A misfiring engine is one more serious car problem that you may come across.
It usually happens on older, high-mileage cars, mostly due to material fatigue of the engine parts as well as the inevitable deteriorating of various engine systems.
But it’s no stranger to newer cars either. A badly manufactured part, poor maintenance, inadequate fuel quality, or else can cause problems.
In this article, we’ll be talking about the meaning, symptoms, causes but also possible solutions to a misfiring engine.
WHAT DOES AN ENGINE MISFIRE MEAN?
To understand this problem, some previous knowledge about how the inner combustion engine works is necessary.
In short, the inner combustion engine uses a mixture of air and fuel which is ignited by the spark plug to push the pistons inside the cylinder.
The pistons then push the crankshaft which is connected, through the gearbox to the wheels of your car.
This is a basic explanation and if you have the time and will, click here for a great, in-depth explanation on Wikipedia.
Anyway, proper engine combustion and thus engine function depend on three key factors:
All three must be supplied in an orderly, quality, and timely manner. Of course, all three factors depend on their separate systems working properly and harmonically with one another.
To conclude on this part: an engine misfire is an interference in the combustion sequence caused by various factors that involve the air-fuel mixture or the spark igniting that mixture.
MISFIRING ENGINE SYMPTOMS
In this part, the symptoms are stated as the problems progress. Of course, every engine is different and the order of appearance may be more or less different.
Also, driving habits and the level of maintenance has a significant role.
But the usual scenario for a misfiring engine is:
This is the first tell-tail sign in most cases.
Instead of smooth idling, you’ll get an occasional fluttering sound and fluctuation in the rev count.
As the misfiring engine problem gets worse, so will the idling get more rough and uneven.
When you press the acceleration pedal, the car won’t speed up as usual.
At first, it will be slightly noticeable, but over time it will get more and more sluggish.
INCREASED FUEL CONSUMPTION
You will notice an increase in fuel consumption anywhere from ten percent, in the beginning, all up to fifty percent if the misfiring engine problem gets out of hand.
If you want to read more on the fuel consumption topic, click here for a separate article.
FREQUENT SMELL OF FUEL
The increase in fuel consumption is usually followed by a strong smell of fuel when the engine is running.
In the beginning, it’s noticeable only when the engine is cold but as the problem worsens, the scent is present always.
This is the case when the misfiring engine is caused by a rich fuel mixture (too much fuel in the mixture).
CHECK ENGINE LIGHT
By the time all of the previous symptoms appear, the check engine light should come up on the dashboard.
In the case of a misfiring engine, the most common cause will be a fault from the O2 sensor (aka oxygen sensor). This is because of the interference in the engine combustion and the exhaust gases will not be as normal.
The common fault codes, if you hook up and OBD tool are p0420, p0141, p0142, p0135, or others all depending on the car model and car manufacturer.
Check engine light may appear occasionally in the beginning while in the later phase will stay lit up all the time.
More on the check engine problem in an article which you can read by clicking here.
I must mention this although this warning light is exclusive to VAG group cars. This warning light stands for Electronic Power Control and is a part of a computerized ignition and engine management system.
It may come up if the engine misfires, either separately or in a combination with the check engine light.
Other cars may have a similar warning light (perhaps with a different abbreviation) so it’s worth checking out.
For more information on the dashboard warning lights, click here for an article on that topic.
Sputtering, poping, and in more severe cases backfiring is what you’ll encounter with a misfiring engine.
The first two usually appear in the early phase of the problem while backfires appear later when things get worse.
Backfiring is the most radical and, in worst cases, it can sound like a very loud bang.
VIBRATIONS COMING FROM THE ENGINE
Once you raise the hood, instead of a calm engine, you’ll probably see one that looks like it wants to jump out of the engine bay.
A pretty frightening scene as the engine will shake and jolt from one side to the other (this happens in more severe cases).
When the situation is not that bad, you’ll only see a mild shaking.
In any case, the engine will produce vibrations that can be felt even in the passenger cabin.
When you press the accelerator pedal suddenly the engine will react with a delayed and indecisive response.
The delay may get worse as the problem progresses.
ABNORMAL EXHAUST FUMES
Normal exhaust fumes highly depend on proper combustion.
If the engine is misfiring, you probably won’t get clear invisible fumes. Instead, you might get pale smoke coming out of the tailpipe accompanied by a strong smell of fuel.
MISFIRING ENGINE CAUSES AND SOLUTIONS
Spark plugs produce the spark needed for igniting the air-fuel mixture.
A worn-out or damaged one will give a weak spark or none at all.
This means the air-fuel mixture inside the cylinder won’t be able to properly ignite and this causes a misfiring engine.
Changing the spark plugs is best. Another option is the cleaning and checking of the electrode gap if the spark plug is still in good shape.
Ignition coils have the purpose of transforming the low voltage of the car electric installation into high voltage.
High voltage is necessary for the spark plugs to produce a good spark.
Change the faulty ignition coil (one or more).
It’s not necessary to change them all if only one is faulty. On this part, using an OBD tool is common practice as it will give you a code for the exact ignition coil that’s malfunctioned.
CONNECTIONS TO THE IGNITION COILS
The plastic connectors on top of the ignition coils are worth the check.
Due to constant exposure to engine heat, they can get brittle, crack, and eventually lose contact.
Another common reason is a build-up of filth and rust inside the connector.
The connector should be removed from the ignition coil and thoroughly cleaned with a contact spray. If the connector is severely damaged it should be replaced completely.
While checking the connectors, a brief inspection of the wiring installation that leads to the ignition coils is certainly a good thing to do.
Although they have good protection, the wires and harnesses are also subdued to constant heat from the engine which can make them brittle.
A rare problem, but does happen, especially on older cars.
It’s also good to check the wiring installation of other components that may cause a misfiring engine.
Cutt out the old wire or wires and replace them with new ones. Soldering the new connection is preferable for better contact and longevity of the repair.
I must add, that this fault may cause more problems with finding the exact damaged wire then fixing it.
Fuel injectors have the function of injecting fuel under high pressure into the engine cylinder.
Although they’re made out of very resistant materials, injecting an aggressive substance like fuel under high pressure will damage the injectors over time.
Instead of producing a fine mist, they start injecting an intermittent stream of fuel.
Replace or refurbish the faulty injector (or all of them).
It’s common practice to replace or refurbish only the faulty fuel injector but if possible it’s better to inspect them all.
ECU-ALSO KNOWN AS THE ECM (Engine Control Unit or Engine Control Module)
A faulty ECU (one or more) causing a misfiring engine is rare.
If it happens, it’s mostly due to some kind of overload in the electric installation or external damage (like swapping the battery terminals by mistake, faulty alternator producing too high voltage, some kind of short circuit, improperly jump-starting the car or else).
Replace or possibly repair the ECU unit.
Buying a new one is expensive for almost all cars so many people resort to specialized shops which can more or less successfully repair the unit.
Once replaced, expertise, tools, and computer software are needed to reinstall and set up everything successfully.
(older type ignition systems)
Ignition leads are cables that connect the spark plugs to the distributor.
Problems usually come from damaged or cracked wires inside the insulation or a weak/broken connecting terminal.
Also, a faulty or torn insulation boot located on the spark plug can cause a problem.
Changing the whole set of ignition leads is best.
One lead can be replaced with a successful outcome, but where one lead is faulty others are soon to follow.
(older type ignition systems)
This is the plastic-like cap located on top of the distributor on to which the ignition leads are connected.
Problems come mostly from material fatigue. The copper terminals and central graphite contact will wear out over time.
One more fault, although rare, is a crack in the distributor cap.
Replace the distributor cap.
(older type ignition systems)
The rotor is located under the distributor cap and is rotated by the distributor shaft.
While turning, it transfers electric power from the ignition coil to each ignition lead.
Material fatigue is the main cause of problems. The connection from the center of the rotor to its tip can crack and lose contact.
Replace the rotor.
(older type ignition systems)
This is a part of the distributor that helps to produce a better spark and reduce damage to contact breakers.
A bad condenser will cause a misfiring engine, mostly in the form of a serious backfire. It malfunctions mostly due to material fatigue.
Replace the condenser.
(older type ignition systems)
The distributor, besides the cap, rotor, condenser, and other ˝electrical˝ parts, has mechanical parts too.
Most problems come from worn out or damaged driving gears, loose plates, worn out bushings for the main shaft, and else.
If you want to know more about the distributor, click here for a thorough explanation on Wikipedia.
Replace or refurbish the complete distributor.
This part can certainly be refurbished and made as new, provided that all the spare parts are available.
But usually, it’s not cost-effective so it’s done only if there’s no other solution (like on old-timers for instance).
A vacuum leak is an air leak anywhere between the MAF (Mass Airflow Sensor) and the engine.
It causes the air-fuel mixture to become leaner as a surplus of air gets into the system sometimes leading to a misfiring engine.
The check engine light may come up too.
Punctured or damaged hoses, tubes or boots, and loose clamps are the common cause of problems.
In more complicated cases the problem may be caused by a faulty brake booster, EGR valve, the crankcase breather system, PCV valve, or else.
In the case of hoses, tubes and boots replace the damaged part. With loose clamps, simply tighten or replace the clamp.
The bigger problem is finding the leak as it can sometimes be a millimeter wide and in hard-to-access places. But there are solutions and methods for successfully finding a vacuum leak.
But with more serious problems first, a diagnostic should be done. Once the problem is found you can either replace or perhaps clean the problematic part.
INTAKE MANIFOLD GASKET LEAKS
Any leak on the intake manifold will cause an improper air-fuel mixture ratio which can lead to a misfiring engine.
The gasket that causes a problem is usually the one between the manifold and the engine. Another place is between the throttle body and manifold.
Replace the gasket with a new one. Here the bigger issue is pinpointing the problem and concluding that the gasket is faulty.
The fuel pump in modern cars is an electric motor that pumps fuel under pressure from the gas tank to the engine.
When it malfunctions the fuel supply may be weak or cut off completely.
Fuel pump problems are mostly caused by material fatigue, namely worn-out pump bushings or bearings, brushes, damaged rotors, or else.
If you want to read more about fuel pump problems, click here for a separate article on that topic.
Replace the fuel pump.
If it’s an external fuel pump, the repair is pretty simple and cheap.
But on a lot of cars, the fuel pump is combined with the fuel gauge sender into one plastic housing that’s submerged into the gas tank.
On some cars, the fuel pump can be replaced separately without replacing the housing and fuel gauge.
On others, it has to be replaced together.
A clogged or dirty fuel filter will weaken the fuel supply and reduce fuel pressure.
Poor maintenance and low-quality fuel are the main causes of these problems.
Replace the fuel filter.
FUEL QUALITY AND OCTANE VALUE
If the fuel is filthy and of poor quality, this will cause all kinds of problems.
Besides a misfiring engine, it will cause clogged fuel filters, damaged injectors, and else.
When the fuel has the wrong octane value it seriously abrupts the ignition of the air-fuel mixture.
Always use fuel from trusted and quality sources. Besides that, use fuel that has octane value recommended by your car’s manufacturer.
If you conclude that the fuel in your gas tank is causing a misfiring engine, best remove it as soon as possible.
By continuing to drive, you may cause engine damage.
Engine compression is a key factor for the combustion process as well as the engine power output (better known as horsepower).
Low compression causes an abruption in the combustion cycle as the air-fuel mixture cannot properly swirl in the cylinder.
With this symptom, the misfiring is not that drastic and is usually accompanied by other symptoms like the engine excessively burning oil, the low power output of the engine, oil on the spark plugs, and else.
This usually happens on older or high-mileage cars or ones that have been poorly maintained.
First, measure the engine compression. If it’s low on one or more cylinders, a complete or partial engine rebuild is in sight for sure.
For more information on engine rebuilds, click here for an article on that topic.
TIMING BELT OR TIMING CHAIN
The timing belt and timing chain has the function of connecting (among others)the camshaft and crankshaft.
They ensure that all the elements work harmoniously and, most important, with correct timing (this is why its called the timing belt).
Otherwise, the piston would collide with the valves during the combustion cycle rendering the engine useless.
Poorly fitted timing belts will cause a misfiring engine because the combustion sequence is not in the right order.
Also, a worn-out belt may skip a tooth and also disrupt the sequence.
Timing chains are a bit more resistant, have longer service intervals but nevertheless can cause problems.
Despite the common belief, timing chains also have a regular service interval and are meant to be occasionally inspected during that period.
If not, it may elongate over time, skip a tooth, causing a misfiring engine or even worse, severe engine damage.
The best solution is proper and quality maintenance.
Once the timing belt or timing chain and the rest of the components are fitted properly and with quality parts, there shouldn’t be any problems.
An important factor is to be aware of the regular service interval and replace the belt or chain on time.
Almost all problems of this sort are caused by either serious neglect or low-quality parts.
If the timing belt or timing chain is worn out, it should be replaced as soon as possible.
CAN YOU DRIVE WITH A MISFIRING ENGINE?
This is a common question and the answer, unfortunately, is not definitive.
Neither a yes nor a no.
It all depends on how bad the problem is. Some of the symptoms mentioned above will prevent you from driving altogether while others will be hardly noticeable, at least at the beginning of the problem.
To be more precise:
A small sputtering, merely noticeable loss in engine power, vacuum leak, or even low compression will probably not prevent the car from being driveable, Even for a longer distance.
Seriuso backfiring, engine vibrations, or problems with the timing belt or timing chain will render the car undrivable. Any attempt to drive the car is both dangerous and will certainly lead to some sort of engine damage.
So, it’ all up to you and your best judgment and that’s the real truth and answer to this question.
In any case, it’s best to tend to the problem as soon as possible and drive a car with a misfiring engine as less as possible.
COST OF FIXING A MISFIRING ENGINE
This is one estimate that’s very hard to give. As you’ve seen the list of causes and solutions is various and pretty long.
Every symptom and solution has a separate price for parts, labor, and a timetable for solving it.
Other crucial factors are the car type, engine type, accessibility for repairs, availability of spare parts, and others.
Simply, sometimes a misfiring engine is solved in an hour and with a small amount of money and sometimes it takes days and a small fortune.
But to give some sort of price span:
From a couple of dozen Euros for smaller repairs up to three or four digits for more serious ones.
Tend to the problems of a misfiring engine as soon as the first symptoms appear.
Most drivers tend to drive the car until the engine completely stops. It’s usually too late by then as the engine may have already suffered severe damage.
Drive it only as much as you have to or in case of an emergency.
Also, when searching for the cause of problems, make a quality diagnostic. This is one more car problem that can easily turn into a time and money pit.
For the end: as with many car problems, regular and quality maintenance may prevent a misfiring engine from happening in the first place.
The truth is that a well-maintained car may never see any of these problems.
Written by: Sibin Spasojevic
Former car technician, life-long car and DIY enthusiast, author for Despairrepair.com