New exhaust gas oxygen sensor on the way

I've almost got this car running nicely. It now starts first time every time. The fuel pressure regulator fixed that. It turned out that the car was running high fuel pressure due to the faulty fuel pressure regulator and flooding the engine on start-up. It was also causing it to run rich.

Now that I've fixed all that, it still runs rough when the engine reaches operating temperature. When I say it runs rough, I mean it idles rough, because when there is even a bit of throttle it runs well and smoothly with full power. Interestingly enough the car runs fine when just started and cold.

When I hooked up the laptop, I noticed that the transition from even to rough running during engine warmup occurs just as the ECU switches from open loop mode (running without using input from the oxygen sensor) to closed loop mode (using input from exhaust oxygen sensor to control mixtures). The voltage from the oxygen sensor seems to be within range, and therefore is not triggering any error codes, but the engine is definitely getting too much fuel, I can smell it from the exhaust, and the engine misses when idleing probably every 8-10 revolutions or so.

Exhaust gas oxygen sensors work by generating an electric current that corresponds with the amount of oxygen in the exhaust stream, which provides a means to determine the fuel/air mixture at combustion. This current decreses with the oxygen content, and rises as the mixture gets more rich. When they fail, they get sluggish to increase in voltage under the rich condition, and the ECU increases the injector pulse width even more, as it is getting an incorrect reading. The result is an engine that runs rich when warm. As VP Commodores idle under closed loop when hot, a faulty exhaust gas oxygen sensor (or Lambda sensor) has dramatic effects on the engine idle.

I have a new sensor on the way that I will pick up from Hunter Holden tomorrow.