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?

My 87 535 is having problems with NOx emissions and appears to run slightly lean. All rubber induction and vacuum hoses have been replaced as well as the fuel pressure regulator. I replaced the catalytic converter and oxygen sensor and am still high on NOx. I've run about 4 bottles of TechRon through in the last year. About the only items left to inspect and/or repair are the injectors and injector O-rings.

Are these easy to remove after 170,000 miles? If I can remove them, is there a preferred method or fluid for cleaning them? For varnished carburetors, I've always soaked them in Chem-Dip (gallon can with strainer included) with good results. Is this fluid OK or will it damage them?

I can get new injectors for $50 each at BavAuto. Is it worth trying to clean them or should I just get new ones?

Any thoughts or past experience?

Thanks.

Your NOx issue is troubling, because the catalytic converter is the one and only NOx control element on this car. Replacing it should reduce your NOx to very low levels. The converter should be a "three-way" catalyst element, capable of reducing nitrogen oxides to nitrogen. The DEC and OEM units are of this type,

As for injector cleaning, that's a matter of choice and budget. 170,000 miles is a lot of service. If you have them reconditioned, you're still left with the original case, pintles, and coils.

As for the oxygen sensor, have you checked for voltage swinging briskly between roughly 0.2 and 0.8 V? Pull back the boot on the spade connector near the rear of the engine, and connect the positive side of a DVM to it; the negative goes to chassis or the negative battery terminal. Car should be fully warmed up, and run it at 1500 to 2000 rpm. You want to see the voltage cycling 1 to 2 times a second. If your car is really running lean, the sensor will be "stuck" below 0.3V.