Engine Idle Problems: Signs, Causes, and What to Do Next

When your car sits at a light and the engine engine idle, the speed at which a car’s engine runs when not under load, typically measured in RPMs. Also known as curb idle, it should be smooth and steady—usually between 600 and 1,000 RPM for most petrol engines. If it’s shaking, surging, or dropping too low, you’re not imagining it. A bad engine idle isn’t just annoying—it’s a warning sign your car’s air, fuel, or ignition system is out of balance.

What causes it? Often, it’s something simple like a dirty throttle body, the valve that controls air entering the engine, or a failing idle air control valve, a small motor that adjusts airflow to keep idle steady. But it can also point to bigger issues: a vacuum leak, worn spark plugs, components that ignite fuel in the engine’s cylinders, or even a clogged fuel injector. Many of the problems linked to rough idle also cause hard starts, poor fuel economy, or stalling—exactly the issues covered in posts about spark plugs, oil changes, and air filters.

Don’t wait for the engine to die at a stoplight. A misfiring cylinder from bad spark plugs? That’s a common cause of rough idle. A clogged air filter? It starves the engine of clean air, throwing off the fuel mix. Even old engine oil can thicken and affect valve timing, leading to unstable idle. These aren’t isolated problems—they’re connected. Fixing one often helps the others. The posts here give you clear, step-by-step checks: how to spot worn spark plugs, what happens when oil sits too long, how a dirty air filter affects performance, and why a failing suspension can indirectly mess with engine sensors.

You don’t need a garage full of tools to start diagnosing idle issues. Listen for changes. Feel for vibrations. Watch the tachometer. If your RPMs jump from 700 to 1,200 and back without you touching the gas, something’s wrong. If the engine stalls when you shift into drive, it’s time to act. Most of the fixes are affordable if caught early. Waiting might mean replacing a $300 sensor instead of a $40 air filter.

The collection below gives you real-world fixes from UK drivers and mechanics. No fluff. No theory. Just what works: how to clean a throttle body, when to replace spark plugs even if the car seems fine, how to check for vacuum leaks with a spray bottle, and why your idle might get worse in cold weather. These aren’t guesses—they’re tested solutions from people who’ve been there. Whether you’re trying to save money or just want your car to stop shuddering at traffic lights, you’ll find the answers here.