Car Shakes When Idle: Troubleshooting Guide

Car Shakes When Idle: Troubleshooting Guide
Car’s shaking when idling is one of the most common and, at the same time, one of the most vague problems that occur. There can be numerous causes and reasons for car shakes however, rough idling should not be taken lightly as, in many cases, there could be serious underlying causes.
Luckily, it’s possible to narrow down the wide variety of causes to several, which occur extremely frequently. Additionally, you can always narrow down the causes with an OBD scanner, the usage of which we’ll discuss in this guide.

What causes car vibrations when idling?

Rough idling will most often happen due to issues with fuel injection and combustion. If there’s too much (or too little) air, not enough fuel injected, or the engine misfires, these will likely cause rough idling.

Outside of fuel injection and combustion issues, there could be issues with the motor mounts or failures in the oxygen sensor.

Extremely minor vibrations when idling, however, is no cause for concern. But if you start noticing increased levels of vibrations or even shaking when idling, stopping, or moving at lower speeds, that may indicate a major issue.

Your car is supposed to remain in smooth operating condition when doing all three. Some other indicators that something is going wrong could be shaking of the steering wheel or vibrations in the engine compartment.

Troubleshooting car vibrations

If you have no experience in car maintenance, it’s often best to bring your car to a professional auto repair service. 

Use these common causes only as pointers towards potential issues.

1. Dirty fuel injectors
As mentioned above, rough idling is often caused by issues with the car’s fuel intake system. Fuel injectors are one such part that’s commonly affected due to prolonged use without maintenance.
Fuel injectors do exactly what they say - they provide the necessary amount of fuel to the ignition system. All fuel injectors are carefully created and optimized to provide precise amounts of fuel to the engine’s cylinder. 
They are relatively close to the combustion source, which emits immense pressure and temperature. Since fuel injectors have tiny nozzles, even small amounts of particles may cause them to become clogged up.
If they fail to dispense the correct amount of fuel, it can cause car shaking and poor performance.
2. Bad spark plugs, wiring, or ignition coil
Another part of the fuel intake and ignition system are spark plugs and ignition coils, which are used to generate an electric charge to create a successful combustion. 

The ignition coil generates a large surge of voltage, which is then carried through the wiring (and a few other parts) to the plug, which generates the spark that ignites the fuel mixture.
Spark plugs slowly collect oil or carbon deposits over time, causing them to become less efficient at producing the electric discharge. Since they cannot properly generate sparks, worn plugs can cause rough idle and misfires. 
Worn spark plugs are also one of the causes for the check engine light to appear.
3. Faulty glow plugs
Another part of the combustion system that’s used in diesel engines instead of spark plugs are glow plugs. If they are worn out, they may fail to heat up the mixture enough, causing misfires or rough idling.
Rough idling isn’t the only effect of worn out glow plugs. In fact, it will often be easiest to notice faulty glow plugs when starting the car in cold weather as they will also cause hard starting.
So, if you have the combination of hard starting and rough idling, faulty glow plugs could be the culprit.
4. Faulty timing or serpentine belts

A faulty timing belt can cause a whole host of other issues, not just rough idling. As the name states, it’s essentially a coordinator for different parts of the vehicle. Faulty timing can cause rough idle due to pistons and valves falling out of sync.
However, the issues may not be directly related to the timing belt itself as there’s numerous parts to the camshaft chain. The tensioner and sensor can also be faulty, both of which bring their own issues that can also cause rough idle.
A faulty serpentine belt can also cause similar issues, although there will be other effects to it such as unusual noises (a squealing or chirping sound), sudden engine performance loss, etc.
5. Loose vacuum hoses
Vacuum hoses connect many parts of the car, providing, essentially, managing energy production by changing the amount of air delivered to them. Even small incisions or holes in them can cause fairly major issues, one of which is rough idling.
A vacuum leak can cause too much air to be delivered to the intake manifold, which can affect the fuel mixture. An improper mixture can then cause misfires and poor performance.
6. Loose gas cap
An incredibly simple, yet unfortunately common occurrence is a loose gas cap. Not tightening it up enough after filling up with gas can cause air to enter the intake system.
It’s also the easiest to fix as all it takes is to tighten up the gas cap. If it’s broken or cannot be fixed into place, a replacement may be necessary.
7. Damaged motor mounts
Motor mounts keep the engine attached and secured in place. If they are damaged or improperly secured, that can cause engine shaking. 
If your car stops shaking when you shift into Neutral, that could indicate that the motor mounts are to blame.
8. Failing oxygen sensor
If the oxygen sensor fails to deliver accurate readings to the vehicle computer, that could cause the fuel mixture to be ineffectively adjusted. As with many other issues, bad combustion or fuel mixture ratios can cause engine vibrations.

Is it dangerous to drive a car that shakes at idle?

Yes, the car is not only more difficult to control, but it could also indicate other problems. Even if you can control the car when it’s shaking, damaged components may cause other unexpected occurrences, which can cause you to crash.
Additionally, most of the car vibrations are caused by various components failing to properly create combustion and improperly providing mixture to the fuel intake system, all of which indicate larger issues with your vehicle. Continuing to drive it, even if successful, can cause component breakdown, which can be costly in the long run.
So, you’re not only risking your own and the lives of others, but continuing to damage a car at a faster pace. It’s best to get the car to a repair service if it starts shaking when idle. You’ll get a full overview that will verify proper functioning of many of the above parts and may include others such as the idle air control system, the throttle valve and many others.

Diagnosing idle vibrations with OBD diagnostic tools

Many of the component failures listed above will have their dedicated fault codes that can be retrieved from the on-board diagnostics system. For example, most misfires will have an associated code P0300 to P0308.
OBD2 diagnostic scan tools can help you retrieve these codes and narrow down the possible causes. For example, in the example above, such an error code would tell you that it’s not the gas cap, motor mounts, or belts. It would also be more likely that it’s related to the intake and combustion components.

OB2 scan tools are relatively cheap and simple to operate. If you’re a car enthusiast that likes to tinker and repair cars, these diagnostic tools can save tremendous amounts of time and, potentially, money.
Get OBD2 diagnostic tool
Disclaimer: Advice, how-to guides, and car care tips on our blog are intended as helpful resources for general maintenance and repairs. While we strive for accuracy, the information is provided to the best of our knowledge and should be used at your own discretion and risk.

Always refer to your vehicle's owner's manual or consult a certified mechanic for specific repair details and safety procedures.