Rick's Free Auto Repair Advice

Wrong gas in car

Wrong gas in car

What happens if you put the wrong gas in car?

I see this question a lot in auto forums. Someone fills with 87 octane in an engine that requires 93. Then they realize the problem and want to know what will happen if they put the wrong gas in car.

Modern cars and compensate for the wrong gas

Car computers can’t tell what gas you put in your tank. It can only determine how well the gas is performing in your engine. If you have a high performance high compression engine and you fill with low 87 octane gas, the computer will detect the difference by way of the knock sensor. As it detects knock signals caused by detonation (gas ignites from the heat of compression instead of from the spark plug), it retards ignition timing to avoid the double ignition–one from the heat of compression and the second from the spark.

But the computer can only compensate so much. Once the computer retards spark timing by a certain amount, if it still detects knock, it will set a trouble code and turn on your check engine light.

What to do if you used wrong gas in your car?

Nothing. Drive less aggressively because pedal to the metal driving will cause more knock events. Your goal is to prevent knock by gentle driving, use up at least a 1/4 tank as quickly as possible and refill with a higher octane fuel. Keep doing that and you’ll be fine.  You don’t have to add octane booster to correct this problem.

Will the check engine light go out on its own?

If the trouble code appeared shortly after you added the wrong gas to your car and you’ve since filled with the right gas, the computer will gradually adjust and the light will go out.

What causes engine knock?

Octane has nothing to do with power. Gasoline with higher octane simply resists igniting too early on high compression engines. In a high performance engine with higher compression, 87 octane will ignite itself from the heat of compression during the compression stroke. The computer will see that as a misfire and the incomplete combustion gases will flow down to the catalytic converter. There, any unburned gases will be burned off. Unfortunately, all that extra unburned fuel raises the temperatures inside the converter—sometimes to the point of melting the precious metals inside.

The detonation of the lower octane fuel will cause the engine to knock and will be detected by the computer’s knock sensor. The computer will retard the timing to try and compensate and more closely match the spark event to the actual combustion. But retarding the spark will result in less power and fuel economy will suffer at the very least. You might also see a Check Engine light with misfire codes. It’s a bad idea all around. You gain nothing and you risk damaging the catalytic converter.

© 2012 Rick Muscoplat


Posted on by Rick Muscoplat

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