A rough idle can be one of the more frustrating automotive problems to diagnose. In minor cases, you may barely notice anything wrong, and someone unfamiliar with your vehicle might not notice any problems. On the other hand, severe idle problems can make a car nearly undrivable, leading to bucking and shaking while you try to take care of your daily commute.
Unfortunately, many potential issues can cause a rough idle, and they often lead to relatively similar symptoms. If you're experiencing poor idling on your car, these three questions may help you better understand what's happening.
1. Is There a Check Engine Light?
Check engine lights get a bit of a bad reputation, partly due to frustration over faulty sensors leading to failed emissions tests or state inspections. However, the onboard diagnostic system (OBD-II) behind your check engine light provides a wealth of information. The diagnostic error codes stored by your computer can often greatly reduce the time and trouble needed to understand a problem.
If you have a check engine light accompanying your rough idle, that's good news. An excellent first step is to have an auto parts store or repair shop read the codes for free. These codes can help you understand the problem, but it's important to realize that they're just a starting point. If you don't have much experience working with cars, you'll need a pro to help you get to the bottom of your problem.
2. Are You Behind On Maintenance?
Severely rough idling isn't usually a maintenance issue, but a few problems can potentially affect your car's idle. The most common and likely maintenance item to result in a poor idle is your spark plugs. Spark plugs provide the jolt that ignites the fuel/air mixture in your car's combustion chambers, and aging spark plugs can cause your car to run poorly.
It's typical to change spark plugs only every 100,000 miles or more on modern cars. Even if you're at or near this mileage, your spark plugs probably won't cause you too many problems. Of course, if you're tens of thousands of miles beyond your replacement interval, they're worth considering. Otherwise, it's more likely something else is to blame.
3. Are You Having Trouble Accelerating?
Many idle issues disappear once you step on the gas, and your car may be perfectly smooth above idle. This behavior often indicates that your engine isn't getting enough air, a situation known as a 'lean' condition. As you increase the throttle, a higher volume of air enters the engine, making the problem less obvious from inside the car.
On the other hand, problems accelerating can point in the opposite direction. If your car bucks and fights as you step on the gas, there may be a fuel delivery issue, such as a problem with your fuel pump, fuel filter, or fuel injectors. While these aren't hard and fast rules, and there are exceptions, they're often a good indication of where the problem may lie.
For more information about auto repair, contact a local mechanic.