What Causes Low Idle Speed?

[DavidLynn]

Low idle speed in a car is caused by low battery voltage. Here is a partial introduction to the effects of low idle speed: 1. Fuel Economy: Since the engine often needs to operate at idle, statistics show that approximately 30% of urban vehicle fuel consumption occurs during idle conditions. Therefore, reducing fuel consumption during idle operation is crucial, which is also a requirement for idle speed control. 2. Emissions: During idle conditions, gasoline engines have a high amount of residual exhaust gas in the cylinders. To ensure stable combustion, the air-fuel mixture needs to be enriched, leading to incomplete combustion and the production of harmful emissions such as CO and HC.

[MacMargaret]

Recently, I also encountered a situation where the idle speed was too low. At that time, the car shook violently during a cold start and almost stalled. After inspection, it was found that the throttle body had accumulated a lot of oil dirt, causing poor air intake. In addition, aging of the ignition system is also quite troublesome. Components like spark plugs and high-voltage wires, when used for a long time, develop larger gaps and make ignition difficult. Vacuum hose leaks are another common cause, especially for cars that are a few years old, as rubber hoses can deteriorate and leak. If the fuel injectors are clogged, insufficient fuel supply will definitely affect the idle speed. More complex issues include sensor failures, such as a faulty intake pressure sensor, which makes it impossible for the computer to determine how much fuel to inject. Finally, it's advisable to check if the engine is under excessive load, such as when the idle speed compensation doesn't keep up with the air conditioning. All these issues need to be checked and resolved one by one; don't delay.

[MaxDella]

Last time during maintenance, the mechanic mentioned that low idle speed is a common issue, mainly due to the following aspects: the throttle body is dirty and the valve is stuck in a very small position, which can be fixed by cleaning; the idle speed control valve is stuck due to carbon buildup and needs to be removed for cleaning; there is a problem with the fuel system, such as a weak fuel pump or a partially clogged fuel injector; the ignition system is aging, with weak spark plug firing energy; and finally, issues with the computer control system, such as the coolant temperature sensor providing false data leading to incorrect computer judgments. Special attention should be paid to vacuum leaks, as aging rubber hoses can create leakage points, directly affecting the engine's air intake. If you want to handle it yourself, you can start by cleaning the throttle body and idle speed motor. If that doesn’t work, you’ll need to use a diagnostic tool to read the fault codes and identify the cause.

[LaPaislee]

The most common causes of excessively low idle speed include several issues: severe carbon buildup in the throttle body obstructing air intake, the idle air control valve being stuck due to dirt accumulation, or erroneous readings from the mass airflow sensor causing the ECU to malfunction. Worn spark plugs or faulty ignition coils can also easily lead to misfires or cylinder failure. Additionally, problems in the fuel system, such as a clogged fuel filter or obstructed fuel injectors, may result in insufficient fuel supply. Pay special attention to vacuum leaks, which could be caused by loose intake manifold connections or cracked hoses in older vehicles. All these conditions can cause the RPM to drop below 700-800. It's recommended to address these issues promptly, as they may lead to stalling when idling.

[JocelynLee]

In ten years of driving, I've encountered abnormal idling a few times, especially when the RPM drops right after starting. There are two scenarios to consider: low RPM during cold starts is likely due to a faulty coolant temperature sensor, causing the ECU to miscalculate the fuel injection. If the car still shakes when warm, it's probably caused by carbon buildup in the throttle body or idle control valve. The ignition system is also crucial—if spark plugs haven't been replaced after 30,000 to 40,000 km, the increased gap reduces ignition energy. Don’t overlook the vacuum system either; for example, a leaking brake booster hose can draw in excess air. Pay attention to sensors like the mass airflow sensor and oxygen sensor—if their data is inaccurate, the ECU's control goes haywire. Lastly, don’t forget the fuel system—low fuel pump pressure or clogged injectors can lead to insufficient fuel supply.