Why Is My Car Shaking at Idle?

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Sitting at a traffic light, why is your car shaking at idle?. hillwoman2 / Getty Images

Normally, on starting the engine, you’d expect a smooth idle, like when it was new, but years and miles tend to wear things out, and your engine might run or feel a little rough. So, why is your car shaking at idle? Here are some possible problems causing car shaking and what you can do about them.

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Engine Mount Problems

Broken engine mount
If the engine mount breaks or collapses, it'll transmit engine vibrations to the rest of the car. Paday / Getty Images

Your engine isn’t connected solidly to the frame, otherwise you’d already feel a lot more engine vibration. Engine mounts are made of semi-flexible rubber, which absorb vibrations, but if they’re worn, cracked, or broken, vibrations can be transmitted directly to the frame. Slightly more complex than rubber mounts, engine dampers incorporate air pressure or hydraulic fluid to reduce engine vibrations. You might also hear unusual banging or bumping while accelerating or downshifting.

On some engines, damping engine vibrations and movement goes a step further with active engine mounts, usually vacuum-operated, controlled by the engine control module (ECM). These are a little more complicated, involving electrical and electronic controls, vacuum switching valves, and vacuum lines and hoses. If the active mount isn’t actuated properly, vibrations might transfer to the frame.

Solution: Repair or replace faulty engine mount, valve, hoses, or wiring.

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Idle Speed Problems

carbon deposits in throttle body
Carbon deposits can obstruct air pathways, affecting idle speed control.

Aidan / Flickr

Most cars and trucks idle between 600 and 1,000 revolutions per minute (rpm), basically fast enough to keep the engine from stalling, smoothing out power pulses from each cylinder, and running the air conditioning and alternator. Idle speed might be controlled by a valve or by the throttle body, stepping up idle speed for high loads. Carbon deposits can clog up the idle air control (IAC) valve, leading to poor control of idle speed.

On some vehicles, idle-up valves are used in addition to electronic control of the IAC, though modern vehicles have completely replaced these with full electronic control of the throttle body. If equipped, the idle-up switch is essentially a vacuum switching valve, opening a vacuum line to increase idle speed, usually mounted on the power steering line. If the valve is defective or the vacuum line is pinched or clogged, turning the steering wheel would overload the engine without “idling up” or increasing rpm to compensate, resulting in low idle speed and more vibrations.

Solution: Clean the throttle body and clean or replace the IAC. Clean, repair, or replace idle-up switch or vacuum lines.

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Ignition Problems

comparing new and worn spark plugs
Worn spark plugs might not ignite the air-fuel mix as efficiently, leading to weak combustion or a misfire.

Jorge Villalba / Getty Images

In each cylinder, one or two spark plugs provide the spark that ignites the air-fuel mixture. Over the life of a spark plug, it might fire 500 million times, each time vaporizing a few molecules from the electrodes and widening the spark plug gap. Oil blow-by, a rich condition, or too much fuel can foul the plugs. If the gap is too wide or the plugs are fouled, they may not fire correctly, leading to poor performance on one or more cylinders.

Most modern vehicles have one ignition coil per spark plug, controlled by the ECM. Older vehicles may have one ignition coil per pair of spark plugs, ECM-controlled waste-spark systems, or mechanically-controlled distributor with a single coil and spark plug wires. In any case, a weak ignition coil may not deliver enough voltage to fire the cylinders properly, leading to weak combustion.

All ignition systems have some kind of spark plug wire, whether they’re long, such as on distributor and some waste-spark systems, or very short, such as coil-on-plug (COP) systems. Spark plug wires use heavy insulation to keep high voltage, in excess of 15,000 V, from “jumping to ground” instead of jumping the spark plug gap, but worn or broken insulation can cause weak spark or cylinder misfires and a rough idle. This can be especially apparent in certain humid conditions and in the rain.

Solution: Replace spark plugs, ignition coil, or spark plug wires. Repair oil or coolant leaks.

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Carbon Deposits

cracked and burnt piston
Carbon deposits can lead to hotspots and serious engine damage.

Raimond Spekking / Wikimedia

The inside of the cylinder can get especially hot, and it is kept more or less under control by the engine cooling system and engine oil, but carbon deposits can lead to hotspots, dieseling, pinging, pinking, or detonation. Normally, gasoline ignites from the heat of a spark, but hotspots can exceed this temperature, leading to premature ignition, excess noise, and engine vibrations. In extreme cases, this can lead to severe engine damage.

Solution: Start with engine top-end cleaning. Severe cases may require engine disassembly.

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Poor Compression

four cylinder head gasket
Poor compression might be linked to valves, piston rings, valve timing, or head gasket issues.

© 2006 Lewis Collard / Wikimedia

When your engine is running, just when the piston compresses the air-fuel mixture, the spark plug fires and ignites it. The ignited mixture rapidly expands, forcing the piston downward, which is converted to rotary motion by the crankshaft. If one cylinder has a leak, though, poor compression will result in less power output, unbalancing the engine and leading to vibrations.

Solution: May require head gasket replacement, valve repair, piston ring replacement, or other engine repairs.

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Sticking EGR Valve

severe EGR valve carbon deposits
Carbon deposits can cause the exhaust gas recirculation (EGR) valve to stick open.

Moosealope / Flickr

To reduce cylinder temperatures and prevent the production of certain toxic emissions, the exhaust gas recirculation (EGR) valve routes exhaust gases back to the intake, diluting the oxygen. At idle, the EGR valve should be closed, but carbon deposits can cause them to stick open. At idle, the diluted intake air doesn’t contain enough oxygen for complete combustion, resulting in random misfires and vibrations.

Solution: Clean or replace the EGR valve.

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Fuel Injector Problems

testing fuel injector spray pattern
A sticking or leaking fuel injector can skew fuel trim and unbalance the engine, leading to vibrations.

kirilllutz / Getty Images

The fuel injectors are responsible for delivering a precise amount of fuel, but contaminants or wear can lead to fuel injector leaks or sticking, injecting too much or too little fuel into the cylinder. Depending on the severity, a fuel injector leak can unbalance the engine or lead to a cylinder misfire.

Solution: Start with fuel injector cleaning. Fuel injector replacement may be required.

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Timing Problems

timing belt and camshaft pulleys
If the timing belt stretches or skips a tooth, it could affect how well the engine runs.

EyeEm / Getty Images

The timing belt keeps the camshafts synchronized with the crankshaft, but at exactly one-half speed. Timing belts and timing chains can stretch, leading to “retarded” valve timing. A skipped tooth—usually only timing belts experience this—can “advance” or “retard” valve timing. If the engine isn’t breathing well, it could cause problems at idle, including misfires and vibrations.

Because engine operating conditions vary according to demand, they need to “breathe” differently in different conditions. Airflow requirements at cruising speed are vastly different than at hard acceleration, and even more different than at idle. Variable valve timing (VVT) can account for some of these differences, enabling the engine to perform its best whatever the driver demands. Sensors and hydraulic valves are used to effect VVT changes, but faults could lead to erroneous VVT application and rough idle vibrations.

Solution: Repair or replace timing components. Clean, repair, or replace VVT valve or wiring.

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Crankshaft Damper

front of engine, pulleys, serpentine belt
This heavy pulley helps dampen engine vibrations before they reach the rest of the car.

EyeEm / Getty Images

Because multiple cylinders fire at different times during each revolution, it’s no stretch of the imagination to conclude that power output isn’t constant, but a pulsation. Every pulsation comes from a different cylinder, smoothed out by the mass of the engine, counterbalance shafts, if equipped, and other damping components, such as the engine mounts discussed earlier. On many engines, the front crankshaft pulley doubles as a damper. The inner portion and outer portion are connected by rubber, which absorbs vibrations, but if the rubber is broken, vibrations won’t be damped, transmitting to the rest of the car instead.

Solution: Replace the crankshaft damper.

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Clutch Problems

technician working on clutch
Clutch release problems might put excess drag on the engine.

GregorBister / Getty Images

On manual transmission vehicles, the clutch is engaged and disengaged by the driver. If the clutch is dragging or not fully disengaging, perhaps due to a hydraulic leak or stretched cable, it induces a load on the engine. Because the engine isn’t idling up to compensate, it can lead to shuddering and shaking your car.

Solution: Repair or replace the clutch or clutch release.

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Dirty Air Filter

severely clogged engine air filter
Ignored, this dirty air filter could choke the engine.

Ploychan / Getty Images

We’ve mentioned engine breathing, and clean air is essential to long-term engine reliability. Over time, the air filter can fill up with debris, dust, dirt, insects, and pollen. In extreme cases, a dirty air filter can obstruct the flow of air into the intake and choke the engine. At idle, at least temporarily, you might experience car shaking at idle, as well as poor acceleration. Unfortunately, continued operation would likely lead to air filter collapse, which would solve the idle and performance problems but allow totally unfiltered air into the engine, which can increase wear.

Solution: Replace the air filter.

As the engine is a complicated piece of machinery, you can imagine these are not the only problems that might cause car shaking at idle. Using these as a guide, you might indeed find something else preventing your engine from idling smoothly. Consult with a trusted mechanic for more thorough diagnosis and repair.