My oldest son has 02,325Ci, 2.6 liter 6 cyl BMW.Its running so bad it will not idle. Happen while driving home. 2 up stream o2 sensors were replaced before this happened. Snap On scan tool used to turn off light and clear codes. The proper software key was used. Next day light set again. A check revealed lean bank 1&2, misfire #5 cylinder. Different mechanic attempted to read codes with Upgraded Snap On tool,but could not read through regulator data port with proper key, so he used a generic key to access data. Same codes as before, codes clear. 5 miles later car ran very bad, would not stay running. We changed fuel filter/pressure regulator. Restart ran a little better but still very bad. Plugs were fouled. Plugs replaced. Restart motor ran perfect for about 5 minutes then plugs fouled again. Could ecw been damaged during data check or is it difficult to accidentally erase programing. Prior to all this happening ,check engine light was on for about a year. But car ran fine. Several weeks before o2 sensors were replaced car ran bad for a day or so then smoothed right up.The car will blow white to grey smoke now while its running no but you can barley keep it running ,only at high rpm. On second restart no smoke till it started running bad, then smoke.

White smoke often indicates coolant being burned, due to a head gasket leak. The cause is usually a warped aluminum head, due to overheating. It's common for accompanying valve damage, which may explain the misfires. Have the cooling system pressure tested and the compression checked. Tow the car for servicing, if you concur. Repairs may run $2,500 to $3,000.

It's important to change the coolant every 2 to 3 years. Use a 50/50 mixture of manufacturer approved antifreeze and distilled water. Minerals in tap water cause phosphate scale, which blocks radiator and heater passage ways.