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.

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Saumil S. Patel