The Dangers of Bad Gas: What You Need to Know

After helping 40,000 customers with repairs and maintenance, we’ve learned at Jerry that the common causes are: 

• Water contamination (rainwaterhigh condensation) in the tank.
• Stale/oxidized fuel (low volatility after long storage).
• Sediment/rust from aging or disturbed station holding tank.
• Ethanol phase separation (water absorbed into E10/E15 separates and falls to low spots in the tank and then pulled in by the fuel pump. Additives won’t “re-mi” it, so draining is required).  

This guide will help you learn how to spot bad gas symptoms, how to fix them, when it’s unsafe to drive and when to call a pro for a repair. Typical solutions range from replacing a fuel filter to draining the tank, but the important thing to remember is that quick action keeps costs down and protects your engine.

If symptoms are severe (knocking, stalling), stop driving to avoid damage. For quick help without the jargon, use the Jerry app to estimate repairs from a national database, find nearby shops, view open recalls, chat with an AI about symptoms and set maintenance reminders in one place. 

Real customers Jerry has helped

Costs for getting bad gas out of your car depend on your exact vehicle, where you live and the quality of parts used. With Jerry, you can quickly see what drivers are paying right now. Below are real examples of customers we’ve helped secure top prices for their bad gas issues. 

Estimates are modeled based on real vehicle and location data; names have been changed. Actual prices will vary by shop, parts, and vehicle condition.

At a glance: Can I keep driving?

Here’s how to judge urgency based on what you feel behind the wheel.

• 🚨 Urgent—turn it off and get help (roadside assistance or tow)
  • Flashing Check Engine Light with loud knocking or pinging under load.
  • Severe bucking, stalling or no-start right after refueling.
  • Strong fuel smell with poor running (possible leak/contamination).
  • Misfueling via diesel in gas car or gas in diesel — do not restart.
• 🕒 Soon—OK to drive but fix this week
  • Rough idle or hesitation, steady (not flashing) Check Engine Light.
  • Hard starts or poor acceleration after a recent fill-up.
  • Noticeably worse gas mileage.
  • Fuel pump whining louder than usual.
• ✅ Monitor—safe to drive
  • Mild pinging sound on steep hills with low-octane fuel (87, 91).
  • Slight roughness that improves after topping off with fresh fuel.
  • No warning lights and normal power otherwise.

Symptoms, likely causes and typical fixes 

Bad gas shows up in a few predictable ways. Use this table to match what you’re experiencing with likely causes and common fixes.

Download the Jerry app to compare local automotive repair quotes side-by-side in minutes.

Risks if you ignore it

Leaving bad gas unaddressed can snowball into expensive problems. Here’s what’s at stake:

Can I repair this myself? (DIY vs. pro)

If you suspect bad gas, act based on severity after running a quick diagnostic check in the Jerry app. Light symptoms can sometimes be improved at home. With a severe knock, flashing lights or misfueling, call for a tow and a pro.

DIY (easy, low risk)

These steps may help with mild octane issues or light moisture, but won’t fix heavy contamination.

• Top off with the correct or higher-octane Top Tier fuel to dilute a suspected bad tank.
• Add a “dry gas” product containing isopropyl alcohol or methanol for minor water infiltration.
• Avoid hard acceleration until you run through most of the tank.
• Note the last station where you filled up and track down the receipt. Ask a shop to take a fuel sample if in for service.
• If the car runs worse after any of the above steps, stop and call a tow.

Pro (recommended)

Shops have tools to confirm contamination and protect your engine. They may:

• Test a fuel sample for water and ethanol separation.
• Drain the tank and lines with proper equipment.
• Replace the fuel filter and inspect the fuel pump.
• Perform fuel pressure and injector balance tests.
• Scan for codes and check misfire/fuel trim data.
• Road test to confirm the fix with clean fuel.

Special notes:

• Diesel engines rely on fuel for lubrication — don’t crank if misfueled.
• Direct-injection/turbo cars are octane-sensitive.
• Many modern cars use in-tank filters that may require pump module replacement.
• For stored vehicles, always use a fuel stabilizer.

Prevention

Preventing bad gas is easier (and cheaper) than fixing the ensuing problems. Jerry customers have learned to build these habits:

What NOT to do:

• Don’t keep driving if you hear a loud knock or see a flashing Check Engine Light.
• Don’t dump random additives.
• Don’t siphon with makeshift hoses — modern tanks have anti-siphon valves.
• Don’t start a misfueled diesel engine — tow it.

JEREL'S CORNER

A few years back, I was assigned a GMC Yukon that was towed in after a major flood event in the area. The owner said it would crank, sputter and stall, then only crank over. The truck had to be pushed into the garage even though the fuel gauge read full, but when I opened the gas cap, I didn’t get the usual raw fuel smell. That was my first clue.  

A fuel sample proved my suspicion right—about half of the sample volume was water. After draining the fuel tank, flushing the pressure line and running a fuel rail mounted injector cleaning service, the 5.3L engine came back to life with little damage. To stay ahead of any future evaporative emissions (EVAP) codes, I suggested the charcoal canister be replaced because it also had to have absorbed moisture from the flood water. 

The complete service totaled $860, which was a lucky break for this Yukon. If the owner was able to drive the vehicle, the contamination may have ended up causing more damage.

Jerel Lawrence

ASE-Certified Mechanic and Contributing Author

👉 Want quick repair estimates, recall checks or local shop quotes without phone calls? Download the Jerry app to get everything in minutes.

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Summary

Our goal is clear, trustworthy information.

Our content is created, edited and fact checked by a team of licensed insurance agents, automotive pros and experienced journalists who know the industry inside and out. Facts are thoroughly researched, verified and kept current, and our editorial team is separate from our business partners, so our opinions & tips are unbiased. All insights drawn from Jerry’s proprietary, aggregated customer auto insurance quotes data provide unique, real-world guidance.

Our experts

Jerel Lawrence

Jerel Lawrence is an experienced and certified automotive technician with a strong foundation in diagnostics, repair and technical mentorship. A graduate of NASCAR Technical Institute in 2010, Jerel began his career at national automotive repair chain locations, where he earned eight certifications from the National Institute for Automotive Service Excellence (ASE). 

From 2017 to 2022, he worked as a General Motors-certified technician at a Cadillac dealership, refining his skills in manufacturer-specific diagnostics and service procedures. Today, Jerel leads as the senior technician and mentor at a family-owned repair facility, where he helps guide the next generation of technicians while ensuring top-quality service.

In addition to hands-on work, Jerel contributes to the automotive community through writing detailed articles and answering technical questions to help vehicle owners better understand and care for their cars.

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Everett Cook

Everett Cook is an award-winning journalist and editor with more than 10 years of experience across a variety of industries. In editing for Jerry, Everett’s mission is to help readers have a better understanding of the costs of owning or leasing a car and to better understand their vehicle in terms of insurance and repairs. Prior to joining Jerry, Everett was an editor for Axios. His previous work has been featured in The New York Times, The Los Angeles Times, The San Francisco Chronicle, The Atlantic, Atlantic Re:think, The Boston Globe, USA Today, and others. He’s also been a freelance writer and editor with experience in SEO, audience building, and long-term content roadmaps. Everett is a proud graduate of the University of Michigan.

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