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"Anytime I see mention of intermittent seasonal behaviour with 240s, I immediately think of possible chassis ground problems. With that in mind, I'd be very, very suspicious of the black wire having a solid chassis ground connection. You can start by checking with a meter, but I'd want to do a proper inspection. I don't recall where the O2 sensor black wire attaches in the LH 2.4 240s, but for a 740 it goes to the ground rails behind the ECU. Make sure all chassis ground connections in such areas are clean and solid - wiggle and remake connections as needed. "
On the 240, a three-wire sensor is used. Its black wire is the sensor output, not a ground. The heater is isolated to the two white wires. The black one connects to the shielded green wire to the ECU below the windshield wiper motor.
In your 7/9 cars have you replaced the original with a four-wire sensor? I think this is a good idea, because it bypasses all the rusty exhaust plumbing originally designated the ground return for the oxygen sensor when these cars were built.
I grounded my four-wire sensor to the same bolt where the radio noise suppression ground strap for the cam cover grounds at the firewall.
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Art Benstein near Baltimore
Why do we leave cars worth thousands in our driveways and put our useless junk in the garage.
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