Clyde,

Thanks for the advice. I measured the OS voltage last Saturday and it was less than .2 volts. Went back tonight to double check and found my multimeter connections are temperamental for this measurement, and when I applied pressure to the connections at the multimeter, the voltage measured about .8 volts. Pulling out the dipstick lowered it to about .5 volts. I am going to buy a new multimeter tomorrow and repeat the test. However, I also found that the OS heater is not getting voltage. I checked at the relay and the motronic unit is providing voltage to relay pin #85 all the time, even when the engine is stone cold. My understanding is if pin #85 has voltage, the relay opens and prevents voltage to pin #87, the OS heater. Could this be a clue? When is the OS heater to be activated and what sensor could be defective to cause the motronic to supply pin #85 voltage all the time? I swapped with the fuel relay just to be sure and the relay appears to be OK.

When I say the engine appears to run lean, I refer to the misfire I get at idle and at no load elevated idle (about 1500 - 2000 RPM). At no load and 2000 RPM you can here a pronounced random misfire at the tailpipes and the engine shakes around a bit. While driving the engine usually runs OK however some days the power surges somewhat like it is too lean.

Anyway, I went to my local BMW dealer who said they did not have an exhaust gas analyzer to measure NOx, and recommended a valve adjustment. The valve lash is fine, I checked it last year and it did not require adjustment.

Another piece of information. When I first bought the car 2 years ago, It had an erratic idle and I brought the car to a recommended BMW specialist who attempted to fix it by "richening" the AFR by changing the flow meter flapper return spring setting. I was not to happy with this because the flow meter is sealed from the factory and it didn't seem like a proper fix. They also unplugged the OS and changed the throttle position sensor initial voltage to 0.5 volts. All of those changes didn't really help the idle problem but did manage to kill the engine power and mess up the automatic transmission shift points. I called him back the next day to determine if he remembered where the flapper return spring was initially. Based on his memory, I have returned it to the "factory" setting. I also plugged the OS back in and reset the TPS initial voltage to 0.7 volts per Bentley. The erratic idle turned out to be the throttle closed micro-switch, used only on auto's.

Anyway, I went through emissions once, a month ago, with the flapper return spring set "richer", but it had zero effect on the NOx.

I'm running out of ideas, any more thoughts?

1. You want a reliable meter and leads; after years of fussing with a Radio Shack meter, I bought a Fluke, and I appreciate it every time I need it.

2. That O2 sensor should be cycling at 2000 rpm; lets assume that is the case, and that you are taking your readings at idle. The pulled dipstick is leaning out the mix. You should be able to do the same thing by adjusting the mixture screw on the bottom corner of the AFM module -- 5 mm allen, I think. Instead of using a CO meter, just adjust the screw for an O2 sensor that wanders around 0.5 volts; probably a bit richer than what you get for optimum CO, but better for an even idle.

3. That occasional popping at idle and even off idle seems typical for my car. From what I read on this board, it's not unusual.

4. The AFM flapper is another matter. If off by a lot, it could keep you from achieving the idle mix you need by use of the CO adjusting screw. Too rich, you get excess deposits and oil dilution; too lean, you burn valves. I'd rather not mess with it.

5. The O2 sensor heater should be powered while the care runs, I believe at all times. I don't have access to my manuals right now (traveling). Where are you looking for voltage? The two heater leads should show maybe 6-12 ohms resistance, engine off. Engine running, you should get 12VDC referenced to ground on one lead, 0VDC at the other. When you buy that Fluke, a set of wire-piercing probes are nice to have, too!!!!! An unpowered heater will delay the turn-on time for the sensor, and may result in sluggish performance at idle -- they don't do much until they reach about 600 degrees F.

Good luck.

Good luck