Will mixed gas hurt a car engine?

Using the wrong type of fuel in your car can cause serious damage to the engine. Most cars on the road today require unleaded gasoline to run properly. Putting leaded gasoline, diesel fuel, or a mix of different fuel types into a gas engine can lead to decreased performance, long-term damage, and costly repairs. Understanding the potential risks of using mixed gas in your car is important to keep your engine running smoothly.

What is mixed gas?

Mixed gas refers to filling up your gas tank with different types of fuel at the same time. This could mean putting in unleaded gasoline and leaded gasoline together, mixing gasoline with diesel or biofuels, or accidentally pumping the wrong fuel at a gas station. Some common fuel mixes that could potentially hurt a gas engine include:

Gasoline and diesel

Diesel fuel has different properties than unleaded gasoline. It is thicker, more lubricating, and compressed differently in the engine. Gasoline ignites more easily while diesel fuel requires hotter compression to combust. Putting gasoline into a diesel engine can cause spark ignition, while diesel in a gas engine will not ignite properly leading to misfires, power loss, and fuel system damage. The two fuel types should never be mixed.

Unleaded and leaded gasoline

Leaded gasoline has been phased out in most countries due to the toxic effects of lead. Mixing leaded and unleaded fuel can lead to build up of lead deposits in the catalytic converter and oxygen sensors. This gradually degrades emission control effectiveness. Prolonged use also increases lead exposure.

Gasoline and ethanol or methanol blends

Biofuels like ethanol and methanol are increasingly added to gasoline. Ethanol can be mixed at up to 10% (E10 fuel) and methanol up to 5% in most vehicles. However, higher ethanol blends like E15 or E85 have different chemical properties and should only be used in flex-fuel vehicles. Improper mixing can damage seals and gaskets in a standard gas engine.

Gasoline octane levels

Using gasoline with the wrong octane rating for your vehicle can also cause performance issues. Low octane gas leads to knocking and pinging, while high octane provides no benefit unless specified by the manufacturer. Mixing different octane gasolines can cause uneven combustion and damage over time.

How does mixed gas affect the engine?

Using mixed or improper fuel can lead to both short and long term consequences for your vehicle’s engine. Here are some of the potential effects:

Lower engine performance

When the fuel does not combust properly, it leads to reduced engine power, acceleration issues, and lack of responsiveness. Knocking or pinging sounds may be audible from the engine. The car will feel sluggish and struggle under load.

Misfires and trouble igniting

The engine needs the right fuel mix to ignite the cylinder charge at the proper time. Mixed gas can cause ignition failure resulting in misfires, especially under load. This leads to jerking and juttering as cylinders fail to fire properly.

Fuel system corrosion

Incompatible fuels or using gasoline with ethanol in an older vehicle can cause corrosion in tanks, lines, and seals. This leads to clogs or leaks that further degrade performance. Rubber lines and gaskets may be degraded.

Sensor and catalytic converter damage

The oxygen sensors and catalytic converter are designed to operate with standard unleaded gasoline. Leaded gas, diesel mixtures, and excessive ethanol can coat or damage these emissions control components leading to check engine lights or efficiency loss.

Engine knocking and pinging

Pre-ignition and abnormal combustion creates knocking or pinging sounds. This indicates poor performance and can damage pistons or cylinder walls over time. Knocking can lead to reduced engine life.

Carbon build up

Improper gas mixtures cause carbon and lead deposits to quickly build up on valves, injectors, and combustion chambers. This build up hurts efficiency and can lead to further damage. Excess carbon emissions may be visible from the exhaust.

Cylinder wear and overheating

Poor lubrication, friction, and abnormal combustion can accelerate cylinder and piston ring wear. Engines may run hotter leading to warping, gasket failure, and additional repairs. Knocking also worsens wear.

Signs of possible mixed gas damage

How can you tell if using the wrong fuel may have damaged your car’s engine? Here are some key signs to watch out for:

– Knocking, pinging or rattling sounds from the engine
– Lack of power during acceleration
– Difficulty starting the vehicle
– Check engine light illuminated
– Fuel or exhaust smells different than normal
– Visible exhaust smoke or emissions
– Increased fuel consumption
– Oil level rising from fuel dilution
– Fuel leaks or corroded fuel system components
– Misfiring, sputtering, or backfiring

If you experience any of these issues shortly after refueling, contaminated or mixed gasoline could be the culprit. Any sudden changes in drivability after filling up likely indicate fuel-related problems.

Can mixed gas damage be reversed?

If using mixed or lower quality gas caused engine problems, is it possible to undo the damage? Here is what you can do:

– Drain the fuel tank and lines to remove any remaining mixed gasoline. This may require disconnecting lines and replacing filters.

– Professional fuel system cleaning to remove any lead or carbon deposits from injectors and the combustion chamber. Fuel additives may help dissolve some deposits.

– Engine de-carbonization to remove built up carbon from pistons, valves, catalytic converter and oxygen sensors if excessive build up was present. May require dismantling some engine components.

– Replacing any damaged sensors or engine parts like seals and gaskets. Rubber components may be degraded.

– Engine tune up including spark plugs, wires, filters to ensure optimal performance

– Switching to higher grade fuel and oil for several tank fills to help dissolve deposits

– Allowing engine computer to relearn proper settings and parameters through driving cycles

While these steps may help restore engine function, some lasting effects like cylinder wear will remain. Damaged engines may never fully regain original power and economy. Preventing mixed gas problems by using only the recommended fuel remains key.

Can mixed fuels be used in any situation?

Are there any cases where mixed gasoline use does not necessarily harm the engine? Here are a few exceptions:

Classic cars designed for leaded gasoline

Vintage cars built before unleaded fuel can often run better on leaded gas mixed with modern unleaded to raise the octane rating. This allows old engines to operate as originally intended.

Dual-fuel vehicles

Some heavy duty fleets use dual-fuel engines designed to run on both diesel and natural gas. The fuels are stored separately and mixed upon injection. These engines are made to handle multi-fuel use.

Small mixes of recirculated diesel

A very small amount (5% or less) of diesel accidentally mixed into a gas engine may be burned off without issue if drained quickly afterwards. This should never be done intentionally.

Higher ethanol blends in flex-fuel vehicles

E85 ethanol fuels can be used in FFV vehicles to provide more ethanol content. The engines are optimized to handle this. But ethanol mixes only work in specified vehicles.

Octane boosters to increase rating

A small amount of octane booster additive is approved for some automakers to increase the antiknock properties. But only the recommended additive percentages should be used.

So there are some niche cases where gas mixes are intended, but these are specialized engines designed for multi-fuel use. For standard gasoline engines, mixed gas or improper fuel leads to both short and long term damage.

Best practices for avoiding mixed gas

Using the correct fuel for your vehicle is essential to prevent problems. Here are some tips:

– Check your owner’s manual for the right octane rating and fuel type

– Never mix gasoline and diesel or leaded and unleaded fuel

– Use a fuel additive only if approved by your automaker

– Fill up from busy stations with high turnover to avoid old gas

– Never use E85 or other blends unless your car is a flex-fuel vehicle

– When switching fuels, use up old gas first before filling tank

– Avoid using different gas stations when refueling to prevent mixing

– If misfueled, drain tank immediately. Do not restart engine.

– Replace old fuel lines and gaskets to prevent degradation

– Pay attention and choose correct pump nozzle. Diesel nozzles are larger.

Following fuel specifications for your car’s engine is the best way to optimize performance and avoid potential damage from mixed gasoline. Be attentive when fueling up.


Mixing incompatible fuels or using the wrong gasoline for your car comes with many risks ranging from decreased economy to extensive engine damage requiring repairs. Gasoline engines are calibrated for a specific fuel grade to avoid pre-ignition, carbon build up, and unnecessary wear. While effects may not show immediately, prolonged use of mixed gas will hurt engine operation and longevity. Pay attention when fueling up and stick to the recommended gasoline grade and type for your vehicle. Be vigilant to identify any drivability or performance changes shortly after filling up that could indicate fuel-related issues. With proper care and maintenance, sticking to a single fuel source will keep your car running smoothly for the long haul.

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