Originally Posted by BimmerFan52

What is the electrical circuit equivalent of "not on the same page"?

Battery? I doubt it. If you turn the key 30 times and it doesn't start because the battery can't deliver enough amperage, what suddenly "energizes" the battery on the 31st attempt?

A bad alternator may not charge a battery, and it may deliver an over-voltage to the electrical system after starting, but it will not prevent a car from starting if the battery has sufficient voltage.

OP - Your phrase "every once in a while" leads me to believe that this intermittent problem (that comes and goes) may be due to a worn component making a poor connection.

But to just eliminate the battery/alternator get a plug-in volt meter. Leave it plugged in and monitor your voltage constantly. Monitor before starting (turn key to on position but don't crank the engine - you want to see 12.1-12.5V) . Then start it and watch the voltmeter for the first 30-40 seconds after starting (you want to see 12v ramp up to about 14.2v) . Then glance at it occasionally during driving (should vary between 13.5V-14.5V).

If you see numbers close to these then start thinking about the starter solenoid. When it won't start have someone else get in the car and try to start it while you have an ear as close as you can get to the starter. If you don't hear a click when the key is turned that means one of two things - The starter solenoid is not engaging when a voltage is sent to it (solenoid on the starter is going bad) or the voltage is never being sent to the solenoid in the first place. If you hear a click but the engine won't crank then you are done troubleshooting - it is the starter motor starting to fail and the starter needs to be replaced. Unfortunately for the E60s the starter placement is such that it is not easy to attach a set of voltmeter leads to the solenoid to see if it is getting voltage when you turn the key.

Intermittent ground connections can also be the source of starting gremlins, although unless you have had recent work done on the car, especially body work or electrical system work, this is less likely.

If you don't hear the click you can also temporarily assume the solenoid is good and try to work upstream. In your case this is not a bad assumption as the solenoid is a coil that acts as an electromagnet. When voltage is applied the solenoid pushes a plunger to engage a starting gear into the engine and closes a circuit to apply voltage to the starter motor itself. Generally if the solenoid is failing but continues to start after multiple attempts, it gets weaker and weaker over a period of months until one day you get nothing, whether you turn the key 10 times or 1000 times. But from your description you can go days with no symptoms.

Keep picking away at all of the things that prevent a voltage from being sent to the solenoid. One example is the switch associated with the brake pedal. If the switch is worn and the computer thinks you are not pressing the brake pedal because the switch is not closing the car won't start. A second example is a worn ignition switch with the contactors needing to be in just the right position before a connection is made. Electronic modules can certainly prevent starting but do not generally come and go as you describe.

Intermittent starting can be as frustrating as a vacuum leak. But stay with it and keep eliminating possibilities and you will eventually arrive at the cause.

Originally Posted by kushane

Not sure about the N62, but on my M54, I had to diagnose a similar intermittent no start issue and I was able to get a electrical multi-tester lead on the positive connection on the starter (and the other lead to a ground). Had someone else turn the key to see if it showed voltage at the starter. It did, and I ended up replacing the starter and problem solved.

Originally Posted by BimmerFan52

Good job. Was it your solenoid or starter motor?

I dread the day I have to replace my N62 starter. A good part of the exhaust system has to be removed to get to it.

Originally Posted by BraveHelios

UPDATE: This morning drove to work, no issues. Parked in afternoon shade. Hi temp in the shade (after 9 hours of sitting) was 68 degrees (via car thermometer). Started car right up... no problem. Car idled for about 1 minute and died. Tried to restart once... nothing; no cranking... nothing. Tried to restart a second time and started up and drove home.

Any votes for a crank sensor considering the new occurrence?