What causes the rough idle of a car engine is a bad valve, bad coil, bad sparkplug wires, or clogged fuel injector if the car is equipped with a fuel injection system.
The engine is rough on idle during startup. What causes a car to sputter and idle rough? A car's engine that sputters and runs rough while idling is an indication of misfiring of one or more cylinders if the cylinders misfire the vehicle's engine balance will be disrupted. When the engine is rough on idling it is an indication of cylinder misfiring. There is misfiring on one or more cylinders that cause unbalance, which in turn, resulted in rough idle of the engine.
The problem here is not the rough idling but rather why the engine misfire, there are several reasons why the engine misfires and it is difficult to test, however, there are two common reasons why there is misfiring on the engine. The first one is a vacuum leak and the other is a bad sparkplug wire.
Another reasons but is rarely happening and happens only to old cars or those cars that have high mileage, that is a burned valve or the valve is out of adjustments, if this happens to the engine the affected cylinders will have low compression when the engine has low compression a misfiring will occur on the affected cylinder.
However, when the car is a fuel injection system the usual cause of misfiring is a clogged fuel injector when the injector is clogged the affected cylinder will be starved of fuel which can then cause the engine to misfire and will result in rough idle which can easily be noticed during startup. Car problem cause: Vacuum Leak or Bad spark plug/wires, for fuel injection engine it is a clogged fuel injector.
Several possible reasons cause the engine to sputter and run rough hat may result in engine misfiring but the most common are:
What causes a rough idle and sputter of an engine
1) Bad valve
2) Bad coil or spark plug wires
3) If the car is equipped with a fuel injector another possibility is that one or more injectors are clogged, which resulted in fuel starvation and creates misfiring.
Car Troubleshooting
If the engine also sputters when the engine is idling but not while driving, then the problem is a bad valve. When the car sputters both when starting and at idle. But, when you start driving, the sputtering goes away. it smoothes out. This is a telltale sign that the valves in the engine are not functioning properly.
What to do?
You need a valve job. Unfortunately, this is usually an expensive procedure. If this vehicle is not under warranty, you'll have to shop around for the best price. Make sure you only have the job done at a shop that specializes in this type of work.
If the engine sputters at most speed. Pull off one of the spark plug wires and carefully lay it close to the ground (any clean, grease-free, metal surface will work). Have a friend crank the engine for a second by turning the ignition key.
As the engine is cranking, look for a spark that should jump from the end of the spark plug wire to the ground. Do this same procedure for all of the spark plug wires but make sure you put the last wire you tested back onto the spark plug before you go to the next one.
If the spark jump to the ground for all the spark plug wires tested, then there is a problem with the fuel injection system. The problem could be that your fuel injection system is not operating normally.
What to do?
Fuel injection systems are complex and hard to diagnose. If this vehicle is not under warranty, take it to a mechanic that has expertise in this particular make/model. We highly recommend you get a second opinion if the first mechanic suggests that you need a new fuel injection system. Very often, the problem is not the fuel injection system itself but rather one of its sensors. A skilled mechanic will be able to determine if all the sensors are working properly.
If the spark does not jump to the ground for all the spark plug wires tested, then the problem is a bad coil or bad spark plug wires.
In the beginning, it was indicated that this car sputters while starting. However, the car also sputters at most driving speeds. This strongly suggests that there isn't ample current reaching one or more spark plugs, thus causing the engine to misfire or sputter. As you check each of the spark plug wires. If at least one of the sparkplugs, you didn't notice a spark jump to the ground as the engine was cranking. This means that either the spark plug wire is bad or the coil isn't providing enough current to the spark plug wire.
What to do?
In this situation, it is hard for anyone but a professional mechanic to determine which is bad: the coil or the spark plug wires. If this vehicle isn't under warranty you'll need to find a mechanic. When talking to a mechanic, explain to him just what this article explained to you.
When you notice your engine to be rough on idle the first thing to do is to replace the spark plug and the spark plug wires, usually, by replacing these parts the problem can be solved. If you do not know how to replace the spark plug/wires, refer to the car owner’s manual but if you’re not sure about replacing the parts by yourself then bring the car to a shop then tell the mechanic about your diagnosis and ask to check the spark plug/wires to confirm that it is the cause of the rough idle and replace the parts to fixed the problem.
If that does not solve the problem the next thing to look at is the vacuum leak. If your car is fuel injectors ask the mechanic to check the injector for any clogging and repair if necessary.
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