Warm start problem - fuel pump?

[Vern]

Vern -- can you please describe the known BMW problem, along with the fix?

Chris, here's the deal...I had a specific Megasquirted car that I set up and tuned. It had a diesel run on condition, much like a '70 LS6 Chevelle on cheap gas. The result was a flooded engine, which would be hard to start when warm, every time the key was turned. So, I am like, "WTF???" This is fuel injection and the engine should not diesel, RIGHT???"

I checked with SF Don and Srennoc to get advice. Tried retarding timing, starving fuel and the B35 motor still ran over. Scratched my head. Scratched my brain through my nose...still could not ditch the dieseling condition. Owner cussed me out ten different ways. Called MS support and got the same lame advice. What did it turn out to be?

Finally found a post about adding a one way diode on the Alternator exciter field wire. You know, the blue wire that makes the battery light turn on? That wire energizes the field coil to power the alternator.

Well...turns out, that power can leak back from that wire into the green switched power circuit and keep power in the injectors as long as the engine turns. This is an in-discrete, or dirty DC voltage leak through the diode pack of the alternator. The fix is to put a 1N4001 diode in line on the blue wire. This allows the battery light to ground out and power the exciter field, but not leak back into the Green circuit.

All 55 pin Moronic systems get ECU Logic power through the green wire at pin 27. This keeps the ECU on just long enough to power the injectors. So, BMW issued a technical service bulletin to install the diode on the blue wire. That is how this became a known BMW issue, but not to us owners.

 

[E9Wayne]

Guys, car is fixed now. It turned out my fuel pump was not getting full voltage based on where the wire was routed and there was some strange cosmic correlation between my pressure in the seat and air temp outside which took things south on a few startups - weird. A new green wire from the relay to the fuel pump fixed things completely and she's back to humming again. It's crazy how much diagnosis I and others put into this situation and it turned out to be a wiring issue, nothing more. Don and my local guy Tom Baruch put her back together and now I'm on the dyno path again after talking with Vern a few times. Blue Belle will be great for Monterey this year...