Reduced engine power can be caused by several reasons.
An undeniable fact is that the inner combustion engine is an engineering marvel, but a complicated one also.
It’s consisted of hundreds of parts that have to form a harmonical and functional whole. Add the fact that the engine is subdued to extreme heat, cold, various weather conditions, driving habits, and other factors.
Also, there is the matter of material fatigue which is inevitable and affects almost every part of the engine.
When all of this is taken into account, it’s only logical that something can go wrong and cause reduced engine power.
This article is focused on the most common reasons as well as some probable solutions for this problem.
COMMON REASONS FOR REDUCED ENGINE POWER
1. CLOGGED OR DIRTY AIR FILTER
The air filter is in charge of cleansing the air that goes into the engine.
It protects the air intake and its elements (which are sensitive) from dust, dirt, or even larger particles.
Without it, the engine would have problems functioning for sure.
Over time, the air filter gets dirty and, if not replaced, gets clogged.
This then leads to a lack of air in the intake which means an improper air-fuel mixture in the combustion chamber and this certainly leads to reduced engine power.
To simplify, the engine is ˝choking˝.
Regular maintenance and timely replacement of the air filter. If you follow you’re maintenance schedule you shouldn’t have problems with this.
If you drive, for instance, on dusty country roads you can even sometimes take out the air filter and dust it out with compressed air. The main point is to keep it clean as much as possible during its service life.
More on the air filter replacement procedure in a separate article which you can read by clicking here.
2. PUNCTURED OR DAMAGED AIR INTAKE HOSES, TUBES, OR DUCTS
Modern engines are very sensitive to the proper air intake.
So much so that even a small puncture or rip in a hose, tube, or duct can cause reduced engine power.
This can be accompanied by a check engine light or reduced engine power light coming up on the dashboard.
All of these parts are made out of durable plastic or rubber (in most cases) so they can last a long time.
Due to various factors like constant temperature changes (heating up, cooling down), taking off when making regular maintenance, unintentional physical damage during a repair, or else can cause the need for repair or replacement.
A complete replacement of the damaged part is the best solution. Mending is possible when there is minor damage like small punctures or tears. These can perhaps be fixed with some hot glue, tape, or else although these repairs tend to be short-lived.
The bigger problem is finding the puncture or tear as they can often be a millimeter wide or less and located in inaccessible places making them very hard to spot.
3. LOOSE HOSE OR TUBE CLAMPS
Loose clamps are a more common case then damaged hoses.
The main reasons would be shrinking of the hoses or tubes over time, material fatigue, or perhaps just forgetting to tighten them while making some other repair.
Of course, tightening or, if the clamp is damaged, replacing it with a new one.
Loose clamps are easy to fix under the condition that you can find and reach the loose one.
4. CLOGGED OR FILTHY FUEL FILTER
The fuel filter has the task of stopping any filth coming into the fuel intake system. The filth may come from impure fuel or residue that settles at the bottom of the gas tank.
Over time, filth gathers in the fuel filter and if not replaced on time, causes a clog in the fuel supply.
The engine will then not get enough fuel and this will reduce engine power.
Regular maintenance. Change the fuel filter on time.
If you drive in countries where the quality of fuel is poor or have an older car where there might be a lot of residue at the bottom of the gas tank, you should change it more often.
When talking about fuel filters, most cars have an additional sock-like filter mounted on the fuel pump-fuel gauge housing. If your car has this type of fuel pump, make sure to check this filter besides the main one.
5. BAD FUEL PUMP
The fuel pump is an electrical motor that provides a stabile fuel supply from the gas tank to the engine.
All modern cars with injection systems depend on constant pressurized fuel supply and without them the engine wouldn’t function properly.
Fuel pumps have a service life as any other part. The electronic motor inside can malfunction over time mostly due to worn-out brushes or bearings.
For more information on fuel pump problems click here for a separate article on that topic.
Changing the fuel pump.
In some cars, the fuel pump is located externally which makes it easier to replace.
On the other hand, some have the fuel pump, fuel gauge sender, and electric motor combined into one housing located inside the gas tank.
On some cars, you have to change the whole part (pump+fuel gauge+electric motor) and on some, you can take the whole thing apart and just change the motor.
Also, be sure that all of the electric installation and connections leading to the fuel pump are OK. Check that the fuel lines are in order too.
6. CLOGGED OR DAMAGED FUEL LINES
Clogged fuel lines are a more rare problem that can cause reduced engine power.
If the fuel filter is changed on time and quality fuel is used, clogs shouldn’t form.
Damaged fuel lines mostly occur in older cars where material fatigue takes its toll. Also, the larger part of the fuel line installation is located under the car making it susceptible to weather, road conditions, or physical damage.
In the case of clogs, best disconnect the fuel lines and used compressed air to blow out all the filth.
With damaged fuel lines, since flammable fuel is in question, best completely replace the damaged part of the installation.
Most fuel lines are not made out of one single line but are a combination of plastic and rubber hoses with connectors.
This makes replacement a bit easier.
Any kind of mending may be possible but is usually short-lived (unless maybe done by a professional).
Mending is not recommended unless there is no other solution or in case of an emergency.
7. FAULTY FUEL INJECTORS
Before fuel reaches the engine it goes through the fuel injectors. They inject the fuel under very high pressure into the combustion chamber.
They must always be in top condition. The injector is supposed to produce a very fine mist, but if they’re faulty they will instead make a stream (instead of mist they will ˝spit˝fuel into the chamber).
Material fatigue is the main reason for faulty fuel injectors. Although they are made from very durable materials, constant injection of aggressive fuel under very high pressures will damage the injectors.
Fuel injectors are meant to be replaced in a regular service interval. Most mechanics and garages change them completely (on all cylinders).
But there are specialized shops that will repair fuel injectors if the build of the injector allows it.
They change only the damaged internal parts (like just the nozzle for instance).
Of course, a new part speaks for itself but a repair can be almost as good.
Consider this if you encounter a fuel injector problem as it can save some serious money.
8. POOR FUEL QUALITY AND WRONG OCTANE RATE
Poor fuel quality is the foundation of most problems with the fuel system and will of course cause reduced engine power.
All of the above-mentioned parts of the fuel intake system will last much longer and cause fewer problems if you use good quality fuel.
Not only in terms of purity and overall quality but in terms of octane value.
It’s simple. The moment you notice that the fuel you’re using is causing problems, change your gas station. Although every shipment of fuel isn’t the same, every gas station should offer constant quality to some degree.
Otherwise, you may end up paying some high repair bills because of the lack of quality.
Also, make sure that you use fuel with the octane value recommended by the manufacturer of your car. It may be more expensive but in the long run, it will pay off through less repair and better engine condition.
9. BAD OR FILTHY O2 SENSOR
The O2 sensor is an electronic device that measures the amount of oxygen in the exhaust gases that come out of the engine through the exhaust manifold.
That’s the definition. In more common language, it makes sure that the exhaust fumes are within the ecological norms. If you want more thorough information, click here for a great explanation on Wikipedia.
It’s located on the exhaust manifold. Most cars have several ones located in different places on the exhaust system for better monitoring of fumes.
Material fatigue and filth are the main reason why an O2 sensor may cause problems.
Both of these will make the O2 sensor send false signals to the ECU causing fluctuations in the air-fuel mixture and can consequently cause reduced engine power.
This is an explanation, in a nutshell, more on this topic in a separate article that you can read by clicking here.
Replacing the O2 sensor is best. Although a pricey part, once replaced you won’t have this kind of problem for a long time.
There’s also the option of cleaning the sensor with special sprays or solvents. Not a bad solution especially if you don’t have the means to buy a new one.
Just have in mind that the material inside the sensor wears out over time and that cleaning may be only a temporary solution.
10. BAD MASS AIRFLOW SENSOR (MAF SENSOR)
This sensor is located between the air filter and the intake manifold. It’s easily recognizable as part of the air intake that has an electric connector on it.
It’s a sensor that measures values like air pressure, amount of air, and air density going inside the air intake and engine. The MAF sensor sends signals to the ECU so it can properly regulate the air-fuel mixture.
The mass airflow sensor has sensitive parts inside. Material fatigue and not changing the air filter on time are among the main reasons for a malfunctioning sensor.
Mass airflow sensors are meant to be replaced completely. This is the best solution if you want quality and longevity of repair, especially if you use quality parts.
As with the O2 sensor, the MAF sensor can also be cleaned with sprays and solvents made especially for this task.
Worth the try as the MAF sensor can be pricey. In many cases, it has a successful outcome.
11. WORN OUT OR DAMAGED SPARK PLUGS
Spark plugs are in charge of providing a quality spark for igniting the air-fuel mixture.
Without it, proper combustion is not possible causing reduced engine power.
Although spark plugs (especially from quality manufacturers) are made to last a long time, material fatigue will take its toll. It mostly shows itself through damaged or weakened electrodes or cracked insulation.
Proper and timely maintenance will ensure that the spark plugs are replaced on time.
Besides this, occasional removing, cleaning, and checking the electrode gap is also common and useful as it ensures the proper function of the spark plug during its service life.
More on the spark plug replacement topic here.
12. FAULTY IGNITION COILS
Ignition coils have the function of providing high voltage electricity to the spark plugs. They are essentially mini electric transformers.
In older cars, there was only one main ignition coil but on almost all modern cars every cylinder and spark plug has an ignition coil of its own. They are usually located just above the spark plug, on the cylinder head.
Because of this location (exposure to heat) as well as constantly producing high voltage the ignition coils seize to function over time.
Ignition coils are meant to be replaced completely. They are made in such a way (sealed as one compact part) that fixing them is almost impossible and not cost-effective.
Most people change only the faulty ignition coil leaving the others intact. If possible, better change the complete set as this will offer better quality and longevity of the repair.
Also, make sure to use ignition coils from quality manufacturers and to the specification of the car manufacturer.
13. DAMAGED WIRING INSTALLATION
The wiring installation is the nerve system of the car.
It provides timely reaction and connection of all the crucial parts of the electric and electronic systems.
In a car, the wiring installation is subdued to all sorts of harsh conditions. Heat, cold, moist, sharp edges, bent spots….all of these are arch enemies of the installation.
A broken cable or one that has damaged insulation can cause a lot of problems that can manifest in all sorts of radical ways. Among them is reduced engine power.
Fixing a wring installation is very simple, Simply replace the broken cable or mend the insulation with some insulation tape.
The biggest problem is finding where the problem is. In most cases resembles finding a needle in a haystack.
Wiring installations have hundreds of meters of cables in them and just one broken or damaged cable can cause havoc.
The problems are of course solvable, but you’ll need a lot of time, patience, and nerves.
14. WEAK, LOOSE, OR RUSTY ELECTRIC CONTACTS
When speaking about the electrics and electronics in the car, bad contacts are perhaps the most common problems of them all.
Every connection and connector is a possible hot-spot for problems. Especially the ones located in the engine bay or the outer parts of the car (like the undercarriage, fenders, bumpers, or else).
Moist, dirt, heat, cold, physical damage will all cause loose, weakened, or rusty electric contacts.
Even in the car interior, problems may be caused by things like moving seats, levers, tight spots, or else.
Just like with broken or damaged wires, a simple bad contact can manifest itself radically like dashboard warning lights coming up for no reason, sudden loss of electric power, or even reduced engine power.
Cleaning connections (with a spray or sandpaper) or completely replacing connectors.
As with damaged wiring, the hardest task is finding the exact bad electrical connection.
Best search suspicious places with rust, moist, connectors in risky places( like where there is heat or motion), etc.
Ground connections are also a common cause of problems. Lots of them are held by nuts or screws to the car body in places that are susceptible to rust. If they lose contact, they can cause an array of problems.
15. FAULTY ECU (ENGINE CONTROL UNIT)
The engine control unit is the electronic brain of your car. There might be one or more depending on the car model.
ECU-s are usually built to last as long as the car making this fault very rare.
They mostly perish from factors like high alternator voltage, improper jump start of the car, or even a bad ground connection.
Engine control units are replaced completely. After that, in most cases, all of the car parameters have to be fine-tuned by a diagnostic program (using a diagnostic tool or a laptop with the proper program installed).
This is a costly and timely repair, especially on more complicated cars.
On this part worth mentioning is that there are specialized shops that can more or less successfully repair an ECU. Mostly depends on the level of damage.
Certainly worth the try as it should cost less and provide satisfactory results.
16. BAD ALTERNATOR
The alternator is an electric generator that provides electricity for your car while the engine is running.
This is an explanation in a nutshell, for a more thorough definition on Wikipedia, click here.
Once functioning it puts a strain on the engine via the serpentine belt.
If it’s faulty (especially when producing higher voltage or when the bearings are bad) it can put extra load on the engine and reduce engine power.
Replacing the alternator, especially if the damage is severe (like a burnt-out stator or rotor for instance).
But if it’s not (like in the case of worn-out bearings, brushes, or a faulty voltage regulator) it can be successfully refurbished.
Both options are good and almost equally effective. Cost-effectiveness is the key factor in making this decision.
17. CAR BATTERY PROBLEM
The car battery always needs to be in good condition.
If not, it will seriously influence all of the other electric and electronic systems in the car.
Mainly the alternator. The car battery and alternator have a combined function for providing a stabile electric supply for electric and electronic systems in the car.
If the battery is faulty and cannot be properly charged it puts an extra burden on the alternator.
This then puts extra strain on the engine which will reduce engine power.
If the car battery is on its last legs, replace it with a new one as soon as possible.
For a separate article on how to change a car battery click here.
On the other hand, if it was a one-time discharge, try charging the battery and if needed, add some distilled water.
The whole procedure for recharging a battery is explained in this article which you can read by clicking here.
18. CLOGGED CATALYTIC CONVERTER
Catalytic converters have the function of cleansing the exhaust gases that come out of the engine.
Over time, they can get clogged due to the build-up of soot and filth. This then blocks the exhaust fumes which leads to reduced engine power.
The main reasons for a clogged catalytic converter are low quality or inadequate fuel in terms of octane value. These cause improper combustion and eventually a build-up and clog.
Otherwise, they usually last as long as the vehicle.
Replacing or cleaning the catalytic convertor.
The first option is best but the most pricey. A catalytic converter is an expensive part mainly because of the materials inside it.
This is why lots of drivers resort to cleaning it. Specialized shops do this for a reasonable amount of money (much less than a new one anyway) and the outcome is pretty good.
19. OVERHEATING CAR PROBLEMS
If the engine temperature is above normal, the engine power will reduce. Excessive heat, which is meant to be relieved via the radiator and coolant system is held up in the engine, so to say.
If not tended to, this problem can cause serious engine damage. Major overheating will cause the head gasket to blow and this means a partial or complete engine rebuild.
There are several reasons for an overheating car. For some of the most common reasons, click here for a separate article on this topic.
Proper diagnostics and replacement of the faulty part or parts. This is a problem that isn’t perhaps that expensive to solve but has major consequences if not tended to on time.
For instance, a loose clamp, punctured hose or a cheap thermostat can lead to an enormously expensive engine overhaul.
The main solution here is to do regular maintenance and check-ups, always pay attention to the temperature gauge, and react on time.
20. LOW ENGINE COMPRESSION
The power output of the engine depends on the engine compression.
Simply put, the compression is the strength of the engine.
Low compression usually happens with high-mileage cars. Material fatigue is the main cause and is more or less inevitable.
In most cases, the piston rings and cylinder walls wear out (among other parts). This is partially why it’s accompanied by the engine constantly burning oil.
Other common reasons for low engine compression are poor maintenance or bad driving habits.
Complete or partial engine rebuild. Engines can be successfully taken apart, cleaned, all the worn-out parts replaced (piston rings, bearings, gaskets, etc) and reassembled.
If everything is done properly and in quality the engine will be as good as new.
There is of course the possibility of completely replacing the engine with a new one or putting a used better one.
A new engine is of course the best but cost-effectiveness plays a key role. For instance, putting a new engine in a 20-year-old car doesn’t seem very wise. Unless you plan to keep it for some reason.
Putting a used engine in the car is only an option if you know the exact mileage and origin of it. Otherwise, you’re probably just throwing money away and buying a heap of junk.
On this part; I would also like to mention various oil additives for increasing engine compression. Not bad if the engine has lost a bit of its strength. If the compression is weak this won’t help nor save you for sure.
21. BAD AC COMPRESSOR
The air-conditioning compressor is engine driven. If it’s faulty in any way it will put extra strain on the engine and reduce engine power.
The most common problems are worn out bearings, faulty pulleys, or the inner parts of the compressor failing.
The AC compressor has a maintenance interval of its own. A lot of drivers don’t know this and that is perhaps the main cause of an AC compressor failure.
Complete replacement is the best solution if the inner mechanism of the compressor has been severely damaged. Repairing is simply not cost-effective in this case.
For other more minor repairs like gaskets or pulleys, there are specialized shops that will successfully fix the AC compressor and make it as good as new.
22. DAMAGED GASKETS
Gaskets have a major role in the engine functioning.
Their main task is to provide a perfect seal between two surfaces.
They are present everywhere, in the air intake manifold, fuel intake, exhaust manifold, between the engine and transmission, between the cylinder head and engine block…you name it.
If anyone of these is punctured, damaged, torn, or loose they will reduce engine power through lower engine compression, it will cause lower oil pressure, lower coolant pressure, leaks, and else.
The best example of how much a gasket is important is the cylinder head gasket. Without it, there is no proper engine compression as well as the fact that oil and coolant will mix.
Replacing the gasket is the best solution, for whatever part of the engine it may be.
The repair all depends on the type, location, and the role the gasket has on the engine. Some are easy and cheap to replace but some demand a lot of work, expertise, and dismantling.
Worth mentioning: if you don’t have the proper gasket you can use various gasket sealants which are also good.
23. FAULTY OR DAMAGED THROTTLE BODY
The throttle body is the engine part that regulates the amount of air getting into the engine.
It replaced the carburetor but unlike it, the throttle body only regulates the amount of air and not the amount of fuel.
It’s connected to the gas pedal with a cable or electric wires (drive-by-wire system). On top of it, there is a throttle plate that turns on a little shaft.
So once the driver presses the gas pedal, the throttle plate opens letting more air inside.
Most problems occur on the drive-by-wire throttle bodies. They have a small electric motor and mechanism on the side of it which turns the throttle plate. The electric motor reacts to signals sent from the gas pedal.
These motors tend to malfunction over time (either the motor itself or the turning mechanism inside). Also, a common cause is physically forcing the throttle plate to open thus damaging the whole mechanism.
Mechanical throttle bodes are much less susceptible to problems. Most problems occur with stuck or loose cables, worn-out springs, or else.
Either way, if a throttle body is faulty then the air intake is not in order. This will reduce engine power for sure.
More on the throttle body issues in these articles:
Throttle bodies that are cable-driven rarely cause problems. Most problems happen on higher mileage cars where material fatigue takes its toll on the part.
If problems occur they are simply solved by changing bushings, plates, springs, holders, or other simple parts. Usually, it’s not a costly or complicated repair.
But with the drive-by-wire throttle bodies, the situation is different. The replacement is not that complicated in most cases but the part is pretty expensive.
Trying to fix the electric motor and mechanism is not an option for most cars as it is sealed and meant to be replaced.
Maybe worth trying but with a very questionable outcome.
It can be opened up but there’s a small chance that it can be successfully put together again. Above that, the probability of it all functioning, in the long run, is small.
Another solution is to try and clean the throttle body without taking it off. We’ve made a separate article specifically on this topic. You can read it by clicking here or you can watch the YouTube video.
24. TIMING BELT PROBLEMS
The timing belt is one of the most important car parts, for sure.
It connects the crankshaft and camshaft and ensures the harmonical functioning of them all (hence the name timing belt).
If the timing belt is not properly fitted the consequence will be massive engine damage. The piston in the upper position reaches the valve in the lower position and collision happens.
In short, this means a total engine rebuild for sure.
But sometimes the timing belt may be off by one tooth. When this happens it is usually accompanied by a rattling sound from the engine caused by the valves as well as possible engine overheating.
In this case, the engine will work but there will be severely reduced engine power.
The most common cause of this is the unprofessional mounting of the timing belt.
The belt can also skip a tooth due to extreme wear. This happens only in case of very poor maintenance.
Violently jump-starting a car can also cause a problem like this, especially in cars with manual transmission.
The best solution is to visit a good and trustworthy mechanic or a garage that will replace the timing belt professionally, in quality, and with a guarantee.
If you encounter this problem and you happen to notice it on time, turn off the engine immediately.
This will save it form further serious damage.
That is if it isn’t already too late.
Preventing this problem from happening in the first place is the best solution here.
25. CLOGGED OR FAULTY EGR VALVE
The Exhaust Gas Recirculation valve is one more part of the car in charge of saving the environment.
It redirects a portion of the engine’s exhaust gases back into the engine cylinders for additional combustion.
Sort of an exhaust recycling machine.
Problems with the EGR valve mostly come form soot and filth forming inside of it. In most cases, this is caused by poor-quality fuel.
Complete replacement or cleaning of the EGR valve.
Cleaning is a more common procedure. Once thoroughly cleaned and returned, it can serve as well as a new one. This is also much less expensive.
Replacement is an option if the valve is severely damaged and cleaning will not help.
The problem of reduced engine power should be taken care of the moment a driver notices it.
Postponing can lead to various problems in terms of driving safety, lost time, huge repair bills, and worry.
Also, regular and timely maintenance will prevent most of these reasons from even happening in the first place. This goes double for older or high mileage cars.
Besides this, knowing your car and paying attention to the appearance of any of these reasons may mean the difference between a minor repair and a total catastrophe.
So, take good care of your car while you own it and you’ll rarely see any problems regarding reduced engine power.
Written by: Sibin Spasojevic
Former car technician, life-long car and DIY enthusiast, author for Despairrepair.com