BMW Help : Engine vibrating after filled with lower octan gas

could be knocking due to the lower octane rating.......ie not burning properly.

Go find a bottle of octane booster and see if that helps

 

Detonation would more likely happen at load. And then only if the sensors were not working.

Likely not octane related IMO.

 

Would it have to do with lower compression leading to offset timing or mixtures? I know the computer should compensate for that, but maybe not in this case as the car is not programed for the lower end fuel?

 

Out of the German cars, the brand that needs the highest octane (91-93) possible is BMW because the engine is tuned for high performance. 91 or higher is required on Bimmers, whereas on Mercs and Audis, I've noticed 89 and higher is required.

 

All of my Mercedes have taken premium fuel, but when I have been stuck with 89 octane they ran fine. On occasion I have tried plain old regular 87 and I actually did notice what seemed to be a rougher idle.

Like was previously posted, the BMW's ECU should monitor detonation and/or fuel quality and adjust accordingly. It might be possible to feel a difference when under a load, but I doubt it. He could probably try a bottle of octane booster, or just drive a hundred miles or so and fill the car back up with premium. He might also find it takes a few starts before it adjusts as well.

It is also possible that he got a bad batch of gasoline, depending on where he bought it. There are differences between brands and sometimes contamination happens.

Off topic, but one interesting thing about my wife's old C-class was the directions to switch to E85. You should drive the car until it was almost completely out of gas, then fill completely with E85. If it ran rough, I found the solution amusing: add three gallons of gasoline.

 

Burn some of the lower ocatane stuff off (1/4 tank) and then refill with premium. Winter blended fuels are notorious for giving more drivability problems than summer blends.

 

Did I not read somewhere that BMW is now using a very advanced and unconventional knock sensor - i.e. that it detects individual pre-ignition on a cylinder by cylinder basis via a light sensor rather than with a single acoustic sensor on the engine block? Maybe it is doing it's thing on just one cylinder because that one has a hot spot rather than pulling back the timing on the engine as a whole?

Please note my anti-hallucinogenic disclaimer --- I could have possibly just dreamed this up.

 

On my car (BMW) it will run less smooth on 87 than it runs on 91. It's noticeable (slightly) at idle- the engine seems to compensate better under load (yes I know thats counter-intuitive but it's what I've experienced).

It could be that he has a marginal spark plug or something causing one cylinder to run a little off - you'd notice that at low RPM. The difference in my car from low to high octane is very very slight. I tend to use lower octane in winter and stick to premium when it's warm out. If it's a very noticeable difference it would be something other than gas, IMO. I would dump in a couple bottles of octane boost just to be sure though - only cost a few bucks and better safe than sorry.

 

Most likely bad gas, water is usually the culprit this time of year. Put in a bottle of dry gas and see if that helps it.

 

BMW uses a system, although more advanced, than the old VW Digifant system, where knock is detected on a cylinder by cylinder basis...and the timing can be retarded by individual cylinders versus all at once, as well. The single knock sensor reads knock only when a specific cylinder fires and adjust that, and only that cylider, if necessary. Hence, a cylinder specific hot spot can be aleviated.

Although this is obvious, if the fuel filler cap is not re-secured after a fill up, these cars will idle poorly...due to a "perceived vacuum leak" in the evaporative system.

Regards,
David

 

How many tanks of regular did he run? One isn't going to cause an issue.

Is the vibration in the steering wheel?

 

US service stations use the Pump Octane Number or PON rather than RON, make sure you're all reading from the same system.

 

A couple of tanks shouldn't be noticeable, also the car will need time to get acquainted with the poorer fuel and adjust accordingly.

 

So what is the outcome? I seriously doubt the fuel octane is the culprit, contaminated fuel perhaps but not an octane issue. The engine will adjust to fuel, besides there is near zero chance of detonation at idle.