Top 5 Causes of a Car Idle Rough and RPM High


When your car is experiencing a rough idle, your engine may chug, vibrate, or shake while it’s running. This will cause your overall driving experience to feel rough and you’ll likely be bouncing a little bit in your seat from it. When the engine idles, there will usually be a misfire that you can feel just from holding onto the steering wheel. Furthermore, the RPM of your engine will be higher than normal when the vehicle is idle. The RPM will usually go back to normal as you accelerate the vehicle, but will increase again after you stop. This is definitely not a normal situation.

Idle Rough and RPM High Causes

If you are experiencing a car idle rough and an RPM high, then there are a variety of reasons for why this could be happening. It is not always easy to diagnose the cause because there are so many factors involved here. Below are the top 5 causes that a mechanic would look for.

1. Bad Spark Plugs

If you have bad spark plugs because they’re worn out or damaged, then it could be causing your engine to be idling rough. Remember that spark plugs are what send a current of electricity into the internal combustion chamber to ignite the mixture of fuel and air. If the spark plugs are bad, then the electrical current won’t be sufficient enough for the ignition. This will cause the fuel and air mixture to burn inconsistently, which will result in an idle rough engine.

2. Vacuum Leak

There are a lot of hoses that help deliver both fuel and air to the internal combustion chamber. One very common cause of idling rough is a problem with a vacuum hose in the vehicle. The hose could be damaged, loose, or worn out, creating a leak. This could result in an abundance of air mixing with the gasoline, which would result in a misfire. Then, the result of that would be a car that is idling rough and having a higher than normal RPM.

3. Carburetor Issues

Carburetors were previously used in vehicles before fuel injectors were invented. Perhaps you have a vehicle which still has a carburetor in it. If so, then it could easily become faulty and cause rough idling of the engine. A prewarning sign of this would be if your exhaust smoke becomes black.

4. Dirty Fuel Injector

Fuel injectors are the modernized way in which vehicles deliver fuel into the internal combustion chamber. The only thing is these fuel injectors can get dirty after a while. If you are not using gas additives or fuel injector cleaners on a regular basis, then you will get a dirty fuel injector. This will result in your car idling rough. You should also see your gas mileage become weaker as well.

5. Clogged Air Filter

The engine needs air from the outside to mix with the fuel in the internal combustion chamber. There is an air filter which prevents debris from the air from getting into the chamber. But if this air filter is clogged or worn out, then there will be less clean air passing through it. This will cause the engine to idle rough.

Read also: 5 Important Tips for Parking a Manual Transmission Car

Things to Remember

Your engine should normally get around 1,000 revolutions per minute after you start the vehicle. If it is over this amount and you experience one or more of the problems listed above, then you will definitely need to make the proper repairs right away. The cost of the repair or replacement job will depend on what the cause was. If you have taken your vehicle to the auto shop quickly, then your engine should be okay. But if you’ve continued to let it idle rough for a long time, then there could be irreversible damage to the engine. This will cost you thousands of dollars in repair or replacement costs. So, don’t wait for this to happen.


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