What Are the Reasons for Unstable Idle Speed and Shaking After Starting the Car?
4 Answers
After starting the car, the reasons for unstable idle speed include errors in the engine control logic and faults in the air conditioning control circuit. The details are as follows: Errors in the engine control logic: It fails to receive the signal indicating the air conditioning is turned on. When unable to determine whether the air conditioning is on, the engine control unit (ECU) mistakenly assumes it is a temporary high load and attempts to increase the engine speed to counteract the resistance at idle. When the engine speed decreases, it cannot handle the load from the air conditioning compressor, leading to another increase in speed. This back-and-forth adjustment results in unstable idle speed. Faults in the air conditioning control circuit: It fails to send the signal indicating the air conditioning is turned on. Similar to the first point, because the ECU cannot determine the air conditioning's status, it continuously adjusts the idle speed, resulting in unstable idle speed.
I've been driving for over a decade and have encountered unstable idle and shaking several times. The most common cause is worn-out spark plugs - they're like the heart pacemakers of the engine. When used for too long, the gap widens and the spark weakens, making the engine breathe unevenly. Last time my car shook so badly my tea cup rattled, but replacing the spark plugs immediately quieted it down. Another frequent culprit is intake pipe leaks - those rubber hoses age and crack after about five years, letting air enter the cylinders irregularly, and of course the engine complains. Also pay attention to fuel quality - small gas stations may be cheaper but often have more impurities that can clog fuel injectors and disrupt the fuel delivery rhythm. If the shaking is severe during cold starts but improves when warmed up, it's likely due to worn engine mounts where cracked rubber fails to dampen vibrations. These checks aren't difficult, but need timely attention - otherwise you might damage the catalytic converter and face much bigger losses.
This kind of issue requires systematic troubleshooting. I usually start with the basics. Use your phone's flashlight to check for oil leaks or seepage in the engine, as oil stains can attract dust and affect wiring. Next, inspect whether the air filter box interface clips are loose—air leaks directly impact the air-fuel mixture ratio. Then, listen to the engine sound; if there's a popping noise, suspect an exhaust pipe leak. After that, press the accelerator to hold the engine at 2000 RPM for 10 seconds, then release and observe if the tachometer drops smoothly—hesitation indicates throttle carbon buildup. Also, check the exhaust pipe inner wall; excessive blackening suggests poor combustion. Battery terminal corrosion can cause voltage fluctuations—just sand it down. If none of these issues are found, it's best to connect an OBD scanner to read fault codes, focusing on oxygen sensor and crankshaft position sensor data streams.
The mechanic at my neighbor's repair shop said the cause of the vibration needs to be diagnosed by elimination. If the vibration is accompanied by flashing dashboard warning lights, prioritize checking the electrical system—test the spark plugs, ignition wires, and ignition coils one by one; the cylinder that fails to ignite is the culprit. If there are no warning lights but the vibration makes the seats tremble, focus on the air-fuel mixture—check fuel pressure, clean the throttle body, and inspect the carbon canister purge valve, as an imbalanced air-fuel ratio is the most troublesome. If the car vibrates when cold but stops when warm, it’s usually due to carbon deposits jamming the piston rings—try running two bottles of fuel additive and driving at high speed. If the vibration persists even when the engine is warm, it might require lifting the engine—either due to broken engine mounts or a displaced transmission bracket. Frequent short-distance driving is the easiest way to accumulate carbon deposits, so it’s recommended to take a high-speed drive once a month. When replacing parts, don’t skimp on cost—aftermarket parts with even a 0.5mm size difference can transmit vibrations into the cabin.
