Ethanol-Blended Gas and Older Cars: What You Need to Know

If you drive an older car—especially one with a carburetor or early fuel injection—ethanol-blended gasoline can cause problems that regular maintenance won’t catch. Modern pump fuel typically contains 10% ethanol (E10), and some blends reach 15% (E15). Understanding how ethanol behaves will help you avoid corrosion, drivability issues, and expensive repairs.

Why Ethanol Is Different

Hygroscopic behavior. Ethanol is hygroscopic, meaning it absorbs moisture from the air. In vented classic-car fuel systems, that moisture can accumulate and promote corrosion in fuel tanks, lines, carburetors, and steel hard lines.

Phase separation. When enough water is absorbed, the water/ethanol mix separates from the gasoline and sinks to the bottom of the tank. Engines may then ingest a water-rich layer, causing hard starts, misfires, rust, and potential damage.

Lower energy content. Ethanol has less energy per gallon than gasoline. Older engines may run leaner on the same jetting or fueling strategy, leading to hesitation, higher temps, or detonation risk if not corrected.

Common Symptoms on Classics

• Hard starting after sitting a few days or weeks
• Rough idle, hesitation, or stumble on tip-in
• Corroded tanks, senders, and carburetor internals
• Swollen, softened, or cracked rubber hoses and seals
• Clogged filters from loosened varnish and debris

Materials and Components at Risk

Older rubber compounds (nitrile, neoprene) and certain plastics weren’t designed for alcohol exposure. Over time, they can swell, soften, or crack. Brass, zinc, and pot-metal carb parts may corrode more quickly in the presence of water-laden ethanol fuel. That's one reason why using a fuel additive containing PEA, like Driven Injector Defender or Driven Carb Defender at every fill up on cars made before 2000 is a must, especially carbureted models.

Prevention and Best Practices

• Use ethanol-free fuel where available, especially for seasonal or infrequently driven vehicles.
• Upgrade fuel hoses and seals to modern ethanol-resistant materials (e.g., SAE J30R9/J30R14-rated hose). Replace accelerator pump diaphragms, needle/seat assemblies, and o-rings with ethanol-compatible versions.
• Re-jet or retune carburetors if needed to correct lean operation caused by ethanol’s lower energy content.
• Add a quality stabilizer like Driven Storage Defender before storage and run the engine long enough to pull treated fuel through the system, if the fuel won't be used within 30 days. For longer storage, drain carb bowls or run the engine dry.
• Keep the tank full to reduce humid air space and slow moisture uptake; use a tight-fitting fuel cap.
• Service on schedule: replace filters more often during the first months after switching fuels; inspect tanks and senders for rust.
• Mind the label: E15 is not approved for most pre-2001 vehicles and many small engines; avoid misfueling.

Storage Tips

For vehicles that sit, combine several strategies: ethanol-free fuel if possible, stabilizer, full tank, drained carb bowls, and periodic start-ups with a gentle drive to operating temperature. This reduces phase separation, varnish formation, and corrosion.

If the fuel is older than 3-4 months and a stabilizer was not used, unfortunately you'll want to pump that fuel out and put fresh fuel in. 

Quick Checklist

• Confirm local fuel blend (E0/E10/E15)
• Inspect and replace aged rubber hoses with ethanol-rated lines
• Refresh carb soft parts (seals, diaphragm, needle/seat)
• Consider minor re-jetting or tuning adjustments
• Use stabilizer for any storage beyond a few weeks
• Replace the fuel filter after the first few tanks and then annually
• Check tank, sender, and lines for corrosion at service intervals

Bottom Line

Ethanol blends aren’t an automatic deal-breaker, but older fuel systems need the right materials, maintenance, and storage practices to stay reliable. Address hoses, seals, and tuning proactively and you’ll avoid most ethanol-related headaches.