I know the feeling….”is it me, or has the car lost some of its Oomph?” You’re right to go with your gut feeling on this one. Your car is giving you the heads up before it eventually stops.
Car feels sluggish? The most common cause of a sluggish car is a misfiring engine, and that is commonly caused by:
- Bad gas
- Faulty plug
- Bad coil
- Fuel pump fault
- Fuel injector fault
- EVAP fault
- Clogged air filter
- Turbo fault
- Faulty MAP sensor
- Faulty MAF sensor
- Faulty EGR valve
- Faulty oxygen sensor
- Faulty knock sensor
- Throttle body fault
- Accelerator pedal fault
- Bad coolant temperature sensor
- Camshaft position sensor fault
I’ve been a mechanic for over twenty years, and while cars have gotten more complex over the years, they still operate under the same basic principles.
A modern car is often sluggish because an engine fault has triggered the limp home mode. The limp mode is a fail-safe. It limits speed to between 30-40 mph.
Symptoms of limp mode include:
- Sluggish acceleration
- Limited speed
- Check engine light on
In this post, we’ll look at the likely causes of engine sluggishness in a little more detail.
So What’s Misfiring?
An engine operates in a set sequence. The cylinders are fuelled with a mix of gas and air and fired by a spark plug in a set order.
When the plug is fired, it ignites the gas/air mix, which sends the piston down the cylinder producing power which is transferred to the wheels by the transmission. However, if one of these mini-controlled explosions fails to occur, it is known as a misfire.
A misfire is a failed opportunity for the engine to produce power. Typically an engine these days is four cylinders, and while an engine needs all of them, it will still run (poorly) on three cylinders.
The symptoms of an engine that’s misfiring are:
- No power
- Slows going up hills
- Poor acceleration
- Rough idle
- Engine vibrating
- Engine noisy
- Engine light on
If your car was made in the last twenty years, it would be OBD 2 compliant. That means you’ll have the benefit of onboard diagnostics.
OBD is extremely useful for diagnosing modern car faults. The onboard computer is programmed to log all faults and to illuminate the engine warning light when a more serious error is recorded.
Some models are programmed to drop into limp mode. As you know, it’s a reduced engine performance mode, and it does this to protect the engine from damage and encourage the driver to visit a workshop.
Having your handheld scanner makes excellent sense. Modern scanners are inexpensive, easy to use, and small enough to fit in your glove box. Knowledge is power. If you know what fault codes are logged and you can obviously use Google, you’re well on your way to diagnosing the cause.
Other Causes Of Sluggishness
While a misfiring engine is, as you know, the most likely issue for the sluggishness in throttle response, some other causes are common too. These include:
- Car in limp mode
- Brake dragging
- Plugged catalytic converter
- Car battery fault
- Alternator fault
- A/c clutch fault
- Intake system vacuum leak
- Evap purge valve open
- Faulty VVT solenoid
- Transmission fault
- Bad gas
3 Things A Car Engine Needs
I don’t want to get into the weeds here, so I will broadly outline what an engine needs to run.
- Correct air fuel mix
1 Air fuel mix:
A gas power engine runs at peak efficiency when the fuel and air are mixed at a ratio of 14.7 parts air to one part fuel. It is known as the air-fuel ratio (AFR). While a modern computer-controlled engine is better at maintaining this ratio than its predecessor, the carburetor, it still doesn’t achieve the correct AFR all the time.
The ratio is constantly being adjusted by the computer as other factors are consistently changing, such as engine load, throttle response, temperature, etc.
The AFR is kept within an upper and lower limit by the onboard computer. If the AFR goes outside these upper and lower limits, the engine runs either too lean (not enough gas) or too rich (too much gas).
Either way, the engine will likely misfire, and you’ll notice a sluggishness in performance.
The onboard computer relies on multiple sensors for information. Wrong information can lead to incorrect fuel ratios. When this happens, an engine light should illuminate to alert the driver and prompt them to visit the workshop, but the engine light doesn’t always warn of a problem.
Spark is also essential, and it’s a simple concept to explain. Gas plus spark equals explosion. No spark, no explosion, and no explosion mean no power.
The main components of the ignition system include:
- Battery – Battery produces the initial energy to run the system.
- Engine control module (ECM) – Controls the timing and firing of the spark plugs.
- Crankshaft position sensor – Crankshaft sensor feeds cylinder identification information to the to the control module.
- Camshaft position sensor – Camshaft sensor feeds information to the control module about camshaft stroke.
- Coil – Coil produces the high voltage needed to fire the spark plug.
- Spark plug – Plug as you know produces the spark.
This is a simple concept too. The combustion chamber must have the ability to contain compression. If the pressure energy from the explosion leaks out through weak valves, cylinder head gaskets, or piston rings, the power to the wheels is lost.
Equally, if pressure leaks from the cylinder, it follows that extra oxygen can be drawn in. Too much oxygen will create a lean condition and a misfire, which we covered earlier.
Misfiring Causes In More Detail
This section covers a misfiring engine since it’s the most common fault. But you won’t know for sure what your fault is until you scan the onboard computer. The first step in the process of diagnosing an OBD compliant vehicle is to scan for current and historical fault codes.
Remember, a car is often sluggish because an engine fault has triggered the limp home mode. This is a mode designed by the manufacturers to allow you to drive your car to the nearest garage. Speed is limited to 30-40 mph, the car feels sluggish, and will likely have an engine light on, also known as MIL (Malfunction Indicator Light).
Some of the faults listed here will in themselves cause sluggish symptoms and, in addition, can also cause an engine misfire.
Fault codes for misfiring include:
- P0300 Random misfire
- P0301 Cylinder #1 misfire
- Po302 Cylinder #2 misfire
- P0303 Cylinder #3 misfire
- P0304 Cylinder #4 misfire
- P0305 Cylinder #5 misfire
- P0306 Cylinder #6 misfire
- P0307 Cylinder #7 misfire
- P0308 Cylinder #8 misfire
Here we’ll take a look at each of the common causes of misfiring. Remember, this is a most common list, meaning other possible causes are possible, just less likely.
1 Bad Gas
Bad gas was once very common, but gas is now usually pretty good quality. However, gas does go stale after about a month when left untreated. If your gas has been sitting in the car for weeks, it’s possibly stale. Old gas could be the cause of your sluggishness.
Top it up with fresh gas and avoid running low on gas. If you don’t use your car a ton, keep the gas tank full and consider using a gas stabilizer additive. The stabilizer is easy to use and keeps gas fresh for up to 2 years. I use a product called Sta-bil. You can check it out here on the Consumables page.
Bad gas commonly causes P0300 (Multiple random misfires) fault codes.
2 Faulty Plug
If your plugs haven’t been changed in the last 20,000 miles, it’s time for a tune-up. Some cars may be fitted with long-life plugs which can last for up to one thousand miles. When removing spark plugs, keep them in order. Knowing which cylinder they came from might be helpful.
Examining all the spark plugs will often tell you which cylinder has the problem. Common plug conditions are as follows:
- Wet plug – Shows the cylinder is getting gas, maybe too much.
- Dry plug – Suggest the cylinder isn’t getting gas, suspect a faulty fuel injector.
- Oily plug – suggests a mechanical problem.
Plugs are pretty basic components and are easy to change. Random misfire or an identified cylinder are both common with faulty worn-out plugs.
3 Bad Coil
Modern cars will have a distributor-less ignition system (DIS). That means no distributor, points, or condenser to diagnose, and that’s a good thing. Modern cars use individual coil per cylinder, they’re known as Coil over plug (COP) and Coil per cylinder(CPC).
Check out “Can I change one coil pack?”.
Check out “Can I drive with a faulty coil pack?”.
COP is easy to change out. It’s a simple job. These guys fail pretty regularly, and a fault code will be set for a particular cylinder. If you need an ignition coil or any parts, check out the Amazon link below.Amazon Ignition Coil
I wrote a post about locating a Coil. The post also covers swapping it out. You can check it out here – Where’s my Ignition coil?
4 Faulty Fuel Pump
A fuel pump sends the gas from the gas tank to the engine. The pump is electric and is usually submerged inside the gas tank. The pump is operated by the control module and is activated with the cycling of the ignition key.
The gas pump can be heard operating for a few seconds at the rear of the car when the ignition key is turned to position two (ignition on). Running a car low on gas will put a strain on the pump and should be avoided. Fuel pumps fail a lot. Fuel pump relays and fuses can cause issues too.
A faulty fuel pump can be replaced at home with regular tools, but it will take patience and good hand skills. A failing pump will cause multiple random misfire codes and possibly a low fuel pressure code. I wrote this post recently about fault-finding a fuel pump. You can check it out here – How to check fuel pump fuse.
5 Fuel Injector Fault
Fuel injectors spray gas into the intake manifold, where it’s mixed with air and fed to each cylinder. Fuel injectors are very precise bits of kit, dirt, or chemical contaminants that won’t pass through the nozzle. As a result, a faulty injector will either supply too much or too little gas. Both will cause an engine to misfire.
If an injector is working, you can try using an injector cleaner in the gas tank. Injectors don’t fail together, and so a single-cylinder misfire code will be identified.
6 Evap Fault
The Evap systems function is to capture and prevent harmful fuel vapors from escaping into the atmosphere. It traps vapors from the gas tank primarily and sends them into the combustion chamber where they’re burnt.
The system is pretty durable. Common issues are system leaks which are detected by a pressure sensor in the gas tank.
However, the only EVAP fault that we’re interested in is a stuck open purge valve. It will cause a lean condition and likely multiple random misfire fault codes. The EVAP purge valve is a simple component to replace.
7 Clogged Air Filter
A clogged air filter will cause the engine to choke and, depending on the system, may over fuel and cause multiple random misfiring. Check your air filter and air box for restrictions. Rodents love to build their homes here. If your air filter is clogged, change it out, this is an easy fix.
8 Turbo Fault
Not all cars are Turbocharged, but they are becoming more common on everyday models, not just sports cars. Turbocharging is an efficient way of getting more power from a small more gas-efficient engine.
If you ram cold air into an engine, it becomes more powerful and efficient. A Turbocharger does exactly this by using spent exhaust gasses to drive a compressor wheel which stuffs air into the engine. Of course, too much air would be a bad thing, known as over-boosting, and so a wastegate opens to release the air from the system.
Wastegates are a common cause of turbo faults, as are wastegate control solenoids, loose boost hoses, and turbocharger failures themselves. A faulty turbo should log a boost fault code and will often log engine misfire codes. A blown or faulty turbo will in itself make your car very noticeable sluggish.
9 MAP Fault
The MAP (Manifold Absolute Pressure) sensor sends information about engine load to the ECM engine control module (computer). The MAP sensor is fitted in the intake manifold or connected to it using a vacuum pipe.
A faulty MAP sensor may cause sluggishness and should log a fault code. A MAP sensor is very simple to change out.
10 MAF Fault
MAF (Mass Airflow Sensor) is housed inside the air-box and is a sensor that measures the amount of air entering the engine. The sensor incorporates an intake air temperature sensor also.
The ECM needs to know how much air has entered the engine in order to calculate how much fuel it should supply (injector duty cycle). The sensors do give lots of trouble and are famous for causing sluggish, hesitant engine responses.
A faulty MAF usually sets a fault code but sometimes won’t. The sensor often responds well to cleaning. Spray some MAF sensor cleaner on the sensor and leave it to dry. No need to rub it. You’ll find the MAF cleaner I use here on the Consumables page. Replacing the sensor is a straightforward procedure.
11 EGR Fault
The EGR (Exhaust Gas Re-circulation) Valves’ function is to reduce combustion chamber temperatures, as this also reduces harmful tailpipe gasses called Nox.
The EGR reduces Nox by diverting some spent exhaust gasses back into the engine cylinders. It does this using a valve which, on most cars now, is electronically controlled.
Common issues with EGR valves include carbon build-up, which prevents the valve flap from opening or closing. A valve that doesn’t close fully will oversupply the engine with exhaust gasses. This will often cause a misfire.
Any fault in the EGR should log a fault code, but in my experience, it doesn’t always work out like that. Replacing an EGR is easy on some cars, and on others, it’s a real pain in the jacksie.
12 Oxygen Sensor Fault
An Oxygen sensor (O2) is positioned in the exhaust system, and it samples the quantity of oxygen. It sends readings to the ECM about once per second.
A lot of oxygen in the exhaust tells the ECM the engine is running lean and a small amount of oxygen tells the ECM the engine is running rich. It uses low and high voltages to signal changes.
The ECM will make fueling adjustments accordingly. This is a process that begins as soon as an engine gets up to operating temperature, known as a “Closed loop”.
Any faulty reading from the oxygen sensor will obviously translate into incorrect fueling, and that can lead to sluggish performance and misfiring.
Common problems include incorrect readings sensors caused by a simple exhaust leak. Old sensors get lazy, which will cause a delayed fueling adjustment. Replacing an O2 sensor generally isn’t too difficult, but some may be fitted in awkward positions. A special open-sided O2 socket makes life a lot easier.
Any faults with O2 sensors should be logged, but I’ve often found a lazy sensor may not log a fault, and a leaking exhaust may not either.
13 Knock Sensor Fault
The Knock sensor is a curious little sensor. Its job is to listen for engine knocks and report them to the ECM. It’s positioned on the engine block so that it can hear them clearly.
The particular sound it’s listening for is a pre-ignition or detonations knock. It’s caused by the fuel/air mix igniting without the help of the spark plug. The knock is damaging to an engine and would eventually damage it.
The problem is solved by adjusting the timing (firing of the plug). An engine will knock when the timing is off, so the ECM continues to adjust the timing (firing of the plugs) until the knock disappears. This is a continuous process.
A faulty knock sensor may not report correctly or slow. Either way, engine performance will suffer, the car will feel sluggish. A fault code should be logged for a faulty sensor, but not always. Most sensors won’t be that difficult to replace, but access can sometimes be a challenge.
14 Throttle Body Fault
The throttle body is responsible for adjusting engine speed according to driver accelerator pedal commands. The throttle body on modern cars is an electronically controlled unit. They are pretty durable but do need to be cleaned from time to time.
A build-up of normal carbon deposits on the throttle plate prevents it from opening and closing smoothly.
Cleaning with a throttle body cleaner usually fixes the job. Poor throttle response and a rough idle are all symptoms of a dirty throttle body plate. A fault code may or may not be logged.
Some cars will require a throttle body relearn procedure after cleaning. This is needed as the ECM makes allowances for the carbon build-up. It adapts throttle position over time. You’ll find the MAF cleaner I use here on the Consumables page.
When cleaned, the ECM will need to relearn its new position.
15 Accelerator Fault
Most cars today have a fly-by-wire setup, which simply means the throttle body and accelerator pedal are connected by wires, not a mechanical link.
The accelerator pedal sends a signal to the ECM proportional to pedal travel. The ECM then adjusts fuel and sends a signal to the throttle body, which opens the flap to adjust airflow proportionally.
If the accelerator pedal doesn’t send the correct signal, the ECM won’t adjust as expected. All accelerator pedals have twin sensors that measure travel, and they must agree, any difference will set a fault code.
A faulty accelerator pedal will cause your car to feel sluggish.
16 Coolant Temperature Sensor Fault
The ECT (Engine Coolant Temperature) sensor sends a reading of the engine coolant to the ECM. It needs this information as temperature plays a large part in starting a cold engine smoothly.
Cold engines require extra gas until they’re up to operating temperature. If the ECM gets a bad reading from the sensor it will under or over fuel the engine depending on the reading.
A faulty sensor will log a fault, and a temp sensor is easy to swap out. I wrote a post about testing and changing out an ECT sensor. You can check it out here – Can you drive with bad coolant sensor?
17 Camshaft Position Sensor Fault
The CKP (Crankshaft Position) sensor has the task of identifying crankshaft movement. This info is sent to the ECM and is mission-critical information. Without the signal, the ECM won’t fire the spark plug or activate the fuel pump.
Related to the CKP is the CMP (Camshaft position) sensor. Its function is to keep track of valve train location. The ECM uses CKP and CMP signals to accurately fuel and time the firing of the spark plugs.
A faulty CKP sensor often causes hard starting issues or rough idling. A fault will be logged. Replacing the CKP sensor is pretty easy on most models. However, some make a real job of it.
Car not accelerating high rpm’s? A car that has limited speed and high rpm has likely entered the limp home mode. Limp mode is designed to protect the car when the onboard computer has diagnosed a fault. The car is safe to drive to the nearest garage.
- About the Author
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John Cunningham is an Automotive Technician and writer on Rustyautos.com. He’s been a mechanic for over twenty-five years and has worked for GM, Volvo, Volkswagen, Land Rover, and Jaguar dealerships.
John uses his know-how and experience to write fluff-free articles that help fellow gearheads with all aspects of vehicle ownership, including maintenance, repair, and troubleshooting.