Setting idle mixture and testing o2 sensor - '87 190e
 

Hi all, you may remember that my new-to-me 16v has problems starting when hot, and likes to stall when hot and coming to a stop. I am trying to diagnose this myself, as I don't have a local mechanic who can work on it for me in any near time frame.

This weekend I have tried to check the duty cycle, as I know the previous owner was playing with the idle mixture screw. I have a meter with duty cycle and these were my readings:

- Car off, ignition on. 50.6%
- Car idling, cold - 50.6%
- Car idling - hot - 95%

Adjustments of the mixture screw do not change this 95%.

It is my understanding that these readings (50%, and calling 95% 100% for fault code purposes) point to a faulty lambda sensor. With that in mind, I set out to read the voltage from the lambda sensor directly. I plugged in to the sensor under the passenger seat with a home-made harness. I got the following readings.

- Start the car (already warm) - 0.48v
- As the car idles, it climbed slowly to 0.78v where it stays steady
- If I give the car gas, it drops in a linear fashion to ~0.7v, then climbs back up to 0.78v at idle.

I am not sure if this indicates a bad lambda sensor or not.

Any suggestions on what I should do next based on these readings sure would be appreciated.

Thank you.

Those readings show that the O2 sensor indeed is inop.

Start with the obvious & replace that sensor

Thank you very much. I have a new 02 sensor, but had a heck of time getting the old one out. Will get it to a shop and report back!

Ok - new sensor is in. Old one was covered in black. Unfortunately, I am still reading 50.6% key on, car off, and not much different when hot and idling. One time I get ~50%, another time I get 95% again.

Any thoughts Doc?

Edit: Voltage at o2 is now 0.49v and doesn't change.