I've been meaning to ask this question since last summer when it happened, but here goes. I took my kids on a road trip from MN to Colorado Springs last summer in my 2006 530i. We took the "long" way there through Kansas, and when I filled up, only 89 octane (I believe) ethanol-blended fuel was available at every gas station I stopped at in Kansas. I've never had less than 91 octane in it (usually 92) but I had no choice. Anyway, after a day in CS, we went to Pikes peak. At that point, I had about 1/4 tank of gas, and it wasn't until I paid the fee that I saw the sign recommending that you have a 1/2 tank of gas minimum to make the round trip. Well the car said I still could drive 100+ miles, so I forged on.

With stopping for pics, traffic, etc - that number started dropping rapidly - but still I was not worried. At least until I got literally 500 feet from the top of pikes peak and my car's warning chime went off indicating a fuel pump failure and it died. Outstanding...right? 3 kids under 10 years old in the car, 500 feet from the top of pikes peak pointing up at about 40 degrees, no guard rail - and my car dies. I'll leave aside the fact that I thought WTF...isn't this car supposed to be able to drive in the freaking swiss alps? Anyway, after contemplating the cost of a tow truck to remove my vehicle from the top of pikes peak, I tried starting it again. I made it about 10 feet before it died again. So there I sat

Kind soul #1 went ahead to notify the rangers, and then kind soul #2 stopped traffic for me, and I did a (very harrowing) reverse 3 point turn to get my car facing down hill. The car started right up again, and this time I was able to make it about 1/4 of a mile before it died again, and I was able to pull over on a flat section that had a shoulder. Of course on the 1/4 mile stretch before it died again, I passed the ranger that was on his way to help and waved him on. Can't make this stuff up. So I let the car sit for 5-10 minutes, started it again and was able to make it all the way down to the bottom and another 10 miles away to a gas station with no fuel pump issues that day or any day since.

I did some quick research, and it seemed that I may have created the perfect storm that allowed my modern fuel injected engine to suffer vapor lock, which I thought had ceased being an issue years ago. High altitude, traffic, frequent stopping and starting, and 1/4 tank of lower octane, ethanol blended gas. Is vapor lock really what I experienced, or was it something else? And don't they sell 91+ octane gas in all states? It's a funny story to tell now, but at the time it wasn't funny at all...

My belief regarding vapor locks is that they take longer to clear than you describe. So, could it be that when facing uphill, your remaining fuel was starving the fuel pump due to the slope of the car, and when facing downhill, the fuel was filling the fuel pump? This could be so if the fuel pump was toward the front of the tank. Also, can't speak for Kansas, but 91 octane gas is the normal "premium" grade in Colorado. I believe it is due to the higher altitudes, and therefore less oxygen per volume of air.

Not sure your problem was caused by 89 octane gasoline.

Is it possible you got E15 gasoline (15% ethanol)? You car probably wouldn't like that much. Normal blend you see is E10. Kansas, being a corn producing state probably has a lot more ethanol variants readily available.

Most of the gas stations in the mountain states have 85 octane as regular. I remember my daughter's old Corolla owner's manual even saying that in high altitude states, that 85 octane was ok. It specifically mentioned western Kansas and Colorado.

Also see http://en.wikipedia.org/wiki/Octane_...nal_variations

"In the Rocky Mountain (high elevation) states, 85 AKI (90 RON) is the minimum octane, and 91 AKI (95 RON) is the maximum octane available in fuel[citation needed]. The reason for this is that in higher-elevation areas, a typical naturally aspirated engine draws in less air mass per cycle because of the reduced density of the atmosphere."

Not sure about 2006, but my wife's 2009 X5 (N52 engine) says you can use 87.

In a different car I once owned, I was low on the tank (lower than 1/4) and on an incline and the engine died.

First, let me say that I am not a mechanic. However, I have the Bentley Service Manual to help with whatever DIY projects I do, so I looked it up.

The fuel tank is not flat-bottomed, it is saddle-shaped. The fuel pump is located on the right side of the tank (filler side). On the left side of the tank is suction jet pump that pumps fuel to the right side for delivery to the engine. The tank is under pressure and the pumps and internal delivery system are designed to work at 50.7 PSI.

My guess is that because the level of the fuel was so low, and because the air pressure at 14,000 feet is well less than at sea level, the system had difficulty moving the fuel to the right (delivery) side of the tank. Don't think it had anything to do with the octane level or vapor-lock.

It's interesting that you mention driving in the Alps, because I was there last month ... drove the back roads through the Alps with a half- to full tank of gas with no problem. But although I don't know to what altitudes the road took us, we never drove to the peak of any mountain, so I doubt we ever hit 14,000 feet like at Pike's Peak. A few years ago drove Pike's Peak, and the only thing that happened was that my wife couldn't write addresses on postcards (oxygen deprivation), and on the trip down we had to stop to let the brakes cool down.

being low on fuel and at a constant angle is not good. and by being low, you had more vapor/air in the tank, which may have expanded as you rose in elevation, due to lower pressure outside.
and with the fuel level being low, the pump doesnt cool off, like it would when its immerssed in fuel.

i think you should have taken the warning sign more serious, and you wouldnt have had the issue.

the indicated range on the OBC is based on your driving of the past 15 miles. If your driving conditions are going to change dramatically compared to your last 15 miles, don't trust the computer!

+1. I had the suction tube crap out on my old e39. Same fuel tank design for weight distribution. I would go around a corner, and all the fuel would slosh into the other side, and couldn't get back. The engine stopped with 3/8 of a tank of gas.

+1

Vapor Lock possible cause of similar incident to Big_Jim's
 

I had a similar (harrowing) thing happen to me yesterday in the Colorado mountains as Big_Jim did.

We had been driving stop-n-go style through Evergreen, CO on the way to Mt. Evans, where the temperature was 80 deg F at 7000 ft, no stalling yet. In fact, I-70 before that at similar but high-speed up-hill conditions we were fine as well; no stalling.

Conditions when it stalled the first time: 2007 BMW 530xi, 1/2 tank of fuel, the usual 10% ethanol E10 91 octane (premium normal fuel for the Denver area), elevation 9,000 feet, 76 deg F, 4% grade up, 35 mph speed, 300 lbs of people (2) on board, only 40 lbs cargo, on the Squaw Pass (Colorado 103) 5 miles from Echo Lake. (Not that stressfull, right?!)

When it stalled the first time, the engine just lost power suddenly. I hit the warning flashers for the one car behind me and coasted quickly to a stop on the incline up. I was only 20 feet past a sufficient pull-in spot so I backed it in carefully to a perfect spot with 3 feet to spare off the road (safe anyway). We walked downhill 1/4 mile to a spot overlooking the I-70 area in the distance to get cell phone bars. It kept dropping calls so I borrowed a bicyclist's phone and called AAA and a tow was 1 hour away.

We walked back to the car and tried to start it again. Twenty minutes had passed and the car started and ran normally. Called AAA and cancelled the tow in plenty of time. We continued up the hill, determined to complete the trip.

We passed Echo Lake and entered the Mt. Goliath area, and the car stalled again on a hair-pin turn 50 feet up hill from the national forest Visitor Center building. Conditions were 73 deg F, 11,000 ft elevation. I backed the car down the hair-pin turn (I know, yeah, fun...) with the hazard flashers going, and parked in a spot right in front of the Visitors Center. Park service employees told us they see vapor lock up there quite frequently, and occasionaly wrecks where power steering and brakes on a dead engine cause accidents. I let the car cool down and now the battery runs down because the electric cooling fan was buzzing away for so long, and a park service truck comes by and jumps the battery. The BMW starts and runs normally all the way down the mountain into the Denver area just fine.

Is this a common problem?
 

Does BMW acknowledge weaknesses in their fuel pumps and or fuel line or evaporative control system pressurization performance? Granted, my '07 530xi has 94,000 miles, so fuel pump wear might be an issue.

My own quick search of the internet shows there have been recalls and Technical Service Bulletins (TSBs) of similar era models like the 04 X3 and later Z4s which might use the same or similar fuel systems. I wonder if this is a frequent problem on 04-11 5-series and other models which use the same 3.0L N52 engine we see all over the place.

I plan to ask Denver-area BMW dealership service advisors (Schomp BMW and Murray BMW) about what their customers have been complaining about, since many go into the mountains around here.