1989 260E. I had a difficult time getting through smog by playing with the mixture with a long 3mm allen. I replaced the O2 sensor and back probbed it. At idle the O2 sensor stays low or high depending on the mixture adjustment. For example a small turn CW and the O2 sensor stays around 0.7v a 1/8 turn CCW the sensor stays at around 0.1V. In either case with the car under load or just keeping steady on the road the O2 reading always hovers between .5-.9V. Why doesn't the O2 sensor fluctuate more? If I create an air leak the O2 sensor drops as expected and if I starve it of air the O2 sensor goes to the high side. There is an 8 pin connector in front of the battery (no LED in it) but the #3 (signal) does not have a connector in there! therefore can't bother with measuring duty cycle. The car is a terrible challenge to get started if i set the mixture too lean. If set richer it starts but does take a few tries and needs a few minutes to run properly without stalling.

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1972 450SL
1982 300D Turbo

have you checked the fuse in the ovp relay? car sounds like in limp mode.if fuse good check and see if working correctly and that the socket with female pins that ovp plugs into is free of corrosion.

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David S Poole
European Performance
Dallas, TX
4696880422

"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
http://www.w108.org/gallery/albums/A...1159.thumb.jpg

The reason its fluctuates is because the computer would be adjusting the mixture up and down all the time.. No fluctuation means computer is not adjusting. The duty cycle IS the diagnostic on these cars.. Various duty cycles mean different things.

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Bill

Backyard Shadetree Mechanic, 30 years running.

1987 300TD
1988 300E (sold, sniff)
1999 S320
1981 Volkswagen Rabbit Truck (Diesel)
2001 Ford Expedition
1988 Suzuki Samurai
On my list to buy: R129, for me, and a R107 for my wife.

Yes, the car stays in open loop so the O2 sensor signal does not fluctuate. I replaced the OVP about a year ago because it was no good and that fixed it then.

I checked the the two fuses and made sure the OVP was not corroded and seating tightly into the socket. Car still stays in open loop. Any thing else to check?

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1972 450SL
1982 300D Turbo

The check engine light goes on for a few seconds with KOEO so that's working OK. Check engine code has not turned on with a fault. With KOEO the O2 sensor (backprobed) only shows a couple mV. I would expect .45V I thought would be priovided by the computer. The O2 wire to terminal #8 on the CIS is Ok (I ohmed it and wiggled it) so the O2 signal should be getting to the computer.

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1972 450SL
1982 300D Turbo