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Hey, Why Does My Car Smell Like Rotten Eggs?

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Four common causes for a rotten egg smell from the vehicle and their related parts.
It stinks to have a car smell like rotten eggs. Since toxic chemicals from the combustion process can be a specific danger to humans and or damage the catalytic converter, it is important to act early. The symptom must be tested even when the check engine light doesn't come on. Besides the foul-smelling gases produced, Sulfur gases, Sulfur Dioxide, Hydrogen sulfide, Exhaust fumes, and other harmful gases can be smelled in the passenger compartment. On the menu, car smelling like rotten egg odor is more difficult to pin down than burning smells. Common symptoms described are a rotten egg or sulfur smell.

Does your car smell like rotten eggs?

A rotten egg smell in cars comes from a gas known as sulfur dioxide. The catalytic converter, fuel pressure regulator, fuel filter, etc., can exaggerate this gas if there's a processing fault, creating an odor.
Engine oil leak: An engine oil leak can cause a rotten egg smell in a car when the leaking oil comes into contact with the hot surfac...read more
Fuel regulator leak: A fuel regulator leak can cause a rotten egg smell in a car due to the presence of sulfur compounds in gasoline....read more
Differential/transmission fluid leak: A differential or transmission fluid leak can cause a rotten egg smell in a car due to the brea...read more
Faulty battery: A faulty battery can cause a rotten egg smell in a car due to the release of hydrogen sulfide gas. When a battery is ...read more
Bad air-fuel mixture: A bad air-fuel mixture can cause a rotten egg smell in a car due to incomplete combustion. When the air-fuel mi...read more

A rotten egg smell in cars comes from a gas known as sulfur dioxide. The catalytic converter, fuel pressure regulator, fuel filter, etc., can exaggerate this gas if there's a processing fault, creating an odor.

The main causes for a rotten egg smell on a car are:

  • Engine oil leak: An engine oil leak can cause a rotten egg smell in a car when the leaking oil comes into contact with the hot surfaces of the engine or exhaust system. As the oil burns off due to the heat, it can produce a sulfurous odor similar to rotten eggs. 
  • Fuel regulator leak: A fuel regulator leak can cause a rotten egg smell in a car due to the presence of sulfur compounds in gasoline. The fuel regulator is responsible for maintaining the proper pressure of the fuel system. If the regulator is leaking, it can allow small amounts of fuel to escape, which can then vaporize and cause the smell.
  • Differential/transmission fluid leak: A differential or transmission fluid leak can cause a rotten egg smell in a car due to the breakdown of the fluid. Both differential and transmission fluids contain sulfur-based additives that help reduce friction and wear. If there is a leak in either system and the fluid comes into contact with hot components, it'll cause a rotten egg smell.
  • Faulty battery: A faulty battery can cause a rotten egg smell in a car due to the release of hydrogen sulfide gas. When a battery is overcharged or damaged, it can produce hydrogen gas as a byproduct of the chemical reactions inside the battery.
  • Bad air-fuel mixture: A bad air-fuel mixture can cause a rotten egg smell in a car due to incomplete combustion. When the air-fuel mixture in the engine is not properly balanced, it can lead to inefficient combustion, resulting in the production of hydrogen sulfide gas. This gas has a characteristic rotten egg smell and can be emitted from the exhaust system when the engine is running.

Toxic combustion chemicals can harm humans and damage the car's catalytic converter, which controls emissions. So, even if the check engine light didn't pop, hurry up and take your car for an inspection.

In short, the car's emission system makes sulfur dioxide odorless. Roughly speaking, the catalytic converter transforms toxic gases and pollutants from the engine combustion process into safer-to-breathe gases.

However, when the catalytic converter has an issue, these gases get a free pass to produce bad smells and even cause allergies.

The symptoms might change if the vehicle's tank contains different brands of gasoline. Moreover, even old transmission fluid and the tailpipe's smoke color are clues of a problem.

Let's Get To The Bottom Of It!

  • Q: Is your car leaving leaks in the garage?
    Leaking gear oil from differential fluid has the same problems and unpleasant odor as old transmi...read more

    Leaking gear oil from differential fluid has the same problems and unpleasant odor as old transmission fluid leaking from other systems like transfer cases or manual transmissions; this might be the case.

  • Q: Is your battery faulty or leaking?
    Failed or damaged battery cases can cause sulfuric acid fluid leaks. Then, batteries with interna...read more

    Failed or damaged battery cases can cause sulfuric acid fluid leaks. Then, batteries with internal failures or overcharging swells create gases that stink like a rotten egg.

  • Q: Have you recently changed the oil?
    As the engine burns gasoline, oil, and transmission fluid, a specific odor generates while the ca...read more

    As the engine burns gasoline, oil, and transmission fluid, a specific odor generates while the catalytic converter turns the exhaust stream into harmless gases. In this process, gasoline quality or engine oil overflow are common culprits of bad smells.

  • Q: Have you noticed an increase in fuel consumption?
    A fuel pressure regulator might fail. The pressure sensor regulates the engine's fuel flow. So, s...read more

    A fuel pressure regulator might fail. The pressure sensor regulates the engine's fuel flow. So, sensors reporting the wrong pressure to the engine control computer produce poor emission control. Also, a lousy sensor in a hot engine bay can result in leaks from faulty parts. A fuel filter can clog up and stop over time, resulting in low pressure. A worn-out filter fails similarly to fuel pressure sensors. A faulty fuel pressure sensor affecting the system's operation interferes directly with exhaust gases. A failing fuel pressure sensor can increase consumption, risking car fire. Typically, code scans can provide a complete snapshot of the vehicle, reporting these types of problems.

  • Q: Did you recently fill the fuel tank?
    Your car's engine having too much fuel might damage the catalytic converter, making the engine em...read more

    Your car's engine having too much fuel might damage the catalytic converter, making the engine emission system malfunction from the fuel overfill and incomplete combustion, resulting in poor fuel mileage. The engine might be burning transmission fluid, impacting your car's catalytic converter. The catalytic converter neutralizes the emission of bad gases, like sulfur gas. But, a faulty catalytic converter won't work properly, setting toxic chemicals free.

Find Professional Help

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If you are a shop owner and sick of business directories that are based on the cost of repair, AutoTechIQ is for you. If your customers receive Digital Inspection results then AutoTechIQ is definitely for you. Check out the certification page or how to rank in your area.

Vehicle Health Inspection Proof

Explore the following typical inspection results that show a potential cause for the symptom and select the one you believe is similar to your vehicle's issue.

Engine code set

During the inspection, the mechanic did a code scan. The code-scanning process relies on data reading from the vehicle's computer; the computer stores a "history log" of everything that happened in the car.

In this "history log," there'll be the occurrence of internal issues, if any. The mechanic's code reading revealed a "Bank 1 System Too Rich" warning, meaning a combustion issue. In this case, the issue concerns a wrong fuel-air mixture during engine operation. So, there's a cylinder delivering too much fuel, becoming excessive or "too rich."

The code was pending, and the driver's dashboard didn't indicate any warning light yet. So, it needs testing since it possibly relates to the sulfur gas smell.

The shop must check the car's computer network on every service visit. In this case, even though the warning light wasn't blinking, it was close to flashing. We'll test the fuel system; poor fuel control may cause foul smells from the car.

Does the issue look like this? if not accessible your shop will document it
add1/5
Engine oil overfull

Part of a digital inspection is checking fluid levels before the service. When checking oil level, mechanics often see if the fluid is at a proper measure, low, or overfilling. Various components use different fluids to operate, like the transmission, cooling, and engine.

The engine uses typical engine oil. In this chapter's proof image, the issue relates to an excessive amount of engine oil surpassing the recommended level. This fluid excess can happen due to a lack of oil draining in oil changes, faulty oil sensors, use of the wrong oil, leaks, and more.

Additionally, leaks into the engine oil from cracked transmission, oil coolers, failed intake gaskets, head gasket failures, cracked blocks, or head castings cause overflow. Although oil leaks often cause a burning oil smell, they might also produce a rotten egg odor if they contaminate the exhaust system.

In this case, the engine oil was overfull due to improper measuring. Additionally, sludge was causing slow drainage between the top of the engine and the oil pan, complicating fluid measurement. Regular oil changes and draining avoid overfilling.

Does the issue look like this? if not accessible your shop will document it
add2/5
Fuel regulator leaking

When opening the car's hood, the technician saw leakage signs on the fuel pressure regulator. This component works by ensuring that the pressure in the fuel rail remains at a specified level; this is essential for the fuel injection and combustion process.

The fuel pressure regulator works with various micro-components, one of which is the vacuum hose. This hose carries a vacuum. Therefore, it balances the pressure from inside the fuel tank to the engine, setting a specific range; this is all due to the physics principle of balancing opposing forces. In this chapter's example image, the vacuum hose was dripping fuel.

A damaged fuel regulator rubber diaphragm allows unmetered gas leaks into the engine. The technician saw signs of fuel seepage and opened the hood, spotting a lousy regulator. Hence, poor fuel mileage, bad vehicle performance, engine oil contamination, and exhaust emissions were causing bad smells from the tailpipe.

Does the issue look like this? if not accessible your shop will document it
add3/5
Differential fluid leak (transmission fluid) create a rotten egg smell because of their sulfur composition

The technician saw a leak under the car after lifting it for an inspection. The example photo from the differential shows gear oil leaks, which make a rotten egg smell.

The differential is an essential part that allows the vehicle to steer and drive safely; it does this by controlling each wheel's rotation speed and frequency according to the car's movement. The transmission system and the differential are closely related since both transfer power to the wheels.

Some transmission and differential fluids have sulfur-containing compounds as additives. These compounds improve lubrication and reduce friction. However, when the fluid breaks down due to overheating, an issue with the output shaft seal, or aging, it can release sulfur compounds, including hydrogen sulfide (H2S), into the surrounding air. The hydrogen sulfide smells like rotten eggs.

Does the issue look like this? if not accessible your shop will document it
add4/5
Worn and old battery leaking chemicals that produce a rotten egg smell

Checking the battery's condition is crucial. In this case, the mechanic opened the vehicle's hood and saw wet spots, case swelling, and gas leaks on the battery. They'll need to replace the lousy battery. Batteries can fail due to age, hot temperatures, and high charges; many things can go bad if the battery leaks.

Batteries have a specific chemical composition that can cause a rotten egg smell. In most cases, a chemical reaction between sulfuric acid, lead dioxide, and lead plates happens inside the battery, causing the odor. Still, a chemical reaction of sorts is only concerning in old and worn batteries.

Additionally, batteries can also contaminate. This type of contamination happens when the sulfuric acid inside the battery breaks, producing hydrogen sulfide gas. This issue typically develops in batteries that are repeatedly charged and discharged throughout their life cycle.

The battery tray prevents damage to the case and acid leaks. If the battery case swells, the mechanic will test the alternator and ensure it doesn't overcharge or overheat the battery.

Does the issue look like this? if not accessible your shop will document it
add5/5

Typical Makes

That need Rotten Egg Smell significantly more often than average vehicle makes

  • Make:

    Honda

  • Model:

    Odyssey , Accord, Pilot, Ridgeline

  • Battery Replacement

Is this the make you are driving?
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Typical Fixes to Address the Cause(s)

The following chapters bases themselves on experiences from our auto repair shop; we'll describe related problems' causes and fixes.

"Car Battery Replacement" fixes "Car Battery Leaking"

  • Battery Replacement

    Battery Main Replacement
    Battery Auxiliary Replacement

  • Related testing, service, or replacement as part of this fix

    Battery Tray Replacement
    Battery Hold Down Install
    Battery Registration To Computer After Replacement
    Charging and starting system testing
    Alternator Replacement

The AutoTechIQ ranking for Safety, Cost Avoidance, and Environmental Impact is

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Hey, Why Does My Car Smell Like Rotten Eggs?
Sometimes a problem is more challenging to describe than it initially looked like. If you are not sure your problem is described by this article, please find below similar vehicle symptoms, which might describe better the issue you are experiencing.

Vehicle Symptoms

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Other things your auto repair shop might talk about:

Too much oil, burned out electrical fuse, odorless sulfur dioxide, car's air conditioner, car's exhaust system. car smells like rotten eggs. worn out fuel filter. malfunctioning catalytic converter. exhaust pipe. broken catalytic converter. cabin air filter. air conditioning system

Typical Fixes Pages:

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