Trouble starting

[OSM e30]

i have a 89 325i and it has trouble starting sometimes like i would turn the ignition and the engine will crank but wont start, but i noticed that when the car is on the "on" position and all the lights on the dash are on, if the check engine light isnt on the car wont start but sometimes the check engine light will light up and then my car will start up perfectly fine, what can this be? because sometimes i just leave the ignition on the on position and wait til the check engine light lights up to start my car and sometimes it takes a while. can anyone help me please? i think it maybe be the crank sensor but i was just wondering what you guys thought, thanks

[jlevie]

That could be any of a number of things, but a problem with the main relay or DME would be at the top of my list of suspects. The check engine light should come on as soon as the DME is powered up and it should stay on until the DME sees timing data from the engine.