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I've been trying to get through New Jersey emissions for the past few weeks. After failing twice I checked my O2 sensor and found it was completely unresponsive. I ordered a new one and went to install it today. I found that the sensor wire (black) had somehow been disconnected. So I repaired it and checked the duty cycle. As soon as I hook up the meter it reads about 30% and climbs steadily all the way to 98.5% where it remains steady. I'm using a Sears DMM with duty cycle. If I connect red to pin 3 and black to pin 2 I get a steady 98.5%. If I only connect the red to pin 3 I do get a fluctuation between 21% and about 47%. If I reverse the leads in pin 2 and 3 the readings also vary between 21 and 47%. The problem is the readings don't change when I turn the mixture adjustment. It does affect the running of the engine but not the reading on the meter. Also the car is now really hard to start and has no power. I can't rev the engine past about 2K rpm even in park. If I go richer on the mixture I get black smoke from the exhaust but will rev to about 3k. Leaner results in even less power and a horrible stumble just off idle. The car is not driveable right now because if I step on the gas it backfires. The only thing that was touched was the O2 sensor. The car ran with the sensor unhooked, but felt a little sluggish because it was running rich. It idles fine. Any suggestions?? Where do I start??

No ideas??

Can you check vaccum at idle ? Should be between 16 and 22 inch Hg. If it is too low, the exhaust system is clogged, most likely your catalytic converter if your car has one.

I agree with saumil, you may have a clogged ‘cat’. I had one on my SL600 and the symptoms were similar to yours, but not drastic as my SL has a pair of catalytic convertors; one for each bank of cylinders. In my case, the car would drive but was very unresponsive. Idle was rough (but tolerable), however getting my SL up to highway speeds quickly overheated the engine.

When the offending ‘cat’ was removed from my car, a ton of fine powder/sand came out of the exhaust end of the ‘cat’. The honeycomb inside the ‘cat’ had somehow become loose and driving vibration caused it to abrade itself inside the ‘cat’. The powder/sand eventually built up enough to block the engine exhaust.

In your case, the problem may have been steadily getting worse and when you changed the Oxygen Sensor, coincidentally, the ‘cat’ may have finally plugged up good to its current state.

Just a thought, but worth investigating.

OK, I'll check for a clogged cat. Seems strange since the car drove fine just 30 minutes earlier. That's about how long it took to replace the O2 sensor. Maybe because it was running so rich with the old sensor being disconnected?? I'll check for vacuum and post whatever I find. Thanks for the tips.

After replacing the O2 sensor, one thing you could do is disconnect the battery for about 10 minutes, incase the computer has to be reset. Try this first.

I checked for vacuum at idle and it shows about 18. I have the car running now but still not right. After doing more research, and playing with the mixture a little bit, the car starts and idles smooth. It drives pretty good but still feels sluggish. I replaced the plugs again and the cap and rotor only have about 3K miles on them. I'm not getting proper readings from pin #3 when i hook up the meter. When i hook up it shows about 50% and varies up and down as it should. But as the car idles the numbers steadily increase all the way up to 98.5% and stays there. It takes maybe 30 seconds to get there. What could cause this??

So the exhaust clog is ruled out. What pin #3 are you meaning ? What is the exact model of your car ? If you have changed the O2 sensor then time to look somewhere else.

Test the coolant temp sensor, on my 87 260E, it is the last sensor on the block. When the engine is cold, you should get resistance of about 2.5 to 3.5K across the terminals. When the engine gets warm to 80 deg-C, the resistance should drop to between 250 and 350 ohms. The exact values may be different, but there should be a drop by a factor of 8 to 10 when the engine goes from 20 degC to 80 degC. An open circuit in this sensor circuit puts the car in a limp mode, so check the cables for this sensor as well.

The next part to look at if you have not already is the EHA. This guy is in charge of enriching the fuel mixture during acceleration. The resistance of the EHA coil should be about 20 ohms. If the coil resistance is fine, then check the current thru it (make sure the meter is in series with the EHA). When the key is on run (just before your start), there should be 10 to 20 ma current (depends on the exact model of your car) should flow. Once the car has started and is at idle, the current will drop. When the car is not yet warmed up (20 deg-C), blip the gas pedal, the current should transiently increase to something greater than 15 ma.

Let us know what you find.