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Old 09-01-2003, 07:50 PM
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zorin zorin is offline
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Join Date: Feb 2003
Location: Ottawa, Canada
Posts: 126
Exclamation Duty Cycle / Bad O2 Sensor ?

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.

'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)

Last edited by zorin; 09-03-2003 at 11:57 AM.
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