Hello group,

The '86 300E is having rought idle . I checked the Duty Cycle on diagnostic port while the Engine was running (operating temperature) , it is stuck at a very low number (7 percent). It used be hovering around 50 percent few months ago. I am not sure what would cause the wrong reading now.

I did the following tests:

- Disconnecting the O2 sensor corrected the duty cycle (showed 50 percent) .
- Disconnecting the temp sensor plug going to ECU made no difference in reading.
- Disconnecting the temp sensor plug going to ignition control unit increased the duty cycle.
- Checked the voltage at the output of O2 sensor , it is about 480 mV which I think is the correct one.

Does this mean I have a bad O2 sensor , a bad temp sensor plug or something else?

Any suggestion is appreciated.

__________________
Zorin

'88 Benz 420 SEL
'79 Porsche 911SC ('87 Carrera Engine)
'00 BMW 540 Sports Package
'99 Land Rover Discovery

86 300E - Manual (sold)
88 260E (sold)
84 944 (sold)