brybo86

I have a vibration somewhere in the dash. This happens only at a low idle near 425ish rpm while completely stopped in gear. This has happened since I got the car last fall. Motor mounts potentially need to be replaced.

Anyways... Previous tank of fuel was 91 octane from early September as we have been gone for a few weeks on vacation.

Last week I filled up at Sam's Club with 93 octane and I'm guessing winter blended fuel.
On my way home from work tonight I noticed that it was no longer vibrating and the idle was a little higher. Instead of the usual 425RPM it was now around 625RPM.



So did my 93 winter fuel cause the increased idle rpm and coincidentally smoothing out my idle vibration?

No recent fuel cleaners.

I did change the oil last week from Amsoil SS 0w30 to Mobil 1 0w30 AFE both using a Fram Ultra XG3600 filter? Could that cause a significant RPM increase?

Or is something else going on?

djamps

IMO probably something else. Fuel octane at idle has nearly zero effect. And you used the same oil viscosity... only thing I can think of is MAYBE the ECU increased timing due to the higher octane (while driving) which led to a higher idle. That said, usually idle timing is fixed and not affected much if at all by learning due to fuel/environment changes. But I don't know enough about the LS400 ECM to know for sure.