I've used this forum for researching a number of items, including the famous (infamous?) P071 and P074 codes.

After cleaning the MAS and checking all the hoses and the like, I was able to clear my codes. But they would come back after less than 200 miles.

Reading one member's comments regarding non-ethanol fuel, I thought I would try it, in spite of its expense ( + .30 a gal).

Interestingly, I found better fuel mileage on the first tank- +2mpg. And the engine seemed to perform better. I liked the response. So I continued to use the more expensive gasoline, and as the tanks went through, I noticed a little more increase in mileage- up a total of 3.5 mpg.
I didn't pay much attention, but I did not have a return to "Limited Performance" mode or "Check Engine".

Last week, I was too low on gas to get over to the Pure station and purchased Shell again. I've heard pinging starting with this tank. And worse, at 160 miles into the tank, the old "Limited Performance" yellow warning came on, as did "Check Engine". A refill with Pure, and within 50 miles, the lights have gone out on their own.

It would appear that at the very least, my XJ8L is not happy with ethanol. With the cost per mile being very nearly the same, I'm going to stick to Pure and pass on the ethanol any time I can.

 

it's odd that it would be so sensitive, my XJR uses 10% ethanol with no issues, maybe you have a sensor that is on the edge of failure somewhere in the exhaust system

 

I agree that a sensor is probably on its way out. My failing MAF sensor caused my car to misfire and ping. When i disconnected the battery to clear the fuel trims, the misfire and pinging got much worse.The MAF itself didnt set any codes. But i had codes indicating a lean burn on both banks.I got a new MAF for $30 off eBay. My transmission actually shifts much smoother with the new MAF.

 

Also the O2 sensors would give you a lean reading, if running the lower octane fuel for a long period of time.

These engines was built to run on fuel with at least 91 Octane. Not regular or 10% Ether, even though Premium cost more, you will get better performance and gas mileage.

You might even get away with 89, which is 2 Octane levels lower than the recommend octane of 91.

For the most part I run 93, but when the going gets tight I run 89.