Re: 745i: A few questions... (archive)

I get about 36psi fuel pressure at idle and about 40psi with the vacuum line disconnected. You may want to verify that the correct fuel pressure regulator is installed on your car. I believe we use a different one than the normally aspirated 735 engine. Not sure which spec is more accurate but you may also want to verify mixture readings at idle and under load. A multi-meter connected to the O2 sensor gives you an idea of that, if you can trust the existing sensor. A dreaded lean condition under boost conditions is definitely not a good idea as the resulting detonation could trash an engine very quickly.

The hunting that your experiencing could very well be due to a lean condition. Motronic with lambda control tends to do that under idle and part-throttle conditions in less than perfect engines. That's why disconnecting the O2 sensor helps in this area. But in the 745 engine the ECU doesn't incorporate lambda control and all 745's that have an O2 sensor had one added after the fact to meet import regulations. I would look for the added black box that has been wired into the ECU and remove it since I'm not sure how it behaves when the O2 sensor is disconnected from it. BTW, my 745 passes emissions just fine without O2 sensor control.

demet

I was fixing up a few interior items on the 745i today. Looking into my shaky idle. Car is hard to start when cold. No vacuum leaks. Loosening up the AFM spring helps smooth out the idle. Removed the vacuum from fuel pressure regulator really smoothed out the idle a lot. Fuel pressure at idle is 30 lb. With vacuum line removed and pinched, 35 lb. So, why is the car idling lean?

Also, another question, while I save up the funds for a chip upgrade, I want to continue to drive the car. Since the wastegate controller is faulty, can I hook up the vacuum line directly to the wastegate? I did this, but I am still cutting out on hard accel. (I hooked up the intake pressure line to the wastegate, but it will cut out faster.) What signal from the control tube opens the wastegate: pressure or vacuum?

Last question. I found a wire in my engine compartment that is not hooked to anything. It is a single white wire with black shroud with a female spade lead. It emerges from an engine harness beneath the rear intake support arm along with the #5 injector wire and another wire that looks like it is hooked up to the post-shutoff turbo cooling fan temperature sender. What is this wire?

Very last question...!!! When cruising 75-85 mph, my engine wants to hunt up and down about 50 rpm. Is this due to a lean running condition. I have disconnected the O2 sensor but it doesn't seem to make a difference. On the 745i, what is the O2 sensor doing? I thought the euro brain did not have an input for it. Am I doing any harm leaving it unplugged? Does it run full-rich with it unplugged? Will it run better?

Thanks fellow bimmer nuts!

Scottie

Anybody bought parts at Allison BMW in Mountain View? Good deals?

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