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My owners manual says don’t use gasoline with ethanol or methanol. But if you must, you can use up to 10% ethanol and 5% methanol with rust inhibitors. The past couple of years I’ve found that my check engine soon light comes on for a week or two after I’ve filled up. Then goes off. Engine runs fine at all speeds and idle, does not overheat. Dealer said they had to reprogram computer. Last time it happened right after a fill-up in Iowa. My mechanic says he has heard that excess levels of ethanol are causing this.

What were the fault codes stored by the computer?
What year is your Infinity?
How many miles?

This is the firat I’ve heard of an Infinity being sensitive to 10% ethanol. I’m inclined to suspect the problem lies aomewhere other than in the gas tank.

It might be as simple as a bad gas cap. That CEL (check engine light) is just a kid in class waving her hand trying to get you attention because she has the answer. You need to have the codes read. Some places will read them for FREE. Try Autozone or Advanced Auto Parts. Get the exact code (like P0123) not just their translation into English and post it back here.

The car is an 01 Infiniti I-30 with 87K miles. My mechanic checked the code and got a “lean exhaust” problem. Then he turned off the service engine light. I crossed the street to a Phillips 66 station (top tier) and put in 10.4 gallons of premium advertised as having no ethanol. On the last fill-up of regular with 10% ethanol I got only 16 mpg. On the interstate at 70 I get 27-29 and around town 23. Something very strange going on. I cannot help but wonder if some brands are using more ethanol than 10% when the price is right.

Comparing the premium to the regular really isn’t doing things apples to apples. Is this car a premium recommended or required car?

Engine is a 10:1 compression ratio and premium is recommended, but selling dealer suggested using mid-grade (89 octane) which I did for several years occasionally using premium. When prices rose I switched to regular and found no pinging or obvious problems. The point is that the service engine light would go off at the next refill. The only variable seemed to be a new gas up. And I notice that at some gas stations, mid-grade is priced same as regular, the only difference being the addition of ethanol. Does ethanol increase octane rating?

Ethanol does increase octane, but the rating on the pump has already taken that into account. If it says 87, it’s 87 regardless of how they reach that rating.

Since your car in premium recommended, that means that the car is designed to run on premium but the computer can de-tune the engine to allow it to run on regular without damaging the engine, but it does so at drastically reduced performance. I’d suspect that you’ve got some sort of borderline running problem that’s only bad enough for the engine to notice when it’s running in detuned mode. Most likely it’s something as simple as just needing a tune up-- the ethanol probably doesn’t factor into it.