I have noc clue whether your O2 sensor is heated or not, so here's a generic discussion.

FOR NON-HEATED O2 SENSOR

Initially when you start your car, the ECU should control the mixture "open-loop" i.e. by dead reckoning rather than using the O2 sensor signal which is essentially absent until the sensor warms up.

Now, how on earth does the ECU know that the sensor is warm enough? Well, it doesn't, but it does know the engine temperature and the time that passes. Usually if the engine is at the operating temperature, the exhaust system (and the O2 sensor) is warm enough. So the ECU really depends on the engine temp sensor (and some time constraints) to go from open loop to closed loop control.

Check your engine temp sensor. The correct resistances are in the FAQ IIRC. Note that there are two - one goes to the instrument panel, the other one to the ECU, make sure you check the correct one.

FOR HEATED O2 SENSOR

Some O2 sensors are heated. Obviously, no heater heats instantly and it still takes some time (maybe 60s or so) for the O2 to get up to 500-600F or so it needs to operate. The ECU is basically preprogrammed to know that your O2 sensor is heated and that 60s after you crank it up, it will start taking O2 sensor's signal into account.

WRAPPING IT UP

The crux of that discussion is: some 1-4 minutes after you start your car, the ECU will try to measure the O2 content in exhaust via the O2 sensor. One of the problems is that if your O2 sensor is connected to the ECU, the ECU thinks it can depend on it. If it's a heated sensor, the ECU will try to read O2 content in your atmosphere and that's not a good thing as you have seen. If it's not heated, the ECU will see a zero voltage essentially and will probably decide that O2 sensor signal is absent.

Since your '91 240 sensor is probably heated, simply disconnect it. The ECU will know not to depend on it and will probably try to implement some backup control, which will probably make your engine run rich (if the ECU knows what it's doing).

I hope I've been helpful.

Cheers, Kuba