caused by low battery voltage; carbon deposits; the control unit receiving incorrect signals and issuing wrong commands, leading to inaccurate control of engine idle air intake and abnormal combustion. It can also result from faulty idle air actuators causing inaccurate idle air control. The impact of idle speed on gasoline engines is as follows: 1. Fuel Economy: Since engines often operate at idle, statistics show that about 30% of urban vehicle fuel consumption occurs during idle. Therefore, reducing fuel consumption during idle is crucial, which is also a requirement for idle control. 2. Emissions: During idle, gasoline engines have a high amount of residual exhaust gas in the cylinders. To ensure stable combustion, the mixture needs to be enriched, leading to incomplete combustion and the production of harmful emissions like CO and HC. Experiments prove that lower idle speeds increase the dilution effect of exhaust gases, further raising CO and HC emissions. Thus, increasing idle speed helps reduce CO and HC emissions. Relevant data and experiments show that increasing engine idle speed from 700r/min to 800r/min reduces CO by 10% and HC emissions by 15%.

[GiannaDella]

When I encounter an idle speed issue, my first thought is to check if the throttle body has too much carbon buildup. Over time, this part can easily get clogged, especially in city driving with frequent stops and starts. Dust and oil residue accumulate, leading to restricted airflow and a drop in RPM. Another common culprit is the idle air control valve (IAC valve), which regulates the idle valve's opening and closing. If it burns out or gets stuck, the RPM won't rise properly. Worn-out spark plugs could also be the issue—weak ignition means the engine lacks power. A clogged air filter can affect the air intake volume too; I once replaced mine, and the problem was immediately resolved. Vacuum hose leaks are another factor—cracks or aging can cause air leakage, leading to unstable idle speed. In short, don’t ignore this problem. At best, it might cause stalling or rough idling; at worst, it could damage the engine. It’s best to get a thorough inspection or have a mechanic fix it for peace of mind.

[Jayce]

In my driving experience, excessively low idle speed is often related to the fuel system. For example, clogged or dirty fuel injectors can cause uneven fuel supply, leading to a drop in RPM. The idle speed sensor may also malfunction, sending incorrect signals to the ECU for idle control. Issues with the EGR valve can also interfere with idle stability if exhaust gas recirculation is not properly managed. On a long-distance trip, I once encountered a similar situation, which turned out to be caused by loose or oxidized wiring connections leading to poor contact. Checking the fuses is also crucial, as a blown fuse can cut off the relevant circuit. I recommend regular maintenance checks, including cleaning the throttle body and fuel system, to prevent most idle-related issues. If your vehicle's speed becomes unstable, be sure to pull over and inspect it to avoid increased fuel consumption or potential accidents. Safety comes first—repair what needs to be repaired.

[McLawson]

From my experience, an excessively low idle speed may be caused by erroneous signals from the engine control unit, such as false readings from the temperature sensor leading to idle speed drops. Issues with the idle air control valve or throttle position sensor are also common culprits. If the mass air flow sensor is contaminated with deposits, inaccurate air intake measurements can cause RPM fluctuations. Additionally, insufficient fuel pump pressure leads to unstable fuel supply and lowered idle speed. Faulty spark plugs or ignition coils can also drag down the RPM. These components should be cleaned when possible, or replaced if necessary - don't invite bigger troubles by trying to save small money.