Car Shakes At Idle But Smooths Out While Driving – Causes & How To Fix

If you’ve found this post you’re probably experiencing some car vibrating or shaking issues. This could be in cases where you’re at the stop light and your steering wheel is vibrating and your car is kind of shaking and buzzing but the rest of the time while you drive, it runs smooth.

There are lots of reasons why this happens and in this post, we’re going to show you the most common reasons, and then, go through some of the most obscure reasons as well.

Now here’s one important thing to note when you experience this sort of problem – if your check engine light is on, you have to start with that. This will point you more specifically to the root cause of the problem.

In this case, you can use your code reader or go to a local auto parts store and have them take the reading for you. It’s usually free.

However, if you do not have your check engine light on, it means you’d have to diagnose the problem manually.

When your car shakes at idle but smooths out while driving, it could be a result of issues with your motor mount, spark plugs, air system, and your fuel system.

Let’s dive into all the common culprits to learn how you can diagnose and fix them.

car shakes at idle but smooths out while driving

Common Reasons Why Your Car Shakes At Idle But Smooths Out While Driving & How To Fix The Issues

1. Unstable Motor Mounts

Your engine is held in place by 3 or 4 points that are supposed to be made of rubber. The function of this rubber is to isolate the engine vibration from the car. When they wear out, they stop performing that function.

Every car is a bit different. But in general, you’re going to have engine mounts on both sides, and the front and the back of your motor.

How To Diagnose Unstable Motor Mounts

You can check the condition of each mount individually. But if you want to have an overall picture of the state of the entire engine mount system, you should follow these steps:

  • Get an assistant
  • Your assistant should start the car and put it in drive with the brake on
  • When it goes into gear, you watch the motor with the hood open and see if it moves back and forth. It shouldn’t move too much but if there’s a lot of motion with the engine overall, you know that your mounts are getting weak.

When you’re done with this, you can still check the individual mounts to see if there’s any wear on them.

Also note that if one engine mount is bad, it can put a lot of stress on the others.

How To Fix Unstable Motor Mounts

If you’re going to work on them, it’s best to have an icebreaker bar and some big sockets because the bolts on the mounts can be pretty large and very tight. Loosen them up and replace them with new ones.

car shakes at idle but smooths out while driving due to motor mounts
Motor Mounts

2. Bad Spark Plugs

The spark plugs burn the fuel and create power for the engine to use in functioning. If they’re worn out or contaminated, they will cause some vibrations.

How To Diagnose Bad Spark Plugs

You’ll need to remove your spark plugs to check them out individually. As you pull them out, you should be checking for things like leaking oil down into the cylinder. This could contaminate a spark plug.

When you get them out you check for things like:

  • Burned out electrode
  • Too big of a gap
  • Too small of a gap
  • Signs of corrosion

How To Fix Bad Spark Plugs

Cars are supposed to have their spark plugs changed at set mileages so if you don’t know when it’s time to change yours, then you should just go ahead and change them to eliminate that problem. They’re quite cheap to get.

A spark plug could be failing or going bad without any physical signs.

car shakes at idle but smooths out while driving due to bad spark plugs
Bad Spark Plug

3. Leaking Air System

The air system is what brings the air in and mixes it with the fuel at the right ratio in order to enable your engine to run smoothly and efficiently.

If your air system starts leaking or gets dirty, it can cause your car to start running rough.

The reason a vacuum leak can cause your engine to start shaking is that air is entering where the system is not expecting it to. This is all controlled by sensors. If the sensors notice that air is coming in at an odd place, it’ll throw off your fuel mixture.

How To Diagnose A Leaking Air System

You can physically see where this issue is coming from if it’s through rubber hoses. This is the most common way to find them.

But sometimes, if they’re really bad, you can hear a hissing noise while the engine is running and that can help you track it down.
Other ways to diagnose this issue would be by looking out for any collapsed rubber hoses, or hoses with cracks or splits.

In addition to having air leaks in your hoses, you can also have them in places like gasket surfaces, on the throttle body, or in your intake manifold.

In cases like these, you’ll most likely not be able to see them. But one trick is to get a carburetor cleaner, and while the car is running, you spray around your engine. This will change the idle of the engine when the leak is present and it sucks that carburetor cleaner in. This way, you can locate leaks in specific places.

Apart from your air system having leaks, it can also be contaminated with oil, water, or dirt. This can mess with different components of the system and cause your engine to run rough.

You can do a general cleaning of the whole air system using a throttle body cleaner or some carburetor cleaner.

To do this, unscrew the main air hose from the air box. Here are a couple of things to note while doing this – Some cars may not let you run with the air hose completely disconnected from the air box so you may have to just leave a little slit for the straw of your cleaner to go in.

Another thing to note is that your car may try to die when you’re spraying your cleaner into the hose so you’d need to have someone inside the car press the gas and keep the car alive during the spray.

How To Fix A Leaking Air System

The leaking hoses will need to be replaced. But in cases where the leak is caused by an improperly screwed hose, all you need to do is tighten the hose.

Related Post: Car Wont Go Over 20 MPH

4. Bad Fuel System

Just like the air system, the fuel mixes with the air at the right ratio so that the car runs properly. If the fuel system gets contaminated, it’ll affect how the car runs.

So what happens with the fuel system is that over time, bad gas or the ethanol in gas will gum the whole system up and the air-fuel mixture will get thrown off.

How To Diagnose A Bad Fuel System

When you start to notice an incorrect amount of fuel will go into your engine, resulting in rough running, lack of fuel economy, etc. Then there’s definitely something wrong with your fuel system.

How To Fix A Bad Fuel System

Get a bottle of a fuel system cleaner and put it into your gas tank to run through the whole system. This will burn up all the build-up on your valves, injectors, and fuel lines. Hopefully, this clears up your idling problem.

It’s important that note that using a fuel system will take a tank or two of gas before you start to feel the result. And you have to follow the instructions on the bottle carefully.

For a fuel system causing rough idling, some of the major culprits are your injectors and your valves. You can go in remove your injectors and clean them up really well.

You can also use a more aggressive valve cleaner that you’ll spray into your intake just like we mentioned for your air system cleaning process.

However, These would require a bit more work so you definitely want to start with that fuel additive since it doesn’t require any disassembling.

car shakes at idle but smooths out while driving due to bad fuel system
Fuel System

Related Post: Car Overheating Then Going Back To Normal


The major reasons why your car shakes at idle but smooths out while driving are issues with your motor mounts, your spark plugs, your air system, and your fuel system. But if you’ve addressed all these issues and you’re still experiencing the same symptoms, there are a number of other causes.

Some of them are easy to diagnose while some are not so easy to diagnose. You can check your timing and accessory belts to see if they’re loose or damaged.

You can also check the rubber at the bottom of your harmonic balancer or crankshaft pulley to see if it’s weak and get it replaced.