E70 N52 Extended Cranking at Startup

[andrewwynn]

"extended cranking" is the primary symptom of a failing starter. So "functioning perfectly" may be inaccurate (though it also could be accurate)

The same symptom also comes from a weak battery or a bad ground or as you surmise: bad fuel pump or FPR.

You can eliminate bad ground by measurement of the voltage from B- jump point under the hood to the engine block while starting. Should be millivolts not say over 0.2-3v.

Usually you can eliminate battery as the cause by a load test.

You can test your theory of fuel supply with the FPR being the most likely candidate of failure by an overnight fuel pressure test. Hook up a gauge to the fuel rail and test it to see if you get 50 psi. Then turn off the car it should drop quickly to ≈ 40-43 but very slowly drop over the next 8-12 hours. It should still read 10-20# overnight.

I am not sure on N52 (I think that's your motor) on the rail pressure it may have been changed since M54 I'm more familiar with but I don't think it uses the stupid high pressure like N55/63.

You can also measure the voltage across B- B+ jump points during starting. I haven't measured that value on my or wife's e70 yet but I could do that for a rough baseline. My customer has an N52 based 328xi I could also measure that value.

What causes the starter related slow crank is shorts in the winding that increase current while reducing torque=power. The increased current drops more voltage (often by as much as two volts) which dramatically drops the power even more. The starting rpm will drop and it takes more seconds for the same number of power strokes to get the car started.

It's more evident on diesels, you read about it very often but will happen on any car.

That said, it's just as likely to be a fuel supply issue so start with checking whatever is the easiest.

You can check the voltage drop from the cig lighter to B+ under the hood and multiply by 300 to get a rough estimate of current. My m54 used to produce 0.75v on a first attempt and 0.625v on a restart (within about a minute).

I haven't done that test on my n55 or wife's N63 but I could do that test on customer's n52 as soon as tonight/tomorrow for an N52 baseline.

One last thought re: fuel pump. They get weaker and weaker over time at end of life but their site demise is masked by the FPR so they seem to fail quickly though they usually fail over 6-12 months.

Ideally a test of pressure at the pump is best but it takes a lot more effort to do that test.

I would measure the current draw to the starter to eliminate that variable since it's rare one that bites people in the ass. (I've read at least 5-6 cases where people replaced their perfectly ok battery to discover 2-5 months later the exact slow crank returns because the problem was the starter pulling 300 vs. 200 amps.)

Take your odometer and divide by your OBC average speed eg 149000/27=5518.519.

If that number is over 5000 the pump is on borrowed time. That number above is close to my actual values meaning i probably should at the least measure my low pressure pump's output myself.