Knock, knock...
 

No, not a joke here, but a question... In part from suggestions from other members of the forum, in part because of a suggestion from a dealership service manager, I started using mid-grade 89 fuel in my car about a year ago, a change from the top grade 91 or 93 I had been using. I couldn't detect any driveability issues, fuel economy remained the same, power seemed unchanged, but oddly enough the engine seemed to run more smoothly on the 89 vs. the 91 gas :dunno . Okay, fine with me, and at about a 10-cent per gallon price differential, I was saving a little money.

Lately, however, I think I'm detecting some knocking when the engine is under load, such as accelerating or climbing an incline. I can only hear the knocking when the sound has something to bounce off, such as tunnel walls or a nearby vehicle. Obviously I'll be reverting to the 91/93 grade for the next coupla tankfuls to see if that eliminates the apparent knocking, but nonetheless I have a few questions:

1) Is 89 mid-grade the wrong stuff to use on a regular basis?
2) How well does the knock sensor compensate for lower grade gas?
3) Does the onset of summer weather, and the seasonal formulation of gasoline, have an effect here?
4) With high mileage on an otherwise well maintained engine (200,000+ miles) is carbon buildup inevitable, calling for higher octane fuel?
5) Might I be hearing something other than knocking (like the marbles in my head for instance) :crazy ?

oh I really don't know how American cars can run on such bad fuel...
the worst quality you get here is 91, and you can't even get that in most countries. Usual you fuel up with "Super" which has 95 octane, "Super +" has 98 octane and V-Power or other HP-fuels have 100 octane.

But to answer your question at least partly, my E30 325i CAN be run on 91, so the knock sensor should in theory not be the problem, but I would recommend 95 because the engine runs a lot more smooth and feels faster

#1. "mid grade" fuel is ok to use, provided you're not towing anything
#2. knock sensors work great - on everything EXCEPT carbon buildup
#3. Summer does have an effect, when warmer the fuel expands at a different rate (generally slower)
#4. carbon buildup is a long term effect on ALL engines - a good dose of Sea Foam & STP fuel treatment then a fuel filter change should make a major difference in it's drivability.
#5. the marbles issue sounds like pinging because of carbon.

Different method of calculating the pump octane in the US vs Germany

V-Power here is 92 octane

wait, i put 89 in all the time, thats what it says on the gas cap, should i be doing anything different?

maybe a little STP gas treatment occasionally (twice a year) just to keep injectors clean and moisture/condensation out.

If the gas cap tells you to put 89 in your car, just use 89.

You honestly shouldn't be putting all of your engine knocking blame on the grade of gas. Even if you are using 87 in general, everything should be fine. Start looking other places other than the grade of gas you are using.

You got a point there, I'm beginning to hear rattling when I turn my head to one side every now and then... :runaway

are you sure its not hte famous bmw exhaust rattle

Actually I was referring to the marbles inside my head :bounce ... I can't say I'm familiar with the exhaust rattle, unless you might be referring to something up by the exhaust manifold, as it seems to me that what I'm hearing comes from the front, not toward the tailpipe. It can be kinda hard to judge sometimes, though...

An update... Ran the previous tank nearly dry (!) and refilled with 91 or 93 stuff (as measured in U. S.; the manual says 89 AKI or 95 RON), and have run the car ~275 miles since then. Haven't noticed any pinging in situations where I did before, and subjectively the car seems to have more power. It could be one of those quirks where at least over the summer, and perhaps due to carbon buildup, the higher octane is what I need, although I'll do a bit more experimentation.