Car shakes at idle but smooths out while driving

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Sometimes problems only show on a car at specific criteria when driving the vehicle and in this case, we will focus on why a car shakes at idle but smooths out while driving.

The most common issues are fouled sparkplugs, failing coil packs, dirty fuel filters, dirty injectors, broken engine mounts, idle control valves, too-low idle, and even performance cams.

Let’s dive into the 8 common issues why a car shakes at idle but smooths out while driving.

8 Common causes why a car shakes at idle but smooths out while driving

1.  Fouled sparkplugs

The basic things an engine needs to function are air, fuel, and ignition and the sparkplug is part of the ignition system. 

The sparkplug is something most people overlook when servicing and a fouled sparkplug can result in poor spark performance it is at its most noticeable at idle and low RPMs.

This can result in the engine shaking but once driving not that noticeable.

Keep in mind a fouled sparkplug (usually black because of soot) is a result of the engine getting too much fuel, so there might be a deeper undelaying problem.

2. Failing coil pack

The coil pack provides the power to the sparkplugs to ignite the fuel so if they are busy failing there will be uncontrolled spark issues.

This can either be intermitted spark or no spark at all, yet again poor spark is most noticeable at idle and low RPMs.

Older vehicles with only one coil and a distributor will most likely not run at all but modern vehicles have a coil per cylinder. 

Good advice is to replace all coil packs if they are older than 10 years if one fails since they are close to the “end of life” for coil packs.

3. Dirty fuel filter

Now we are focusing on the fuel side of things, and a dirty fuel filter can result in inadequate fuel supply to the engine.

A dirty fuel filter will not be able to provide the fuel needed for the engine to run right and at low RPM the engine will stutter and shake because there is too little fuel to combust.

This is a bad situation for boosted engines, turbo or supercharged, because with too little fuel the engine will run in a lean condition that can cause engine damage (sometimes so bad the only option is replacement) since detention (pinging).

4. Dirty Injectors

If the car is diagnosed with a dirty fuel filter the chances are pretty good that some contaminants have found their way to the injectors. 

There can be two situations with dirty injectors, they can be clogged so the fuel cant flow through them, or they can be stuck open resulting in flooding the cylinder with fuel, and fouling the sparkplug.

In both conditions at low RPMs like at idle, it will be noticeable, and the engine will run irregularly.

This is a danger to any engine since no fuel and too much fuel is an engine killer.

5. Idle control system failure

Situated close to or inside the throttle body of the engine you will find the idle control system of the vehicle, and its job is to control the idle speed of the car in any situation.

When the idle control does not function properly the car will idle either too low, which can cause the engine to vibrate, or too high which won’t cause the engine or the vehicle to shake.

At speed, you won’t notice it has a problem since the system only functions at idle.

6. Idle adjustment is too low

While most modern vehicles have electronic throttles there are older cars that have a manually operated throttle body that has an idle adjustment screw.

It has the same job as an Idle control system but in this case, you control it with a screw that presses the throttle blade open to raise the idle speed and closes it to lower it.

When the car idles too low it can vibrate and shake because it is on the verge of stalling.

7. Broken engine mounts

The job of the engine mount is to dampen the vibrations of an engine and thus if it is broken it will send the vibrations through the chassis.

Once the vehicle started driving and the engine goes in the higher RPMs it will smoothen out since an engine is prone to vibrate at its worst at idle.

8. Performance cams

This isn’t a problem at all but you might have bought a nice performance car, especially a muscle car like a Ford Mustang GT350, Dodge Charger SRT Hellcat, or a Chevrolet Corvette ZR1, that is equipped with a performance camshaft from the factory.

The car will vibrate idle and not while moving since the engine will run rough since the camshaft duration and valve lift are very aggressive resulting in that typical “muscle shake”.

Final thoughts on cars shaking at idle but smoothing out while driving

While most of the problems are related to the engine not being able to ignite the fuel properly, not getting adequate fuel, or getting too little air the chances are also there that your engine mounts have failed.

Hopefully, you found the article helpful in diagnosing your vehicle’s problem and getting it fixed.


How does a car act when spark plugs are bad?

The vehicles will stutter and shake at idle since the fuel cant be ignited correctly (misfire), poor acceleration, bad fuel economy, and difficulty starting.

How is a rough idle diagnosed?

Rough idle is usually identifiable when the vehicle has a shaking or bouncing sensation that is felt throughout the vehicle.

Can you drive with broken engine mounts?

Yes, but it will not be advised to drive long distances or continuously. Drive it to the nearest repair shop or dealership and get it replaced because it can cause further problems like bending lines or ripping hoses to and from the engine.