My '88 240DL failed its' emission test because of high NOx (4.7 vs limit of 3.0) The HC and CO were way below their limits. I found that theO2 sensor output voltage was low(..0.1V) and not cycling so I replaced it. No change. I replaced the intake manifold and throttle body gaskets, and checked all the vacuum lines for leaks but still no change. I found that increasing fuel pressure by disconnecting its vacuum line did cause the O2 sensor voltage to cycle normally. Also.disconnecting the air mass meter caused the O2 sensor voltage to go high (steady at about 0.9 volts) Changing the mixture adjustment on the air mass meter had no effect. Any suggestions on getting this lamda system working will be much appreciated. JHall

 

Are you sure it is the correct Mass flow?
Maybe a bad new sensor?

 

I'm sure that the air mass meter is the right part no. but I'm not sure it'sgood. I believe the new O2 sensor is OK because it does respond when I cause a rich mixture by increasing the fuel pressure or by disconnecting the air mass meter. Thanks for the fast response. JHall

 

I would look at the Mass Flow meter.

We had one a couple of weeks ago that had the wrong Mass Flow.
It ran like Crap. The O2 was pretty much stuck at like 5 Volts. Come to find out it had the wrong Mass Flow sensor installed.

 

I didn't think the O2 could go that high. The highest I've ever seen is about 1 volt. JHall

 

Sorry meant .500. I always forget about the Decimal point. LOL