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3 Common Causes of Rough Idle

Techs at Evans Tire Working on Car

A regularly maintained automobile engine should run smoothly and efficiently, purring away beneath your hood with few noticeable problems. Yet as an engine falls into disrepair, it often begins to shake and vibrate. You may not notice such issues while you drive, since vibrations coming up from the roadway often mask a vibrating engine.

Yet once you have stopped your car, you may find it idling with an uncharacteristic and often worrisome roughness. Fortunately, a mechanic can often alleviate rough idling without much trouble. First, however, they must determine the precise cause. This article takes a closer look at three common causes of rough idle.

  1. Engine Misfires

Perhaps the single most common cause of rough idle comes in the form of a misfiring engine. As you may know, any automotive engine contains multiple cylinders. Each of these cylinders contributes to powering your car by combusting a mixture of gasoline and air. A misfire happens when combustion fails to occur in one or more of your cylinders.

In most cases, a misfire has one of three causes. The first involves a loss of spark. Your spark plugs hold the responsibility of triggering combustion by creating a spark that ignites the air-fuel mixture. A dirty or damaged spark plug fails to create a spark. Likewise, bad wiring and crack distributor caps can prevent spark plugs from operating correctly.

Engine misfires may also stem from an incorrect air-to-fuel ratio. If the mixture contains too much air and too little fuel, it may fail to combust. This issue, known as lean fuel, happens even when the spark plug fires normally.

Finally, misfires may happen because of a loss of compression. Leaky exhaust valves and blown head gaskets often lead to this issue.

Regardless of the cause, misfires throw an engine out of rhythm. In some cases, the roughness may become so violent that your engine actually stalls out. Contact a professional as soon as possible to determine the underlying issue.

  1. Vacuum Leaks

In order to perform correctly, an engine must carefully regulate the amount of air it takes in. Unfortunately, air leaks sometimes occur in an engine. Such leaks, which go by the name of vacuum leaks, throw off the air-to-fuel ratio. Specifically, by allowing excess air into the system, vacuum leaks create a lean fuel scenario.

As covered above, lean fuel can lead to misfires which in turn lead to rough idle. In this case, resolving this issue requires that a mechanic correctly identify the source of the vacuum leak and seal it off. Leaks commonly occur around carburetors, throttle body gaskets, intake manifold gaskets, and vacuum fittings.

Other problems often masquerade as vacuum leaks. For instance, stuck EGR valves or incorrect PCV valves create very similar symptoms. Car owners should also be aware that vacuum leaks may lead to other engine idle problems. In particular, if you have noticed that your car consistently idles at a high speed, you may have a vacuum leak.

  1. Dirty Fuel Injectors

Another common cause of rough idle involves your car’s fuel injectors. As you can probably guess, the fuel injectors disperse carefully metered amounts of gasoline into your engine at specific times. When fuel injector nozzles become dirty, they often fail to supply an adequate amount of gas.

Without a consistent supply of gasoline, an engine’s RPMs may fall dangerously low. The dirtier the injectors become, the more pronounced your engine roughness gets. At some point, your engine will become so starved for fuel that it stalls entirely. Fortunately, you can often resolve the issue by adding liquid fuel injector cleaners to your gas tank.

Rough idle creates a lot of stress for car owners, especially when it becomes severe. For more information about how to resolve your car’s rough idle, please contact San Diego’s automotive maintenance pros at Evans Tire & Service Centers.