Rough Idle / Misfire

When you’re driving, few things will get your attention more quickly than an unexpected change in the normal smooth running of your vehicle’s engine. When it happens, regardless of driving conditions, it commands your full attention. Your first thought is likely to be, “Now what?” followed shortly by a vision of dollar signs flying out of your wallet.  

 A rough idle/misfire usually starts out as being merely a nuisance, but if the causes are not identified and corrected, it may result in your engine not starting, or stopping altogether when you least expect it.

A misfire will cause the engine to momentarily stumble, or lose rpms, and then regain its normal engine speed. The misfire will usually reappear, either under specific operating conditions, or randomly. A misfire may occur when your engine is idling, causing a rough or uneven idle.

What to Look For

The first thing to do if you’re confronted by a misfire or rough idle, is to gather as much information as you can to assist your mechanic in diagnosing the problem. Visually inspect the engine area of your vehicle—when the engine is not running, of course. You don’t have to be a trained mechanic, but look around to see if anything strikes you as being wrong, such as obviously loose or unconnected wires, broken parts, fluid leakage, or anything else that looks out of place and make a written note of it.

Also, keep a written log about the problem, noting specifically the circumstances of the misfire such as whether the engine is cold or has warmed up; the speed it occurs (low speed or high speed); the frequency it occurs; and if it occurs only when accelerating or at a steady speed.

Causes

Many system malfunctions can cause misfires or rough idling. Ignition systems, including spark plugs, plug wires, ignition coils, and ignition timing are major players, as are lean air/fuel mixtures and mechanical issues within your engine. Worn piston ring, worn or broken valves, and worn cylinder walls, as well as worn cam lobes may contribute to the problem.

Another issue that is especially difficult to track down and identify are vacuum leaks in the intake manifold or throttle body that can cause misfiring. 

What to Do

Many things may contribute to the problem and misfiring could be caused by a combination of issues. Your best bet to identify and cure the problem is scheduling a visit to your trained auto mechanic who has all of the diagnostic equipment and tools necessary to do the job.  

This is where your written notes and logs about the misfire/rough idling will come in handy. Demonstrating to your mechanic the details about how and when the problem occurs can give him/her a head start on solving the problem and get you back on the road as soon as possible.

Spark plugs play an important part in narrowing down the cause of your misfire. By removing the spark plugs, a trained mechanic can get good clues as to the cause by ‘reading’ the firing ends of the plugs. The color, wetness, and physical condition of the plug tips, will give your mechanic hints about the cause—whether it’s electrically related, fuel/air mixture related and which cylinder is misfiring.  

If the resolution of your misfire requires new parts, such as spark plugs and/or an air filter, insist on brand name parts from manufacturers who have a long history and have established a reputation for brand quality, such as Champion spark plugs and filters.

If it’s time, or getting near the time for your vehicle’s regularly scheduled maintenance, this is a good time to attend to it is while your vehicle is already in the hands of your mechanic. 

Results

Getting your vehicle back after the rough idle/misfire has been cured and with all of your scheduled maintenance up to date will make you think you have a new vehicle. No more suffering with a misfire and no more stress about what is wrong and how you’re going to have it fixed. It’s like having a new vehicle, but without the payments.

The content contained in this article is for informational purposes only and should not be used in lieu of seeking professional advice from a certified technician. We encourage you to consult with a certified technician if you have specific questions or concerns relating to any of the topics covered herein.