What is the problem with no idle speed?
4 Answers
The reasons why a car has no idle speed are as follows: 1. Air intake pipe leakage: According to the principle of idle speed stability control of the engine, under normal circumstances, the opening degree of the idle speed control valve and the intake air volume strictly follow a certain functional relationship, that is, when the opening degree of the idle speed control valve increases, the intake air volume increases accordingly. If there is air intake pipe leakage, the intake air volume and the opening degree of the idle speed control valve will not strictly follow the original functional relationship, meaning there will be sudden changes in the intake air volume as the idle speed control valve changes. The air flow meter cannot accurately measure the actual intake air volume, causing the ECU to inaccurately control the intake air volume, resulting in unstable engine idle speed. 2. Switch not closing properly: Fault analysis: When the idle speed contact is disconnected, the ECU determines that the engine is in a partial load state. At this time, the ECU determines the fuel injection quantity based on the signals from the air flow meter and the crankshaft speed. However, the engine is actually operating under idle speed conditions, with less intake air volume, causing the mixture to be too rich and the speed to rise. When the ECU receives the "mixture too rich" signal from the oxygen sensor, it reduces the fuel injection quantity and increases the opening degree of the idle speed control valve, which in turn causes the mixture to be too lean, leading to a drop in speed. When the ECU receives the "mixture too lean" signal from the oxygen sensor, it increases the fuel injection quantity and decreases the opening degree of the idle speed control valve, causing the mixture to be too rich again and the speed to rise. This cycle repeats, resulting in unstable engine idle speed. When operating the air conditioner, turning the steering wheel, or turning on the headlights during idle speed, the engine load increases. To prevent the engine from stalling due to the increased load, the ECU will increase the fuel injection quantity to maintain smooth engine operation. If the idle speed contact is disconnected, the ECU will not consider the engine to be in idle speed conditions and will not increase the fuel injection quantity, thus the speed does not increase.
It's really frustrating when the car suddenly loses idle speed and stalls as soon as you release the throttle, right? I figure the common issues could be these: the throttle valve is stuck, most likely due to excessive carbon buildup; the idle speed control valve is broken and can't regulate the air intake properly; there's an air leak in the intake system, like a cracked hose; the spark plugs are worn out and can't ignite properly; or the fuel injectors are clogged or the fuel pump pressure is insufficient. The most annoying part is that the engine control unit (ECU) might occasionally malfunction and cause this too. I suggest first checking if the throttle valve is dirty—try cleaning it yourself—and then inspect if the idle speed control valve is stuck. Those rubber intake hoses are the most prone to aging and cracking, so give them a thorough squeeze check. If that doesn’t fix it, you’ll need to examine the fuel system and spark plugs. If you still can’t figure it out, head to a repair shop immediately to read the fault codes—driving with this issue is too dangerous.
Last month, my old car had the same issue—the tachometer was jumping erratically at red lights before finally stalling. The mechanic said it was mainly due to a failed idle control system, possibly caused by carbon buildup jamming the idle air control valve, an air leak in the pipe behind the air filter, or a faulty manifold absolute pressure sensor. Once, my friend’s car had a small hole in the vacuum hose, and the ECU couldn’t calculate the air intake correctly, leading to stalling. For fuel-injected cars, a faulty throttle position sensor signal is especially critical. I tried removing and cleaning the idle air control valve with carburetor cleaner myself—many cars run fine after this. If there’s an air leak in the intake pipe, you’ll hear a hissing sound. For older cars, it’s best to inspect the entire throttle body assembly, as aged rubber seals causing leaks can be a real headache.
Currently, the car's lack of idle speed is mostly due to electronic control system malfunctions. For example, the idle speed motor could be stuck by carbon deposits, preventing it from adjusting the air intake, or the intake air temperature sensor might send distorted signals, causing the ECU to misjudge. Issues with the crankshaft position sensor can directly affect ignition timing. It's also common for the mass airflow sensor to get contaminated by oil, leading to inaccurate readings. In a previous case I encountered, oxidation on the engine wiring harness connector caused poor contact, preventing idle signals from reaching the ECU. For such electronic faults, the most reliable approach is to use a diagnostic tool to read the fault codes and check the corresponding sensors based on the prompts. Trying to fix it yourself without proper knowledge can often make things worse. If it's a mechanical issue like valve leakage, it becomes much more complicated and requires professional intervention.
