Q: I own a 2003 BMW 3 Series with 90,000-plus miles. No real issues with the car ever, until this June. When I was pulling out of my complex, the car died, but started right up. Then, two days later it happened again. Then...it happened three weeks later. I took it to the shop and they kept the car for days and then responded that the ethanol levels in the gas tank were too high. So, I let the tank run empty and placed high-octane gas in the car - which was two weeks ago. But then today the car died again, minutes after starting...Any ideas?? - Stacy S., Minneapolis

A:Looking at the big picture, your car needs air, fuel and spark to run. Interrupt the flow of any of these items for more than an instant and the car will stall. A plugged air filter or air passageway on the intake side, or a catalytic converter that gets blocked up on the post-combustion side, can cause your car to stall. Such blockage typically would not come and go; it would be there and cause its problem fairly consistently.

If there were a spark problem - an ignition component failure - this likewise would typically appear and stay, rather than arise infrequently with normal operation otherwise. Overheated parts can cause stalling, but you mentioned that this happened minutes after starting.

A fuel problem is a likely culprit based on your facts, and ethanol can be the source. Fuel manufacturers add ethanol to our fuel by government mandate to reduce pollution. A problem arises if water gets into the fuel. Ethanol-blended fuels can dilute a small amount of water per gallon. Should the amount of water exceed that amount, however, the ethanol molecules will bond with the water and separate from the gasoline, a condition known as phase separation. (Water is in the air and can enter fuel from, for example, condensation in storage tanks.)

The water/ethanol compound is heavier than gasoline and sinks to the bottom of the tank, the area from which the fuel system draws it to feed to the engine. This condition can produce stalling. Fuel that has separated in this way can also, over time, cause damage to fuel system components. Particulate matter in the fuel lines, such as rust, can lead to blockage, including intermittent blockage, which will also cause your vehicle to stall.

If you've run your tank almost to empty and refilled it with good gasoline, the problem could involve some blockage in the fuel system. Check your injectors, fuel pump and fuel filter. Also, try buying gas from a different station.

One more thing: Fuel systems are vented so that, as your car drains the tank, a vacuum doesn't form that stops fuel flow. Occasionally the vents can become blocked. Check to ensure that this hasn't happened as it, too, could cause the vehicle to stall.