Why Your Car Has Smelly Exhaust Fumes & How To Fix

You shouldn’t be able to smell your car’s exhaust fumes inside the cabin. Your tailpipe emission is toxic, so it’s best that you address the problem of unpleasant exhaust odor right away. Smelly exhaust fumes are quite a common problem for car owners, so it’s one of the basic car maintenance tips to know the reason for this health hazard, how to deal with each scenario and how much the repair or replacement would cost.

Smelly Exhaust Fumes In The Cabin: Most Common Reasons

Driving in a car filled with exhaust smell is highly unpleasant, but what’s more is it’s unhealthy, even toxic. Your vehicle’s exhaust gasses contain a number of poisonous compounds, including carbon monoxide, formaldehyde, nitrogen oxides, sulfur dioxide, benzene and soot. All these compounds are harmful to the human body, especially if inhaled in large quantities over a prolonged period of time.

Particularly troublesome among them is carbon monoxide, since it stays for many hours once entered the human body. This means repeated exposure over time has a detrimental cumulative effect. This potentially fatal gas is an odorless and colorless byproduct of combustion. Even a few minutes of inhaling this toxic gas can cause symptoms including dizziness, nausea, headaches and drowsiness. 

There are many reasons for exhaust fumes to make their way into a car’s cabin. Depending on the particular type of smell as well as other visual and audio signs, you can narrow down or pinpoint the culprit to have the problem looked at. Below are the most common causes of exhaust fumes smell in the cabin and what to do to resolve the problem.

smelly exhaust reasons
Exhaust fumes can have different smells and accompanied by sounds or color, depending on the source of the problem. Photo credit: Toyota of Clermont

Rotten Egg Smell: Failing Catalytic Converter

The smell of rotten eggs is a sign of a compound called hydrogen sulfide, which can be traced back from the sulfur in your gasoline. When everything functions properly, the small amount of sulfur that is present in the fuel is converted to sulfur dioxide, particularly thanks to the working of the catalytic converter. 

The catalytic converter is a vital part of a car’s anti-pollution system. It’s designed to clean the harmful compounds in the exhaust fumes to make them less harmful before they are released into the environment via the tailpipe. It is a standard feature in modern automobiles, and is legally required in most US states. In fact, it is illegal to remove this component.

When the catalytic converter breaks down or its filtering layers have worn down, the sulfur in the fuel is not converted into the odorless form of sulfur dioxide, but instead into the rotten egg-smelling hydrogen sulfide.

Like everything else in your car, the catalytic converter can wear out over time, become overheated to the point of breaking down, or be contaminated with gasoline in the exhaust. When this happens, you’ll notice a rotten egg smell coming from the exhaust and sometimes this smell can even enter the cabin. 

Other symptoms of a failing catalytic converter is sluggish engine performance in general, particularly reduced acceleration performance and poorer fuel economy. 

What to do

In this case, you’ll need to have the catalytic converter inspected and replaced if needed. If ignored, a failing or clogged catalytic converter would cause your engine to run at suboptimal conditions, which is not ideal for engine performance and lifespan, and also you may not pass emissions testing. 

The cost

The most expensive repair related to the exhaust system would be a catalytic converter replacement, which is in the center of the system. Most other exhaust repairs are relatively inexpensive. 

The cost for replacing the catalytic converter can vary widely, as the price for the part itself depending on the quality ranges from under $100 to $2,250, which might be close to an old car’s value. Regarding labor cost, depending on the make and model of your car, the job can take from one hour to two hours, with repair shops charging anywhere between $70 per hour to nearly $130 per hour. So including parts and labor,  a catalytic converter replacement can easily cost you between $950 and $2,500.

Smokey Smell: Exhaust Leaks

If the irregular smell in the cabin you notice is just vaguely musty and smoky like regular exhaust fumes from cars’ tailpipes, there might simply be an exhaust leak somewhere behind the catalytic converter, like a cracked exhaust pipe or a damaged muffler that’s allowing exhaust gases to escape the system early and make its way to the cabin. 

Exhaust leaks can also occur where the exhaust manifold attaches to the engine, which is located at the upstream of the exhaust system, under the hood.

There are other symptoms for exhaust leaks to look out for, the most common being a loud rattling, rumbling or hissing sound as you drive. This is because, for instance, the exhaust sound is muffled by the muffler, so an exhaust leak before the muffler will cause such loud noises. Note that sometimes exhaust leaks noise can be dampened by noise from the engine or your tires while you’re driving. 

In addition, you will likely notice poorer fuel economy. Exhaust leaks mean that your engine needs to work harder, and sub-optimal engine operation directly translates to it eating up more fuel to do its job.

A leaking muffler can be replaced, and an exhaust leak elsewhere can simply be patched up. But there are two good habits that you should adopt to make sure this problem won’t happen any time too soon. That is, try to combine errands so that every time you drive, you’re driving for longer, and regular servicing of the exhaust system.

This is because the exhaust systems will rust over time, and if you mostly take your car out for short trips, this rusting will happen sooner. Short trips don’t allow the exhaust system to get sufficiently hot, thus water vapor generated during the combustion process cannot vaporize but instead remain in the muffler and the exhaust system. This excess moisture accelerates corrosion. 

This type of corrosion takes place from the inside of the exhaust system due to internal condensation build up, thus is more serious than external rust that’s on the surface only. When a part is rusted through from the inside and its structural integrity is compromised, it would need a replacement instead of simply patching up the rusting holes.

What to do

If your vehicle is more than 5 years old, you should have the exhaust system inspected periodically, ideally every year or two to check for exhaust leaks, loose parts, corroded parts and other damages to parts. Minor damages and leaks in the exhaust system should be addressed before it escalates to more expensive problems.

Do note that rust can form in places that are hard to see, such as on the top of the muffler, the catalytic converter or an exhaust pipe. Also, if upon inspection, you find that the muffler and exhaust pipes are in good shape, remember to check the connections between parts carefully, as they can rust or come loose from driving on rough terrain. 

The cost

cracked muffler
A minor damage on the muffler might be taped off, but most of the time it would require a replacement. Photo credit: Blog – Rainbow Muffler

If the leak is at the exhaust manifold, you’ll be paying between $20 and $50 for the gasket itself, plus some $160 to $330 for labor. Meanwhile, a muffler replacement can cost anywhere between $75 and $750 for both part and labor, depending on whether you get a generic muffler or a performance muffler. Generic aftermarket models are only priced at $25 to $50 for the part alone, while a mid-range, medium quality muffler will range between $50 and $125. But if you own a luxury vehicle, you will most definitely need to get a high-end performance muffler, which would cost from $300 to $500. And add $80 to $100 of labor cost on top of that.

Small cracks or pinholes on the exhaust pipe is a pretty easy repair, which can be sealed using just epoxy or exhaust tape. You can get a patch kit for typically less than $15, which are widely available in most hardware stores. Meanwhile, the average cost for exhaust pipe replacement is between $760 and $800, with parts usually priced at around $700 while labor costs range from $60 and $100. 


Sweet Smell: Burnt Coolant

If you’re noticing a somewhat sweet smell in the cabin, it’s most likely engine coolant or antifreeze. There are many reasons for burnt coolant. Coolant can contaminate oil, which is usually caused by a cracked or leaking head gasket, and the coolant mixing with oil will be burned off inside the engine’s combustion chamber. Another common reason is that a pinhole leak in a coolant hose can spray coolant onto a hot engine component, which boils the coolant away.

If you notice your heater is producing a sweet smell that is somewhat similar to cinnamon, maple syrup or butterscotch, it is an indication that coolant is leaking into the heating system. There might be broken seals in the heater core, the heater can be cracked, or there is a leak from a hose to the heater core. In this case, you will normally notice coolant dripping onto the ground under your car or running down the side of the heater core. Healthy engine coolant is usually green or orange in color.

What to do and The cost

The leaking head gasket or coolant hose need to be repaired or replaced if the crack is serious. A cracked and leaking head gasket can be time-consuming and expensive to repair. Repairing a cracked cylinder head will cost at least $1,000 for both parts and labor. For a total replacement, the part itself would typically cost between $700 and $850, while labor cost an average of $900 to upwards of $1,000, since it’s a time-consuming job. 

Meanwhile, replacing the coolant hose is straightforward. The average cost replacement is between $130 and $160, of which parts are priced at around $50. 

Gasoline Smell: Rich Air and Fuel Mixture

You should never smell gasoline while you’re driving. In modern cars, it’s rather rare for a fuel leak to happen. The most likely culprit of that familiar, acrid aroma of a filling station from the exhaust is a rich air and fuel mixture. 

In this case, you might also see black smoke coming from the tailpipe. Do note that gasoline that gets into the exhaust system can also cause a backfire, or damage to the catalytic converter.

On diesel-powered vehicles, the exhaust smell will be much stronger than in the case of a gas-powered car. You’ll also notice that the exhaust fumes are thicker and darker, especially on large pickups with powerful diesel engines.

This might be due to your fuel injector sending too much gas through the engine, so not all of this is burned off during combustion. Either the engine is receiving too much gas or not enough air, and the engine control unit is unable to compensate.

There may be various reasons for this off-balance air and fuel ratio. The most common causes are leaking or clogged fuel injectors, clogged air filters, a bad oxygen sensor, a faulty mass airflow sensor, or damages to the throttle body. Another cause of this gasoline smell is a stuck choke if your car has a carburetor.

One often overlooked source of problem is using the incorrect brand of gasoline for your vehicle, which might cause the engine and the exhaust system to work in suboptimal conditions.  

What to do and The cost

The cost to clean a clogged fuel injector ranges from $50 to a bit over $100, depending on the make and model of your vehicle. In addition, you might need to pay an additional $50 if your fuel filter needs to be replaced.

Similarly, a clogged air filter can be cleaned or replaced. They are easily accessible under the hood and usually require no tools at all to replace, so you can do it yourself. The cost for air filter replacement runs from $50 to $90 for both parts and labor. The cost for cleaning can range from $20 to $70. 

The average cost for oxygen sensor replacement is between $330 and $400, including parts and labor. Meanwhile, mass airflow sensor replacement cost is between $220 and $330, the majority of which is the cost of the part itself. 

The average cost for a throttle body replacement is between $580 and $700, of which parts cost between $490 and $580.

Smelly Exhaust and Bluish Smoke: Burnt Oil

If you smell strange exhaust fumes in the cabin and notice blue-tinted smoke coming from the tailpipes, it’s probably due to old engine oil coupled with an overused oil filter that is burning excess oil in your engine, which in turn leads to excess exhaust. 

What to do 

Engine oil lubricates and keeps the vital components in your engine clean and free from corrosion, prevents overheating and minimizes excess wear due to friction. To prevent burning oil as well as to make sure your engine is running in tip top shape, make sure to change your oil and oil filter periodically as recommended in your manual.

The cost

An oil and oil filter change will cost between $35 and $75, if you’re using conventional engine oil. If your vehicle requires synthetic oil, you would be paying more, typically from $65 to $125.


Other Things You Can Do To Reduce Smelly, Toxic Exhaust

Change Your PCV Valve Regularly: Your engine’s positive crankcase ventilation (PCV) valve helps control emissions that are the by-product of engine combustion. The PCV valve can get dirty or clogged with oil and debris. Therefore, regular inspection and cleaning, or replacement if needed, is required to keep this part running properly, which would minimize your smelly and harmful emissions.

Change Your Fuel Filter and Air Filter Regularly: To run smoothly with the least emission possible, the engine requires clean air and clean fuel for combustion. Fuel filters and air filters are included to guard off dirt and debris, but if the fuel filter is too old and clogged, it cannot clean the fuel that enters the combustion chamber. And when the air filter is dirty or clogged, it’s hard for enough air, and clean air at that, to enter the engine. This means your engine will require more fuel to run, and more fuel, especially dirty fuel, leads to more smelly and toxic exhaust.