Over the past year or so I've occasionally noticed some pre-ignition sounds under a load or hard acceleration and would back off the throttle. It might happen several times in a week and then I wouldn't notice it again for a month or more. The computer has not registered any codes during the past three years. When it has happened, I've experimented a few times with bottles of octane boost products from the auto parts store and found that a sufficient dose will prevent any pre-ignition.

It started me wondering if the gas stations that I've been using have been cheating on the octane, pumping 87 even though I'm pressing the button and paying for the 93. I always use Exxon, Mobil or Chevron because those are the cards I've got. My other cars either call for 87 or perhaps are not as sensitive to a lower rating so I can't confirm it with another car.

So has anyone else experienced what you thought might have been a tank full of the wrong octane? A quick google search found lots of very old articles like it was once an issue but not much recent on the topic. On the other side of the coin, if a knock sensor is going out, will the x308 definitely throw a code or can it fail in such a way that the computer doesn't know it has failed?

 

The knock sensors have a specific tightening torque.

It is part of the design/calibration intended to give
the sensors the correct degree of sensitivity.

If the sensor is loose, it will not be as sensitive and
your ECM will not compensate correctly for load and
fuel.

 

I'm sure you meant detonation and not pre-ignition. The two terms are not interchangeable.

If you've got no fault codes pointing towards the knock sensor or ignition and are consistently using gas with an octane rating equal or above the Jag recommendation of 91 AKI, then it's probable that the noise you hear is not detonation.

Off the shelf octane booster, at best, will raise the rating of the fuel by .1 point. 93 octane becomes 93.1 and not 94 as the misleading advertising states.

 

I struggled for a year with detonation in my XJ8.
It only happened incidentally, and in the usual detonation area, low rpm under load.

I did a cleaner run several times, and only used the highest octane of gas.
As it was incidentally, it was hard to judge it worked, but the sound always came back at a certain time, so obviously it didn't.
Then, cleaned the MAF sensor, and ~1 year on, it disappeared forever, even with lower octane gas I tried.

The funny thing is we had the same problem on our SLK280 (not uncommon according the SLK forum), and there it also disappeared after cleaning the MAF and throttle body.