Question: I have a Ford E-150 and a Silverado 1500, both with V-8 engines. Both engines idle so smoothly you cannot tell they are running with good no-alcohol gas, and both engines have a rough idle when using E10. As Nebraska is a corn-growing state, it pushes E10 (15 cents a gallon cheaper), but it comes with added problems. The Ford gets 13 mpg and the Chevy gets 15 mpg with pure gas, and they both lose 1.5 mpg with E10 (that's 10 percent). I buy the higher-priced no-alcohol regular. I know some areas only sell that blended junk some call gas. Just some added info from Nebraska.
Answer: I have yet to hear a complaint about a rough idle while burning E10 compared with ethanol-free gasoline. But I've also not driven too many Ford or Chevy trucks in which you can't feel the engine running. The U.S. Department of Energy states that E10 will drop fuel economy 3-4 percent compared with straight-up gasoline. I suggest getting a gasoline/ethanol test kit. These kits were previously expensive, but now are readily available for less than $20. It's simply a test tube where you mix water and gasoline, then read the alcohol content at the separation line. No matter what it says on the pump, they may be more than 10 percent ethanol -- especially in the winter fuel blends. Ethanol is actually a higher octane rating than gasoline (used in race engines), but the problem is it has a significantly lower energy density. Simply put, you need more fuel (richer mixture) to produce the same power as gasoline. And that's what causes the drop in mpg. But keep in mind that today's non-flex-fuel PCMs are more than prepared for E10. They just richen the fuel mixture to compensate. But when someone accidentally fills up with E85, it produces a lean condition out of range for the PCM to deal with. This causes lean drivability symptoms, and the MIL comes on with a too-lean diagnostic trouble code. Back to the subject: Have you noticed the rough idle shortly after refueling with E10, coming off straight gas, but it eventually improves? That may be an indication you're getting a temporary lean condition prior to the PCM adjusting fuel delivery.
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