(Updated on August 18, 2020)
Combustion engines operate on the principle of burning things to go faster. Ironically, you’re not supposed to smell anything burning while you’re driving down the road.
If you do encounter a burning smell while in or around your vehicle, it’s important to identify the source and correct the problem. Some burning materials are very dangerous and can even cause a fire.
We’ve done our best to describe these different smells in words, but smells can be very subjective and difficult to describe. If you’re still not sure what the source of your burning smell is, it never hurts to consult your local trusted mechanic for their advice on the matter.
Causes of a Burning Smell
Brakes work to slow your vehicle by converting your vehicle’s kinetic energy into thermal energy. Naturally, excessive heat buildup can cause a burning smell to come from your brakes.
Burning brakes smell a bit like burning carpet. It’s an organic yet metallic smell that gets stronger the closer your nose is to the wheels. A burning smell emitted from your brakes is not to be taken lightly, so it’s important to figure out why your brakes are heating up so much.
Brakes Working Hard
Even if your brakes are working perfectly, your brakes will start to stink up the cabin if you’re working them really hard. Some examples of working your brakes include descending hills, repeated hard stops, and even a single panic stop from a high speed.
If you start to notice a brake smell after working your brakes hard, try to cool them off before you drive too much further, especially if more heavy braking will be required in the near future.
Failure to let your brakes cool could lead to warped rotors and brake fade. Brake fade is a reduced ability to stop as the braking components become heat soaked. In extreme cases, brake fade could even cause a complete loss of braking.
If you’ve experienced brake fade, you should replace your brake fluid. Boiling brake fluid introduces air into the lines, and the existing brake fluid will never be able to resist heat quite as well as it did before.
As brake components wear, their ability to resist heat diminishes. Brake fluid is hygroscopic and absorbs moisture over time. This reduces its boiling point. Liquids are virtually incompressible, but gases can easily be compressed.
As brake fluid boils, the liquid brake fluid turns into a gas. This gas is not able to provide the clamping force requested by the driver because most of the pedal travel is spent compressing the gas, rather than squeezing the calipers. This leads to extreme brake fade, where you can push the brake pedal all the way to the floor and barely slow down.
Brake pads help dissipate heat through the pad material. If you run your brake pads down to the backing plate, they will have significantly less heat resistance than they did when they were new.
Brake rotors also lose their ability to dissipate heat when they have been resurfaced too many times. This is why each vehicle has a minimum rotor thickness requirement. When rotors get too thin, they are at an increased risk for excessive lateral runout. This is commonly known as warped rotors.
A seized caliper means that a caliper is unable to release its clamping force on the brake pads for a given wheel. This keeps the pad partially or completely engaged with the rotor, causing a constant braking force on one wheel as you’re driving down the road.
The braking components under this constant braking force never get a chance to dissipate their heat, which will make the caliper, pad, and rotor on that corner extremely hot.
If your caliper is seized, you may see smoke or even a small fire from one corner of the vehicle due to the intense heat generated by the pads being held against the rotor.
Bad Brake Hose
If a brake hose is pinched, it may allow pressure to enter the caliper, but prevent it from leaving. This would hold the pad against the rotor and exhibit similar symptoms to a seized caliper.
Bedding your brakes is the process of intentionally heating up the brakes to transfer a thin layer of pad material from the new brake pads to the rotors. This pad material is a necessary part of the adherent friction force.
There are actually two types of friction between the brake pads and rotors that are used to slow your vehicle: abrasive and adherent friction.
Abrasive friction is what most people think of as “friction”. Abrasive friction is the breaking of molecular bonds in the pad material and brake rotors. This causes brake rotor and pad wear as the two surfaces are essentially whittling each other down as they rub against each other.
Adherent friction is an “adhesive force” between the pad material and itself, and requires a thin layer of the brake pad material to be placed around the surface of the rotor. As the brake pad rubs against the rotor, it adheres to this thin layer as the rotor passes through the pad, and breaks the bonds as it moves away.
If you just replaced your brakes and are bedding them for the first time (which you should always do with a new set of pads), your brakes will almost certainly smell during the bedding process. This is normal and expected.
Your brakes will heat up considerably when you bed them in properly, so remember to let them cool before applying the handbrake so your rear pads don’t stick to the rotors.
2) Clutch (Manual Transmission)
Nobody likes a burnt clutch smell. Since clutches are hard to access, the smell of a burning clutch may as well be the smell of burning money.
A burning clutch smells like burning rubber – a bit like burning brakes, but more organic and less metallic. Think burnt rubber with a bit of burnt popcorn mixed in. You will often smell this inside the cabin, but may smell it outside the vehicle as well.
Excessive clutch slipping may also yield smoke from the engine bay.
You are most likely to smell your clutch when it is slipping excessively. Perhaps you are slipping the clutch intentionally when the clutch pedal is partially depressed. This may be done to creep up a steep driveway, for instance.
Sometimes a clutch is slipping when it should be fully engaged. This usually happens when the clutch is worn and needs replacing, but could also happen if the clutch pedal needs to be adjusted. A slipping clutch can often be identified by rising engine speeds, where your ground speed remains unchanged.
3) Burning Electrical System
Modern vehicles have many computers that require a labyrinth of electrical wire to connect them to fuses, sensors, and other computers.
A fuse protects an electrical system from excessive current. When the current draw is higher than the fuse is designed for, the fuse will break. This prevents the flow of electricity and saves the electrical component.
If you have a fuse that keeps blowing, there is a problem with the electrical system along that path. Grab a wiring diagram and see which components connect to that fuse.
Next, grab a multimeter and start testing various parts of the electrical system that share the blown fuse to ensure they are within factory specifications.
An electrical short means the path of electricity is completing the circuit on a different path than the circuit was designed for. This is likely to blow a fuse, as it could leave a system with excessive current draw.
Exposed wiring could cause an electrical short if electricity is able to pass between two wires with insufficient insulation.
An electrical short may also heat up a wire enough to melt insulation if there is no fuse to protect against the short.
Wires in a vehicle’s electrical system are wrapped in a wiring harness to protect them from the hot engine bay and ambient conditions.
If part of the wiring harness were to touch a hot component of the engine (such as the exhaust manifold), the insulation may melt. Melting insulation smells like burning plastic and may cause an electrical short as well.
Arcing electricity has a distinct electrical smell that is caused by the formation of ozone gas. This smell could be described as burnt chlorine.
Electricity may arc if a high voltage component lacks sufficient insulation and is close enough to jump to a nearby conductive surface, such as metal. Spark plugs, coil packs, and spark plug wires are three components that are designed for high voltage.
Be careful with arcing electricity. It is strong enough to leave you with a nasty shock and could easily cause an engine fire if it ignites something flammable like fuel.
The HVAC system is connected to the vehicle’s cooling system via the heater core. It also links to the air conditioner if one is equipped. Burning smells coming from the HVAC system could indicate a problem with one of these other systems.
Musty Smell on Startup
While not necessarily a burning smell, it is common for HVAC systems to smell a bit musty when you first turn on the blower motor.
Sometimes the HVAC system needs to clear out dust and other particles for a moment before you can drive with a stench-free experience. This is especially true when you first run the car heater.
Bad Heater Core
A heater core is a tiny radiator that uses hot coolant from the engine to warm up the cabin.
A coolant leak in the heater core may cause fogging windows, overheating of the engine, or a burning smell that smells a bit like burning rubber.
Dirty Cabin Air Filter
Most modern cars come with a cabin air filter, but these filters are often forgotten and neglected. A dirty cabin air filter will cause all sorts of smells, none of them pleasant.
Replacing a dirty cabin air filter is a cheap and easy way to refresh the interior of your car.
Bad Blower Motor
A bad blower motor or blower motor resistor could be receiving excessive voltage or insufficient resistance, causing a burning smell as the fan spins faster than it was designed to. A melted housing may smell like burning plastic, or you may experience an electrical smell.
Overheating A/C Compressor
If the refrigerant is too low, the A/C compressor will have to work harder to pump the remaining refrigerant in the system. If the system overheats, it may produce a burning smell.
5) Foreign Objects
Perhaps your vehicle is perfectly fine, but you picked up a plastic bag that was floating down the highway, and the bag got caught on your exhaust.
If you smell something burning and you don’t recognize the smell, it’s always a good idea to do a quick inspection of the vehicle to make sure something isn’t dripping or caught on a hot part of the car.
The exhaust system has some of the hottest components on your vehicle, so that is a good place to start.
6) Engine Oil Leak
Oil leaking onto the pavement is a nuisance that will stain your driveway. Oil leaking onto your exhaust is a serious concern that could start an engine fire.
Burning oil smells like the burnt version of what’s on your oil dipstick and is pretty easy to recognize.
While engine oil has a relatively high flash point compared to gasoline, it can still burn given the right conditions. If your car smells like burning oil, do an inspection to make sure that oil isn’t a fire hazard.
Please note: oil leaking out of your vehicle is not lubricating your engine. If you know you are leaking or burning oil, check the engine oil level more frequently. Starving an engine of oil will cause catastrophic damage to the engine.
Recent Oil Change
If you or a technician recently changed the oil, some oil may have leaked from the oil filler cap to a warmer part of the engine. If the leak is very small, it may burn off eventually on its own without causing a problem. However, larger leaks should be cleaned up.
Common Oil Leak Locations
Oil may leak from a number of places, the most common being:
- Valve cover gasket
- Oil pan gasket
- Drain plug
- Oil filter housing
- Camshaft seals
- Front and rear crankshaft (main) seals
7) Melting Drive Belt
A melting drive belt (also called a serpentine or accessory belt) smells like burning rubber and is often accompanied by a shrill squealing noise. A drive belt may melt from engine components that are too hot or a seized pulley.
When an accessory pulley has seized, it creates a large amount of friction against the drive belt, heating it up. This belt slip is the cause of the awful squealing noise.
If you experience belt squeal, check to make sure the accessory belt tension is adjusted properly and that all accessory pulleys spin freely.
8) Loose Hose
If a fuel or vacuum hose comes in contact with a hot engine component, it is likely to melt. This may smell like burnt rubber or plastic.
Engine hoses are supposed to be secured using clips from the factory to prevent melting and excessive vibration. Sometimes these clips are lost or broken on older vehicles.
Replacement clips may be purchased from a dealership or an aftermarket company. As a quick substitute, zip ties do a great job of securing hoses.
If you are using a zip tie near a hot component, check the temperature rating for the zip ties to make sure they can withstand the ambient temperatures they will experience. Once the zip tie is tightened, cut the excess length. Do not place a zip tie where it may contact the exhaust or other hot component.
9) Clogged Catalytic Converter
A catalytic converter converts toxic gases which produce that rotten egg smell into less harmful emissions.
Catalytic converters have an operating temperature in excess of 800 degrees Fahrenheit (427 degrees C). If a catalytic converter becomes clogged, it has the potential to start a fire since the exhaust system is so hot.
A clogged catalytic converter may exhibit other symptoms such as sluggish engine performance. It will often glow when it’s too hot.
10) Misfire or Partial Combustion
Partial or incomplete combustion usually smells like gas and the source of the smell is often the tailpipe. However, you may also smell gas from inside the cabin.
Incomplete combustion could be caused by many factors and is often accompanied by a check engine light on newer vehicles. You are also likely to experience sluggish acceleration.
Poor Engine Calibration
If your engine is tuned improperly, your vehicle is probably running with a suboptimal air fuel ratio. An air fuel ratio that is too rich will smell like gas.
Leaking fuel will likely cause the car to run lean and throw lean check engine lights. This is because fuel is spilling out before it reaches the combustion chamber. Leaking fuel is very dangerous because it could easily ignite and start a fire.
11) Gear Oil Leak
Gear oil is the stuff that lubricates your transmission or differential. It has a distinctly different smell than engine oil, both when it’s fresh and when it’s burning. Burning gear oil is acrid like a burning clutch, but smells a bit more like very burnt bacon.
Some exhaust systems are placed in close proximity to the transmission or transaxle. If you’re leaking gear oil and it drips onto the exhaust, it will likely stink up the cabin every time you come to a stop.
Locating the Source of the Burning Smell
There are several steps you can take yourself to locate the burning material.
1) Inspect the Exterior
If you are driving and you smell something burning, pull over as soon as it is safe to do so. Next, get out of the car and do a quick walk around the vehicle. Look for anything obvious like a fire.
2) Inspect the Engine
If you see any smoke coming from under the hood, do not attempt to open the hood. Opening the hood will provide a fire with more oxygen and create a dangerous situation.
Open the hood if you feel it is safe to do so. Look for anything melted, smoking, or smoldering. The smell should be stronger near a melted component.
3) Perform the Sniff Test
While working your way around the vehicle, try to locate where the burning smell is strongest.
To narrow the problem down, start at the engine and smell the air around you. Next, move to the back of the vehicle and take a sniff. Which side is stronger? Keep splitting sections of the car in half until you reach a point where you have found the strongest smell.
Can You Drive With A Burning Smell?
Look around the source of the smell for anything that is obviously wrong. If the burning smell is strong and you’re not sure if it’s safe to drive the vehicle in its current condition, have the vehicle towed to a shop for a diagnosis of the problem.
For milder smells, you may be able to drive the vehicle to a shop safely under its own power. Only do this if you’re comfortable with the risk, the vehicle otherwise drives fine, and there is no visible smoke. If the smell gets worse or you see smoke, stop driving immediately.