I don't think it was the gas/octane.

When you leave the gas cap too loose the yellow check engine light came on. I bet you didn't put the cap on properly the first time and tightened it properly when you bought your second tank.

I don't know about the V8 in a 540, but the I6 in my 323i doesn't like anything lower than 91.

91 will derive overall better performance out of your car versus 89 so I would stick with it. I think Desertnate is correct about your gas cap being the culprit of the light being thrown. Very common.

ive filled my tank more than 10 times the same way and have had the same problem until i filled it with 89 octane so im pretty sure the cap was on correctly. Its a mystery!!! I think im just going to have to put some new o2 sensors in just to be sure!

Bentley has a good section on the function of the check engine light (CEL); how it turns on, when it turns off, and how long it stays in memory. To badly summarize about 5 pages of text, the CEL comes on for a miriad of causes. If it is a serious cause, it comes on immediately. If it is a less serious cause, it can take a few drive cycles to confirm the problem. If the problem goes away, then the light will stay on a few cycles until the problem is confirmed to have gone away. The error then stays in memory for 40 to 80 drive cycles before getting erased. (Think of a drive cycle as a startup from cold, get up to operating temperature, and then have the Engine Management system complete its diagnostic test.)

What this means is that the problem went away a few drive cycles before you switched gasoline. It was likely just a coincidence that the light went out at that time.

I would not replace the O2 sensors... yet. You should have the code read, or invest in an OBD II or Peake Reader and do it yourself. It would be a shame to spend a few hundred $ on sensors when they may not be required. Believe me, there are many causes of a CEL.

SoCaLE39 said:

ive filled my tank more than 10 times the same way and have had the same problem until i filled it with 89 octane so im pretty sure the cap was on correctly. Its a mystery!!! I think im just going to have to put some new o2 sensors in just to be sure!

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Yeah, these things work for diagnostics and will give you a better idea of the cause of the CEL. Also, there are definitely A LOT of reasons the CEL would turn on. You have to remember that it is basically a dummies guide to some mechanical issue that could be anywhere in or around your car. Although I do find it funny that it is still called "Check Engine Light" even after all these years.

rtw_travel said:

I would not replace the O2 sensors... yet. You should have the code read, or invest in an OBD II or Peake Reader and do it yourself. It would be a shame to spend a few hundred $ on sensors when they may not be required. Believe me, there are many causes of a CEL.

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