8 reasons why your car is shaking at idle!

(Last Updated On: August 5, 2023)

When your car shakes at idle but runs smooth when driving, it can make you a little nervous. You know something is about to go wrong, but it may not be so bad. So, it brings on to answer why is my car shaking at idle?

To start with, just to set the tone, a shake could be described by one person as a vibration and by another as a violent wobble to the vehicle when idling. So, this article was put together to cover all of the bases. To start with, any minor vibration that isn’t normal through to the nasty shake, one of the eight reasons below, will likely cause the issue.

8 Possible reasons why your car is shaking at idle

Belts and pulleys

All belts used in the engine should be in good visible condition with no cracks or tears and not stretched in any way. With the motor running, you can usually visibly see that a belt is stretched or just not under any tension. You can generally see the belt vibrating as the belt wobbles on the pulleys. All straps should be under pressure, and there will only be a small amount of play. I wouldn’t recommend trying to tighten or adjusting the tightness of belts if you aren’t sure what they are or what they do. If you notice a problem, have your mechanic take a look, who will be able to advise you.

All engine pulleys should spin perfectly straight with no noises, screeching, or distortion in the pulley. Again, with the engine running, you can see a pulley that is faulty by eye; if the pulley is distorted or the bearing in the pulley is gone, you will see it vibrating.

Engine Misfire

Some engine misfires are so insignificant that they are only apparent when idling or revving the engine when idling, but as soon as you start to drive, the misfire disappears, and the engine runs smoothly. This can be any number of a reason and will probably already be on this list.

But the way to diagnose the fault and be sure is to get the vehicle plugged into a vehicle diagnostic machine. A diagnostic machine can read every parameter in the engine, such as the o2 levels, fuel temperature, DPF or catalytic converter is blocked, and so on.

Spark plugs

Dirty, incorrectly gapped, or just burnt-out spark plugs or plugs can cause a vibration. This may feel like a misfire. It may just make you notice that something isn’t right. They won’t necessarily all be at fault at the same time. It may only be one of them.

You will need to remove the spark plugs to check they aren’t covered in oil, eroded, and still in serviceable condition before refitting or changing to rule them out as a possible problem. While removing the spark plugs, check that you can’t see a puddle of oil in the cylinder to rule out engine issues.

Coil pack

The coil pack sends the spark to the spark plugs to ignite the air and fuel in the combustion process. Coil packs are notorious for “breaking down,” They either don’t send power to the spark plug or send an irregular spark. They are one of the most common causes of engine vibrations, typically a misfire.

Coil packs, when they first start to go wrong, are usually evident in a minor misfire or a light engine hesitation when under load. It isn’t the most common issue to hear a shaking at idle only; it will usually be evident when driving as well. But it has happened that they can only be apparent at idle.

Motor mounts

The motor or engine mounts connect the engine to the car’s body. Because the engine moves around slightly, they aren’t rigid mounts but are made with a tough rubber called ….. Over a period of time, these mounts become worn and allow the engine to move excessively, especially when idling.

They also have other symptoms, such as a shake when starting the engine, switching off the engine, and driving at high speeds. You may also hear knocking noises under hard acceleration, and the motor mounts can get so bad that the car will jump out of gear.

To check the motor mounts, you only need to try to move the engine with a lever bar or move the engine with your hands; it shouldn’t move around freely. You should be able to see the excessive play in the rubber motor mount. Also, with the engine running while stationary, you can visibly see the engine vibrating more than expected.

Electrical sensors

Almost all of the electrical sensors in an engine bay will cause a misfire if faulty. However, there are the off few, such as the mass airflow sensor or MAF for short, which can cause the engine to run rough at idle but clear when driving. Sometimes these sensors can have a dirty pick-up that requires removal and cleaning to clear the fault.

It’s difficult to determine which sensor is faulty without connecting the vehicle to a fault code reader. All mechanics will use one but charge to read the faults (as you would expect, the machines aren’t cheap and require constant updates, which aren’t free).

Vacuum hoses & air leaks

There are vacuum hoses that run around the engine bay that are under pressure; any splits to these hoses can cause the engine to run rough. However, it doesn’t usually clear when driving, but if it’s only a minor leak, you could potentially only feel the shaking at idle.

Air leaks from intake hoses will do precisely the same and cause the engine to run rough. You can sometimes hear air leaks as you will hear the leak whistling or hear the hose trying to draw in the air. So, it can be straightforward to self-diagnose.

Fuel injectors

Fuel injectors are some real heavy-duty mechanical faults that require a mechanic to diagnose. Dirty or blocked injectors can give an irregular spray pattern which causes problems for an engine during combustion. They need ultrasonic cleaning to rectify this type of problem.

Electrical problems with fuel injectors can be picked up by a diagnostic machine and cause vibration, depending on the type of fault, this is certainly one that can cause the engine to shake at idle, but the engine will run completely smooth when motoring along.

engine shake at idle

Is it safe to drive when your car shakes?

Like with most faults found on vehicles, it’s challenging to say it’s safe to drive without knowing precisely what the issue is. As you can see from the list above, some problems will be safe to go on a short journey. However, none of these faults allow you to drive for a long time without needing repair, so you do so at your own risk. Not only can you cause more damage to other vehicle components some of the faults can put you in a dangerous situation when driving.

Is it normal for the engine to shake a little?

An engine contains many moving mechanical components, so it is normal for the motor to shake a little. Engines shouldn’t jerk, knock or vibrate out of rhythm. You will feel the engine running, but it still should feel smooth.

It’s usually evident that your vehicle engine is shaking too much. If you have to question if it’s shaking excessively, it probably is. But, if you aren’t sure, get a second opinion.

Final thoughts

A car shaking at idle is a good indication there is something wrong with the vehicle. If you can’t determine from the list above exactly which part of the car is causing the problem, have a mechanic take a look. An engine misfire or a faulty motor mount is one of the more common areas for the engine shaking at idle.

My name is Tom although my friends call me Tommy. Messing around with cars and bikes has always been a hobby of mine even from a young age. So I made it my day job 17 years ago. I am a fully qualified mechanic as you would expect. I've worked in all different areas of the motor trade, valeting, panel beating, engine repairs, I'm sure you get the idea. I enjoy sharing my wealth of knowledge and experience with others, which is the reason I spend a lot of time here writing for this website.

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