Hello there today we will be going over why a vehicle may misfire at idle.

Any car that has been maintained well should run and drive great. Even though as time goes on the engine falls into a time of needing repairs.

When the cars engine begins to vibrate and shake there may be some noticeable issues while you drive. The shaking may be hard to notice from the engine because the road may feel bumpy.

Though once the car has been stopped, the idling may feel very bad or super rough. The good thing is a Auto mechanic can fix this with not much trouble.

Here are the 3 biggest reasons as to why a engine may have a rough idle.


The biggest cause and easiest to fix is an engine misfire. This is probably the main reason as to why most engines will have a rough idle.

Every car contains at least 4 cylinders some up to 8 cylinders. Each of these cylinders provide the help of combustion of gas and air.

A misfire will occur if the combustion fails to occur in one cylinder or more. Most times a misfire will happen in only a few ways.

The main is loosing the spark as time goes on the spark plugs begin to wear. The spark plugs take the responsibility of the combustion creating a spark that ignites the air-fuel mixture.

A spark plug that is damaged or dirty will start to fail. Also bad wiring or cracked distributor caps will prevent spark plugs from operating properly.

A misfire may also stem from the incorrect air to fuel ratio. If the mixture contains too much fuel, this is know as running rich.

If the engine has to much air and to little fuel this is known as running lean. Misfires also happen because of a loss in compression.

An exhaust valve and a blown head gasket can often start to lead this occurring issue.

No matter what may cause the misfire will through the engine out of sync. This could become so bad that the engine may stall out.

Vacuum Leaks

For an engine to run correctly, the engine must properly control the amount of air the system takes in. Sadly sometimes there may be air leaks that occur in the engine.

The leaks are called vacuum leaks because it throws off the air to fuel ratio. This causes the excess air to go into the system causing a lean fuel condition.

As said earlier in the article a lean ratio will cause a misfire leading the car to have a rough idle.

For a vacuum leak this calls for a mechanic to identify the correct source of the vacuum leak. Leaks will occur mostly around carbs, throttle body gasket, intake manifolds, and other vacuum fittings.

Other problems such as a EGR valve and a PCV valve could make these same exact issues occur. Be aware that vacuum leaks may lead to other idle problems.

If a car begins to constantly idle at a high speed there may be a vacuum leak.

Fuel injectors

Other issues cause rough idle may include the fuel injectors. Fuel injectors let certain amounts of fuel into the engine causing combustion.

When the fuel injector begins to get dirty, it will begin to not give the correct amount of fuel for the combustion.

If the gas going in isn’t the correct amount the RPM of the vehicle will go dangerously low. The worse the injector is, the rougher the engine becomes.

Soon the engine will be starved for the fuel it desires and will stall entirely, luckily fuel injector cleaners work wonders.

Rough idle creates a stressful situation for any car owners so make sure to get it checked out when you can!


Misfire at idle only

This concludes the article please if you have any questions or concerns please email me. Thank you for reading.

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