Why a Car Idles Rough But Drives Smooth (What To Do!)

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So, what could be the reasons why a car idles rough but drives smooth? It’s quite simple, you’ve got some broken motors. Rough idling doesn’t just happen; it occurs when one or more components in the engine bay are faulty.

It is actually advisable that you take rough idling as a serious issue and ensure to fix the contributing (faulty) component as early as possible. Virtually all components that make up a car can cause rough idling when faulty.

However, the commonest culprits behind rough idling include spark plugs, fuel delivery systems, bad drivetrain, and sensors.

In this article, we will look at the various possible reasons why a car may idle rough when at low RPM but drive smoothly.

Why a Car Idles Rough But Drives Smooth?

Why a Car Idles Rough But Drives Smooth

Most definitely, this happens when the motor mounts are badly damaged or faulty. The motor mounts are used to hold the engine in place – inside the engine bay.

If the mounts go bad, the engine may be unbalanced, and as such, it may shake when you try to start the car or resume driving after being stuck in traffic.

The engine shakes when you try to start the car because of the force generated by the starter –> crankshaft —> engine.

If there’s a broken component in the drivetrain or the engine, this force would act on the component; thus, causing you to feel a quick, or steady shake/vibration before your car gets back to normal as you start driving.

Idling simply refers to the state of the engine while the car is not in motion. That is to say when you’re stuck in traffic, or you’re steaming your vehicle early in the morning, at the moment (when you’re not driving but stuck in one place), your engine is idling.

Now, rough idling refers to the uncommon shake, bounce, or vibration you may feel while your car is not moving. For example, if you’re in stop-and-go traffic, apparently, you’d be switching from the parking gear to the driving gear quite often.

If you notice that your car shakes or vibrates each time you switch to the parking gear, or when switching from one gear to another, that means to say that your engine is rough idling.

Rough idling mostly happens at low RPMs; in most cases, once you start driving, the shaking/vibration/rough idling stops.

This happens because while driving at a good speed, most of the components that cause rough idling go to rest (not really like they stop running, but their input becomes a minimum requirement to keep the car going).

What To Do If Your Car Rough Idles Constantly

The places and components to check immediately include the following:

What More?

Mainly, the most possible reason why a car idles rough but drives smooth is due to a broken part. It could be broken mounts, valves, sensors, or clogged injectors/carburetors. You need to troubleshoot your car, detect the exact faulty component and have it replaced.

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