The Mustang O2 seensor trick works fine, sounds like you have another issue. It might not be realated to the slow issue, but if it is always reading about .5v DC it is a problem. The .5 volts is the center baseline for the computer to receive until the o2 Sensor warms up tp temperature and begins to relay accurate readings to the ECU computer. The O2 sensor needs to warm to about 300-600 I think before It is providing accurate readings, hence the two white wires from the Mustang o2 sensor. They are the heater wires and I dont think they matter which is pos or neg, they are reversable and work correctly as far as I can tell, I have used the sensor myself.
I guess my thought for your issue is that it might not be coming out of the "wait for the o2 sensor to warm up to temp and until then send a constant .5 volts" mode. My thoughts would be to first check to see that the MB o2 wire connections are providing a ground at the brown wire and voltage at the other wire in the two contact connection that went to the OEM o2 sensor, without a ground and the voltage, the o2 sensor will not heat up...as fast. Now, eventually, once you are driving, the sensor, if the trouble is just the heater circuit, the sensor would warm to operating temp just from the heat of the exhaust and should begin to give the computer readings. If the heater wires are woring correctly and the signal wire ( black) is connected correctly to the OEM harness green wire, You might have an ECU problem, but that is beyond my diagnosis abilities

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Christopher Henkel
1990 190E 2.6 - Arctic white SOLD
1986 190E-16v - Blauswartze
1993 300CE - SOLD
2003 W208 CLK 320 Cabriolet - Magma Red