Skip the auto shop - Our mechanics make house calls

Q: RPMs fall when using higher octane fuel, 2005 Hyundai Elantra

asked by on

I just refueled my engine with 91 Octane fuel today. It sounds much different and in my opinion better and also feels as if it has much more "pickup" when I stomp on the gas pedal at low speeds, but after 80 MPH on the interstate with the pedal to the floor, the RPMs begin to slowly fall. Is this possibly the higher octane fuel? Maybe spark plugs? Any ideas?

My car has 69700 miles.
My car has an automatic transmission.

As you may know, the compression ratio of your engine determines the octane rating of the gas you must use in the car. Higher octane fuel prevents the air-fuel mixture from igniting before the spark plug does it. Firing the air-fuel mixture at the proper time gives you the maximum power your engine was designed to get. What may be happening when you are giving the engine more fuel by peddling it to the floor is providing the engine with more fuel than it is able to burn at higher speeds. This is a result of the car’s ignition timing being designed to use 87 octane fuel. When you provide more fuel to be ignited given a certain ignition timing, this can cause a slight delay in the ignition of the fuel resulting in the drop in performance as you have described. For more assistance determining the best fuel for your vehicle, YourMechanic has trained technicians who can travel to you and assist in any repairs that may be needed.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: Car won't crank, even after replacing parts.

First, thing you need to find out is where you're losing your spark from. That will consist of taking the wiring harness connector and testing voltages on the ignition control module in the distributor. I believe that may be where...

Q: My Hyudai elantra 2014 stalled and stopped after the battery light, engine light and oil light came on

Hello, thank you for writing in. With check engine lights on, the first step is always scanning the vehicle for codes. These help direct your attention and ease diagnostics. The main concern is that the engine is making a loud...

Q: Bad oil pressure sensor

A common failure in your vehicle is the oil pressure sending unit. Once the sensor is replaced, these codes should go away. I would ask for help from one of the mechanics at YourMechanic to replace your oil pressure sensor...

Related articles

What Causes Hoses to Leak?
While the largest part of your engine is mechanical, hydraulics plays a significant role. You’ll find fluids at work in a number of different areas. Your car's fluids include: Engine oil Transmission...
How to Avoid Back Pain in a Car
If you have back problems, sitting in a car for an extended period of time can be excruciating. Even without back problems, you could experience discomfort and soreness from...
P2103 OBD-II Trouble Code: Throttle Actuator Control Motor Circuit High
P2103 means there is a fault with the throttle actuator control motor circuit, likely due to a defective electrical component or part.