Why Does a Car Have Unstable Idle Speed?

[OEmersyn]

Reasons for unstable idle speed and shaking in cars include: 1. Severe carbon buildup in the engine. 2. Dirty throttle body or carbon deposits in fuel injectors. 3. Faulty spark plugs, high-voltage wires, or ignition coils leading to poor ignition system performance. 4. Unstable fuel pressure, insufficient fuel pump supply pressure, or incorrect readings from the intake pressure sensor. 5. Clogged fuel lines causing inadequate fuel supply. 6. Aging engine components. Solutions for unstable idle speed and shaking: 1. Clean carbon deposits from the engine and idle speed control valve. 2. Clean the fuel system. 3. Replace spark plugs, high-voltage wires, and ignition coils. 4. Clean the air filter and throttle body. 5. Replace aging engine components.

[OSilas]

My friend just ran into this issue! His older Japanese car had erratic idle speeds, and it turned out to be three common problems: First, the throttle body was as dirty as a lump of coal, with sludge blocking the air intake, causing inconsistent airflow. Second, the spark plug electrodes were worn out, with two out of four cylinders misfiring intermittently. Third, the oxygen sensor was 'poisoned,' with lead deposits in the exhaust pipe sending false signals to the ECU. An extra point that’s often overlooked: If an older car shakes right after cleaning the throttle body, it’s likely because the ECU wasn’t reset—just use an OBD scanner to recalibrate it. I’d recommend tackling the first two issues first. Cleaning the throttle body costs a few hundred bucks, and a pair of spark plugs is around a hundred—after that, the idle will be as smooth as a new car.

[OChloe]

As someone who has repaired hundreds of cars, I classify unstable idling into two categories: mechanical faults and electronic faults. For mechanical issues, focus on checking the vacuum hoses—those finger-thick rubber tubes will cause air leaks if cracked, making the tachometer jump erratically like at a disco. For the electronic control system, pay special attention to the idle speed control valve. Carbon buildup in this thumbnail-sized component can cause severe RPM fluctuations during cold starts. Last month, a German car exhibited idle hunting, which turned out to be due to a rodent-chewed wiring harness on the intake pressure sensor connector. Here’s a handy diagnostic tip: shift to neutral, rev the engine to 3000 RPM, hold for ten seconds, then suddenly release the throttle. If the RPM doesn’t settle back to around 800, there’s a 90% chance the idle control system is faulty.

[JonathanLynn]

Simply put, the engine is struggling to breathe! Especially for cars with over 80,000 kilometers, clogged fuel injectors can cause uneven fuel spray, and a dirty fuel filter that hasn't been replaced for too long can lead to insufficient fuel pressure. Both issues can make the engine shake like an asthma patient. Personal experience: Last year, my car's RPM fluctuated wildly between 500 and 1500 when idling with the AC on at a red light. Replacing the crankshaft position sensor fixed it. Special reminder for turbocharged car owners: oil leakage from the turbocharger's wastegate pipe can also cause idle fluctuations—be vigilant if you see oil stains on the intake pipe's inner wall. During routine maintenance, it's essential to have a mechanic inspect the combustion chamber's carbon buildup with a borescope.

[LeGabriella]

The most dangerous aspect of unstable idle is sudden stalling at red lights! Common causes include: ignition coil leakage leading to misfiring (you can feel obvious shaking by touching the engine cover), malfunctioning air flow meter causing improper air-fuel ratio (black smoke will puff from the exhaust pipe), and the most troublesome issue—engine mount deterioration. Don’t underestimate the rubber mounts; if they fail, the entire engine will shake like it's tap-dancing. A typical case last week: a car owner opted for cheap 92-octane fuel from a private gas station, and impurities caused the fuel pressure regulator to jam—only after cleaning the fuel system was it restored. Prioritize checking trouble codes; a basic OBD scanner (costing just a few dollars) can read critical codes like P0171/P0300.