Engine Misfires and Smells Like Gas - Top Causes, Fixes & What to Do

Certified Professional Automotive Mechanic – Owner and Editor of VehicleRuns

Last Updated: August 31, 2026

Safety note: Troubleshooting guidance can help you narrow down likely causes, but it cannot replace an in-person inspection. If the vehicle feels unsafe, warning lights are flashing, you smell fuel, see smoke, notice overheating, or have problems with braking, steering, or control, stop driving when it is safe to do so and have the vehicle inspected.

If your engine is misfiring and you also smell gasoline, the two symptoms usually belong together. In many cases, fuel is getting into one or more cylinders but is not burning the way it should, so the engine runs rough and raw fuel odor shows up at the tailpipe or under the hood.

The most common causes are ignition problems, leaking or over-fueling injectors, and air-fuel control issues. A worn spark plug, weak coil, vacuum leak, bad sensor reading, or poor fuel can all create a cylinder misfire and leave a fuel smell behind.

The pattern matters. A misfire at idle points in a different direction than one under load, after a fill-up, or only on cold starts. This guide helps you sort out the most likely causes, what to check first, and when the problem is serious enough to stop driving.

Start by noticing where the gas smell is strongest and when the misfire happens. A tailpipe fuel smell usually means unburned fuel, while fuel odor under the hood raises concern about an actual leak.

What you notice	Most likely cause	What to check first	Urgency
Flashing check engine light and rough running	Active ignition misfire	Scan for misfire codes and inspect plugs and coils	Stop driving
Gas smell strongest at the tailpipe	Cylinder not burning fuel	Check for misfire counts, wet plugs, or weak spark	Can worsen
Fuel smell strongest under the hood	Leaking injector or fuel seal	Inspect injector area and rail for wetness	Stop driving
Rough idle with hissing intake noise	Vacuum leak	Inspect intake boots, PCV hoses, and vacuum lines	Diagnose soon
Problem started right after refueling	Contaminated fuel	Avoid topping off and consider fuel quality issue	Diagnose soon
Hard cold start with fuel smell	Flooded engine or rich mixture	Check for wet spark plugs and sensor-related rich codes	Can worsen

Best first move: If the smell is from the tailpipe, begin with misfire codes, spark plugs, and coils; if the smell is under the hood, first rule out a real fuel leak.

Safety note: Do not keep driving with a flashing check engine light, visible fuel wetness, smoke, or a strong raw fuel smell near the engine bay. Unburned fuel can damage the catalytic converter, and an external fuel leak is a fire risk.

Table of Contents

Most Common Causes of an Engine Misfire With a Gas Smell

A fuller list of possible causes appears below, but these are the three issues that most often explain an engine that both misfires and smells like gas.

Worn Spark Plug: A fouled or worn spark plug can fail to ignite the air-fuel mixture cleanly, leaving that cylinder misfiring and sending raw fuel into the exhaust.
Failing Ignition Coil: A weak coil often causes an intermittent or load-related misfire, and the unburned fuel from that cylinder can create a noticeable gas smell.
Leaking Fuel Injector: An injector that drips or over-fuels can make one cylinder run rich, foul the plug, cause a rough misfire, and produce a strong gasoline odor.

What an Engine Misfire and Gas Smell Usually Means

When an engine misfires and smells like gas, the simplest explanation is that fuel is present but combustion is incomplete. That can happen because spark is weak, the air-fuel mix is wrong, or a mechanical problem is keeping one cylinder from burning normally.

If the fuel smell is mainly from the tailpipe, think first about unburned fuel leaving the engine. Spark plug problems, weak coils, injector faults, and mixture control issues are common reasons. This version often comes with rough idle, hesitation, reduced power, and a check engine light.

If the gas smell is strongest under the hood, shift your focus toward a leak rather than only a misfire. A leaking injector seal, fuel rail connection, or cracked hose can create both a fuel odor and a drivability problem. That pattern matters because it raises the safety risk immediately.

Cold-start misfires often point toward fuel control or enrichment issues, while a misfire that shows up under acceleration leans more toward ignition breakdown or a fuel delivery imbalance. A single-cylinder misfire usually feels sharper and more rhythmic than a general rich-running problem affecting the whole engine.

Possible Causes of an Engine Misfire That Smells Like Gas

Worn Spark Plug

A worn, fouled, or fuel-soaked spark plug may not fire the mixture consistently. When that happens, the cylinder can skip combustion and send raw fuel into the exhaust, which creates a gasoline smell and rough running.

Symptoms to Watch For

Rough idle that improves slightly off idle
Misfire on one cylinder
Poor fuel economy
Hard starts after the engine sits
Black or wet spark plug tip

Moderate Severity

A bad plug may not strand you immediately, but continued driving can overwork the ignition system and send unburned fuel into the catalytic converter.

How to Confirm: Remove and inspect the suspect spark plug after scanning for cylinder-specific misfire codes.

Typical fix: Replace the worn or fouled spark plug set and correct any plug-gap or fouling issue.

Failing Ignition Coil

An ignition coil that is weak or breaking down may fail under load, heat, or moisture. That leaves one cylinder with poor spark, causing a distinct misfire and a raw fuel smell from incomplete combustion.

Symptoms to Watch For

Misfire gets worse during acceleration
Intermittent roughness after warm-up
Check engine light with cylinder-specific code
Jerking or hesitation under load

Moderate to High Severity

A coil problem can quickly worsen and may cause severe misfire, catalyst damage, and poor drivability if ignored.

How to Confirm: Use a scan tool to identify the misfiring cylinder, then swap that coil to another cylinder and see whether the misfire code follows it.

Typical fix: Replace the failed ignition coil and any related boots or seals.

Leaking Fuel Injector

A leaking injector can drip fuel when it should be closed or deliver too much fuel while running. That can flood one cylinder, foul the plug, and produce both a misfire and a strong gasoline odor.

Symptoms to Watch For

Fuel smell after shutdown
Hard restart when warm
One cylinder plug looks wet or unusually dark
Oil level rising or oil smelling like fuel
Rough idle with black exhaust odor

High Severity

A leaking injector can wash down cylinder walls, dilute engine oil, damage the catalytic converter, and create a real fire risk if fuel is leaking externally.

How to Confirm: Check fuel pressure bleed-down after shutdown and inspect injector balance if equipment is available.

Typical fix: Replace or professionally service the faulty injector and renew any leaking injector seals or O-rings.

Vacuum Leak or Unmetered Intake Air Leak

Extra air entering after the airflow meter can lean out one bank or disturb idle fuel control enough to cause a rough misfire. On some engines the computer over-corrects, which can leave a fuel smell even though the root issue started with unmetered air.

Symptoms to Watch For

Rough idle that smooths out at higher rpm
Hissing noise from intake area
Lean condition or fuel trim codes
High or unstable idle speed

Moderate Severity

Many vacuum leaks start as an annoyance, but they can make the engine run poorly, trigger more codes, and complicate fuel control if left alone.

How to Confirm: Inspect intake boots, PCV hoses, vacuum lines, and manifold connections for splits or loose fittings.

How to Find a Vacuum Leak in Your Car

Typical fix: Repair the leaking hose, boot, gasket, or intake connection and restore proper sealing.

Contaminated or Low-Quality Fuel

Bad fuel can upset combustion quality across multiple cylinders. Water contamination, the wrong fuel blend, or severely stale gasoline can cause rough running, misfires, and a fuel smell from incomplete burn.

Symptoms to Watch For

Problem began right after refueling
Multiple misfire codes
Poor acceleration across the rpm range
No obvious single bad cylinder

Moderate Severity

Contaminated fuel may not be a safety emergency by itself, but it can cause stalling, repeated no-starts, and catalyst damage if the engine keeps misfiring.

How to Confirm: Look at when the problem started and whether it appeared immediately after a fill-up.

How to Tell If You Have Bad Gas in Your Car

Typical fix: Drain or dilute the contaminated fuel, replace the fuel filter if applicable, and refill with quality gasoline.

Faulty Oxygen or Air-Fuel Ratio Sensor

A sensor that reports the mixture incorrectly can drive the engine too rich or too lean. If the mixture goes rich enough, the engine may stumble, misfire, and produce a noticeable raw fuel smell.

Symptoms to Watch For

Strong fuel smell without an obvious external leak
Rich mixture codes
Poor fuel economy
Sooty exhaust tip or blackened plugs

Moderate Severity

Mixture control faults can make the car drive poorly and may harm the catalytic converter over time, but they are usually less immediately dangerous than an external fuel leak.

How to Confirm: Use scan data to compare sensor response, short-term fuel trim, and long-term fuel trim with engine operating conditions.

Typical fix: Replace the faulty oxygen or air-fuel ratio sensor and correct any mixture-related contamination issues.

Internal Engine Mechanical Problem

Low compression from a burned valve, worn rings, or another internal issue can keep one cylinder from burning properly even when spark and fuel are present. That leaves a persistent misfire and unburned fuel odor.

Symptoms to Watch For

One cylinder misfire that stays after plug and coil swaps
Low power all the time
Popping through intake or exhaust
Abnormal compression reading

High Severity

Mechanical faults usually do not improve on their own and can lead to severe drivability problems or expensive damage if the engine continues running poorly.

How to Confirm: Perform a compression test and, if needed, a leak-down test on the suspect cylinder.

Typical fix: Repair the internal engine fault, which may involve valve work, head repair, or more extensive engine rebuilding.

How to Diagnose the Problem

Confirm where the gas smell is strongest: tailpipe, under hood, or around the fuel tank area.
Scan for stored and pending trouble codes, especially cylinder-specific misfire codes and rich or lean mixture codes.
If the check engine light is flashing, stop driving and address the misfire before extended operation.
Inspect spark plugs for fouling, fuel wetness, heavy wear, or one plug that looks different from the rest.
Check ignition coils and coil boots, especially on the cylinder flagged by the scan tool.
Look for obvious intake leaks such as split air ducts, disconnected vacuum lines, or damaged PCV hoses.
Inspect the fuel rail and injector area for wetness, staining, or fresh gasoline odor under the hood.
Consider fuel quality if the problem started right after refueling or affects multiple cylinders at once.
Use live data to review fuel trims, oxygen sensor behavior, and misfire counts at idle and under light load.
If ignition and fuel control checks do not explain the issue, perform compression or leak-down testing.

Can You Keep Driving If the Engine Misfires and Smells Like Gas?

Important: The guidance below is general and cannot confirm that your specific vehicle is safe to drive. If a symptom affects braking, steering, handling, fuel, overheating, smoke, visibility, or vehicle control, treat it as potentially serious and have the vehicle inspected before continued driving when appropriate. For more context, see our Automotive Safety Disclaimer.

Whether you can keep driving depends on how severe the misfire is and whether the fuel smell points to unburned exhaust fuel or an actual leak. This symptom can range from an urgent repair to an immediate stop-driving situation.

Okay to Keep Driving for Now

Only mild intermittent roughness with no flashing check engine light, no obvious fuel leak, and no strong under-hood gasoline smell may be manageable for a short period while you schedule diagnosis. Even then, avoid heavy throttle and long trips.

Maybe Okay for a Very Short Distance

A noticeable but steady misfire with a tailpipe fuel smell, reduced power, or a solid check engine light may be okay only for a very short trip to a shop. The main risk is catalytic converter damage from unburned fuel.

Not Safe to Keep Driving

Do not keep driving if the check engine light is flashing, the engine is shaking badly, power is dropping sharply, fuel smell is strongest under the hood, or you see liquid fuel. Those patterns raise the risk of fire, stalling, or expensive exhaust damage.

How to Fix It

The right fix depends on whether the engine is failing to ignite fuel, getting too much fuel, or receiving incorrect air-fuel control information. Start with the fault pattern, then match the repair to the confirmed cause.

DIY-friendly Checks

Read trouble codes, inspect intake hoses and vacuum lines, look for visible fuel wetness, and remove spark plugs for condition comparison. On many vehicles, basic plug replacement is also a reasonable DIY task.

Common Shop Fixes

Shops commonly replace failed coils, install spark plugs, service or replace leaking injectors, repair vacuum leaks, and diagnose fuel quality or sensor-related mixture problems.

Higher-skill Repairs

Injector balance testing, live-data diagnosis, leak-down testing, wiring diagnosis, and internal engine repair usually require more experience, better tools, or professional equipment.

Related Repair Guides

Typical Repair Costs

Repair cost depends on the vehicle, local labor rates, and the exact root cause. The ranges below are typical U.S. parts-and-labor estimates, not exact quotes for every engine.

Spark Plug Replacement

Typical cost: $120 to $400

This usually applies when worn or fuel-fouled plugs are causing the misfire; engine layout strongly affects labor cost.

Ignition Coil Replacement

Typical cost: $150 to $450

Cost varies based on whether one coil or several coils are replaced and how easy they are to access.

Fuel Injector Replacement or Service

Typical cost: $250 to $900

A single injector near the top of the engine costs much less than multiple injectors or hard-to-access direct-injection work.

Vacuum Leak Repair

Typical cost: $120 to $500

Simple hose or intake boot repairs are cheaper than intake manifold gasket or deeper smoke-test-driven repairs.

Fuel Drain and Refill for Contaminated Gasoline

Typical cost: $150 to $500

This range usually fits bad-fuel cases where the tank must be drained and the system reset with fresh fuel.

Oxygen or Air-fuel Ratio Sensor Replacement

Typical cost: $200 to $550

Upstream sensors are often the most important for mixture control, and seized threads can raise labor time.

What Affects Cost?

Whether the misfire is one cylinder or several
Engine layout and access to plugs, coils, and injectors
OEM versus aftermarket parts choice
Need for extra diagnosis such as smoke testing or live-data analysis
Catalytic converter damage from prolonged driving

Cost Takeaway

If the problem ends up being plugs, one coil, or a simple vacuum hose, the repair is often on the lower end. Injector problems, repeated rich running, or deeper diagnosis usually move the job into the mid range. If the misfire has been ignored long enough to damage the catalytic converter or reveal an internal engine problem, cost can rise quickly.

Symptoms That Can Look Similar

Parts and Tools

FAQ

Can Bad Spark Plugs Cause Both a Misfire and a Gas Smell?

Yes. If a spark plug cannot ignite the mixture reliably, that cylinder can send unburned fuel into the exhaust, which often creates a gasoline smell.

Is It Safe to Drive if My Engine Is Misfiring but Still Running?

Only sometimes, and only briefly. A mild misfire without a flashing check engine light may make it to a shop, but severe shaking, a flashing light, or fuel smell under the hood means you should stop driving.

Why Does the Gas Smell Seem Stronger at the Tailpipe than Under the Hood?

That usually points to unburned fuel leaving the engine rather than an external leak. Ignition misfires, rich running, and injector problems are common reasons.

Can a Fuel Injector Cause a Single-cylinder Misfire?

Yes. A leaking or over-fueling injector can foul one spark plug, flood that cylinder, and create a rough, repeating single-cylinder misfire.

Will a Misfire and Gas Smell Damage the Catalytic Converter?

Yes. Unburned fuel can overheat and damage the catalytic converter, especially if you keep driving with an active misfire.

Final Thoughts

An engine that misfires and smells like gas usually means fuel is not being burned correctly, not just that the engine is running a little rough. Start by separating tailpipe fuel smell from under-hood fuel smell, then narrow the problem with codes, plug condition, coil behavior, and signs of injector or intake issues.

The best first checks are usually the most common ones: spark plugs, coils, visible vacuum leaks, and injector clues. If the misfire is severe, the check engine light is flashing, or you suspect an external fuel leak, treat it as a stop-driving problem and get it diagnosed before more damage happens.