I want to buy a bmw 318i e46 2004 mdelby my friend but it cut out while driving.

SOURCE: Idle problem

I've had an Idling problem with my 1993 318i E36. First, I would suggest getting the fault codes for your car. There are places where you can have these codes scanned for free such as autoparts stores. The fault codes will tell you which components aren't working properly. Based on my fault codes, I replaced the Idle Control Valve, the oxygen sensor, the camshaft position sensor, and the air intake boot. I could easily install all of these parts in less than an hour. All of these replacements cost me a total of about $200 and solved the problem completely. Granted, I did this myself. Look forward to the dealer charging you over $500 if you have as many problems as I did.

SOURCE: the rev is going up & down on idle & when driving

have you checked your air inlet manifold for any leakages by spraying WD40 around each portall bolted to the cylinderhead gasket,check for vacuum air pipes deterioration.make sure the engine is running first ,then spray WD40 to check that the engine speed should increase.it sounds like your engine is haveing problems breathing due to heavy hydro carbons increasing./and your CO2% is dropping.

SOURCE: BMW 318i wont start engine

check the starter

SOURCE: battery drains in my 1984 BMW 318i

I have the same car I had the same problem.If you have the correct battery for your car with the same problem you have a short or positive ground in your dash/electric fuel pump/fuse box also be carefull of how you take the key from the ignition and if they're old replace them.

SOURCE: Would a crankshaft sensor fault cause my BMW to just cut out?

Short answer is yes it would. Not the only thing it could be but to answer your question, yes.