BMW Repair: 325 Starting Problem, blue spark, distributor cap

Question
Hello Tino,

A friend of mine borrowed my 1987 325 manual and, needing to refill the tank, put in 87 octane instead of 91.  The car ran fine for a while but, suddenly, starting loosing power and stopped running altogether in traffic.  It was able to restart, but soon died again - this repeated about 5 times.  With the car back and after siphoning out the 87 and putting in 91, the car ran fine for a couple days, but the engine will not turn over.  Headlights are bright as normal indicating the battery is ok.  I'm getting bright blue spark on the wires and replaced the distributor cap.  Any ideas on what might get the car up and running again?  My friend is beside herself, so I'm hoping you can help (for her sake and the car's!).  Thanks!

Answer
Hello David,
Just to clarify things, does the starter crank but does not start the engine? I am a little confused by your statement that "... the car ran fine for a couple of days, but the engine will not turn over." How can you have blue spark on the wires if the engine does not run?
I think your friend has not done anything wrong. It may just be that the car took a dive at about this time.
Tell me more of the circumstances please. And is it a 325e?
Regards to your friend,
Tino